<p>I don't know if this was thought up ahead of time, but it sure makes sense in hindsight.</p>

<p>The reason 21 million is the right number is because people don't know how to value currencies.</p>

<p>For instance, right now a Euro is worth $1.30 USD and a Japanese yen is worth about a U.S. penny.   Ask someone which currency they would rather hold right now and most will answer Euro, because $1.30 is worth more than $0.01.</p>

<p>Of course, that relative unit value means nothing.  Ask most currency traders which currency is better to hold and most (today) would probably say Japanese yen, because what matters is whether the value will go up or down, relative to the other being compared.</p>

<p>When bitcoin hit parity with the U.S. dollar in Feb 2011, it gained a sense of legitimacy that helped propel it on a tremendous pace, rising over 30X that level just four months later.</p>

<p>If there had been more than 6 million coins issued by then, the total dollar value of all bitoins would probably have been about the same, and thus the exchange rate would then have been lower.   So let's say there were instead 60 million coins issued by Feb 2011, and each one worth a dime.  That 60M X $0.10 is the exact same total dollar valuation the 6M X $1 has ($6M) for all bitcoins combined.  Tthe difference is that because it had become "worth more than a dollar", and as a result people attributed greater interest and respect for it.  Had it not been "worth more than a dollar" so early, it might have taken a whole lot longer to get the name recognition and attention it did that has helped attract the participants that Bitcoin has today.</p>

<p>Bitcoin today stands on "dollar parity"'s shoulders.</p>

<p>That may sound bizarre but ask that Euro / Yen question to different people yourself and then ask the reason why their answer was given.   </p>

<p>So the number needed to be a low enough number so that while total dollar valuation of all bitcoins was still in the single-digit millions it would reach parity with the U.S. dollar at some point.</p>

CC BY-SA 3.0
2014-06-09T10:00:49.493Z
So why not 16 million (3y cycle)? Why not 26 million (5y cycle)?
Pacerier
CC BY-SA 3.0
2014-06-09T10:05:24.967Z
And why not 17 million (40 btc reward)? Why not 25 million (60 btc reward)?
Pacerier
CC BY-SA 3.0
2014-07-26T22:33:38.243Z
Why not a 3 yr cycle? Perhaps because that would cause the first & second halvings to occur too soon (and thus too high of a concentration of coins going to those who mined or bought early).  Without knowing in 2008 what the level of traction Bitcoin would have a few years later, using four years made sure that if it took three years for Bitcoin to get any publicity (e.g., what happened with the Gawker Silk Road story in May, 2011) there was still a good year left of mining at the 50 XBT/block rate before the first halving -- thus distributing Bitcoin to a bit wider set of early adopters.
Stephen Gornick
CC BY-SA 3.0
2013-03-17T17:28:06.927Z
2014-07-26T22:23:34.783Z
2014-07-26T22:23:34.783Z
Stephen Gornick
<p>The exact number of Bitcoins is not important.  Whether the end result is 1 million or 100 billion makes little real difference.</p>

<p>The important aspect here is the <strong>process</strong>, not the quantity.  </p>

<ol>
<li>New Bitcoins enter the system in an orderly, predicable way.</li>
<li>Outside forces cannot arbitrarily flood the currency with new money.</li>
<li>An incentive is provided for people to apply their CPU power to make the currency more secure.</li>
<li>Eventually, Bitcoin has to be self-supporting through transaction fees.  Hence the tapering off of blockchain rewards.</li>
</ol>

<p>Having said all that, there are some psychological advantages to having a low limit like 21 million.  It was inevitable that people would see Bitcoin as being "more valuable" if the exchange rate for a whole Bitcoin was over $1.  Ensuring this "high exchange rate" but making the coins highly divisible was probably a conscious design decision.</p>

CC BY-SA 3.0
2014-06-09T10:03:25.560Z
This doesn't actually answer "Why 21m?"....
Pacerier
CC BY-SA 3.0
2014-06-12T14:54:00.727Z
And the answer is "why not?".  I was just trying to explain why the exact number doesn't matter.
slashingweapon
CC BY-SA 3.0
2013-07-18T16:02:18.770Z
2013-07-18T16:02:18.770Z
slashingweapon
<p><a href="http://en.wikipedia.org/wiki/Gold" rel="nofollow noreferrer">Wikipedia (Gold)</a>:</p>
<blockquote>
<p>A total of 174,100 tonnes of gold have been mined in human history,
according to GFMS as of 2012. This is roughly equivalent to 5.6
billion troy ounces or, in terms of volume, about 9261 m3, or a cube
<strong>21.0 m</strong> on a side.</p>
</blockquote>
<p>Bitcoin is often compared to gold, the total number of bitcoins matches the total amount of gold mined in human history which can be imagined as a cube <strong>21 m</strong> on a side.</p>
<p>As it is not entirely so important how many Bitcoins will exactly be mined. Satoshi could have easily chosen almost any number. He could just adjust block reward halving (210 000 blocks), reward sizes (50, 25, 12.5 ...), etc. to match some particular number.</p>
<p>Gold is considered <strong>a standard for currency</strong> and was used as such for thousands of years until recently <a href="https://wtfhappenedin1971.com/" rel="nofollow noreferrer">it was abandoned in 1971</a>. And that could be the reason why Satoshi maybe wanted Bitcoin to resemble gold at least in this unimportant property.</p>
<h2>EDIT</h2>
<p>Satoshi Nakamoto himself compared Bitcoin to gold.</p>
<p><a href="https://bitcointalk.org/index.php?topic=583.msg11405&amp;source=post_page---------------------------#msg11405" rel="nofollow noreferrer">Satoshi Nakamoto @ bitcointalk.org:</a></p>
<blockquote>
<p>As a thought experiment, imagine there was a base metal as <strong>scarce as gold</strong> but with the following properties:</p>
<ul>
<li>boring grey in colour</li>
<li>not a good conductor of electricity</li>
<li>not particularly strong, but not ductile or easily malleable either</li>
<li>not useful for any practical or ornamental purpose</li>
</ul>
<p>and one special, magical property:</p>
<ul>
<li>can be transported over a communications channel</li>
</ul>
</blockquote>

CC BY-SA 3.0
2015-03-01T19:29:59.470Z
M, million or meters, that is the question
saloalv
CC BY-SA 3.0
2016-05-10T16:03:59.403Z
And why not *5.6 billion* troy once eq. ?
fduff
CC BY-SA 3.0
2017-11-30T03:56:35.993Z
Did he not account for the fact that total bitcoins in circulation will never be equal to the total bitcoins mined/ mine-able? Some of them are lost forever..
Deepak Gupta
CC BY-SA 3.0
2017-11-30T12:04:19.397Z
Yeah, also some gold is lying at the bottom of the sea. Some is burried etc.
Ondrej Bozek
CC BY-SA 4.0
2014-01-01T15:16:57.450Z
2022-02-23T10:37:31.080Z
2022-02-23T10:37:31.080Z
Ondrej Bozek
<p>It is the result of a 50 bitcoin reward half life of 210,000 blocks.</p>

<p>Reward starts out at 50 bitcoins and halves ever 210,000 blocks. This works out to be 2.1 quadrillion monetary units of currency (satoshi). This is probably the largest number estimated to be needed for a global currency and some padding for attrition.</p>

CC BY-SA 3.0
2014-06-09T10:16:30.903Z
Why 50 and not 55 or 60 or 45?
Pacerier
CC BY-SA 3.0
2013-03-17T16:18:00.277Z
2013-03-17T16:18:00.277Z
Tuxavant
<ul>
<li>42 is the Answer to Life, the Universe, and Everything.</li>
<li>Bitcoin block rewards are cut in half every 4 years.  Half of 42 is 21.</li>
<li>Nearly every person can understand what x "million" means, but comprehension breaks down rapidly with larger numbers.</li>
</ul>

<p>Therefore 21 million.</p>

<p>;-)</p>

CC BY-SA 4.0
2018-05-16T11:36:56.240Z
Haha!! I just answered a similar question with the same answer:-)!!! And myself totally agree with the "wink" at the end ;-) ;-)
behkod
CC BY-SA 3.0
2017-09-07T03:10:42.857Z
2017-09-07T03:10:42.857Z
Jonathan Cross
<p>This was done based on production rate mostly. They did take some things into account but the number doesn't have a real economical explanation rather than the fact that they had to stop production somewhere to maintain a value. </p>

CC BY-SA 3.0
2013-12-03T03:23:47.287Z
2013-12-03T03:23:47.287Z
Tracks
<p>I suspect it has something to do with the M1 USD supply at the time Satoshi was developing Bitcoin.  If you look at the decade prior to the white paper, inclusive of the quantitative easing period commencing with the 2008 housing bubble, the M1 money supply shows a trendline with a slope of ~58, using years as 4 digit integers.  This means, that if predicting M1 over the next decade (from 2009 to 2019), there would be an expected M1 of 2.1 trillion USD, at which point 90% of all bitcoin will have been mined.  Because Bitcoin is designed with an 8 decimal fraction system, the 21 million total BTC can actually be expressed as 21,000,000.00M = 21,000.00Billion = 21.00Trillion.  So roughly 9x functional BTC will exist relative the USD.  Using the M1 trendline to predict M1 in 2140, we see an expected ~10Trillion, which relative to the 21Trillion functional BTC, still gives us a 2x cushion on volume.  And mathematically, we've seen noted elsewhere in this Q&amp;A that the 4 year halving schedule of the coinbase gives us 21Million BTC.  So, there's a convenient convergence of predicted volume needs &amp; reward schedule @ 21Million BTC.  (But realistically, this is probably just me seeing signal where I want to see signal, and not actually Satoshi's anticipation of volume.)<br><br>
I think another curious question is why the 4 year halving schedule? Is this based on national US elections?  Is it based on a calendar Leap Year?  Is it Satoshi's approximation of a reasonable re-evaluation period for BTC participation rates?<br><br>
<img src="https://i.sstatic.net/QpV4t.png" alt="M1 Supply &amp; Trend from St Louis Fed (FRED)"></p>

CC BY-SA 3.0
2015-04-11T18:23:24.063Z
2015-04-11T18:23:24.063Z
Adocracy
<p>Here's something Nakamoto wrote on the subject</p>
<p>From <a href="https://www.bitcoin.com/satoshi-archive/emails/mike-hearn/11/" rel="nofollow noreferrer">https://www.bitcoin.com/satoshi-archive/emails/mike-hearn/11/</a></p>
<blockquote>
<p>From: Satoshi Nakamoto satoshin@gmx.com<br />
Date: Mon, Jan 10, 2011 at 4:34 PM<br />
To: Mike Hearn mike@plan99.net<br />
Subject: Re: More BitCoin questions</p>
</blockquote>
<p>...</p>
<blockquote>
<blockquote>
<p>I wanted something
that would be not too low if it was very popular and not too high if it
wasn't.</p>
<p>It'd be interesting to see the working for this. In some sense the
number of coins is arbitrary as the nanocoin representation means the
issuance is so huge it's practically infinite.</p>
</blockquote>
<p>It works out to an even 10 minutes per block:
21000000 / (50 BTC * 24hrs * 365days * 4years * 2) = 5.99 blocks/hour</p>
<p>I fudged it to 364.58333 days/year.  The halving of 50 BTC to 25 BTC is after &gt;210000 blocks or around 3.9954 years, which is approximate anyway based on &gt;the retargeting mechanism's best effort.</p>
<p>I thought about 100 BTC and 42 million, but 42 million seemed high.</p>
<p>I wanted typical amounts to be in a familiar range.  If you're tossing around &gt;100000 units, it doesn't feel scarce.  The brain is better able to work with &gt;numbers from 0.01 to 1000.</p>
<p>If it gets really big, the decimal can move two places and cents become the &gt;new coins.</p>
</blockquote>

CC BY-SA 4.0
2023-06-16T09:30:43.533Z
2023-06-16T09:30:43.533Z
RedGrittyBrick
<p>The number comes from 3 decisions:</p>
<ol>
<li>Time between blocks: 10 minutes</li>
<li>Initial issuance to be 50 BTC</li>
<li>Halving every 4 years</li>
</ol>
<p>From 1) we know each hour should have 6 blocks, so the number of blocks in 4 years is 4 * 365 * 24 * 6 or 210.240</p>
<p>Satoshi was practical and round it to 210.000</p>
<p>Now, if you add all block issuances, starting with 50 and halving every 210.000 blocks, the total will be 21 million</p>
<p>Satoshi was pretty sure about the 10 minutes between blocks and the 4 years halving period, but he thought on using 100 BTC as initial issuance, but drop it because “42 million seemed high”</p>

CC BY-SA 4.0
2024-12-20T22:50:57.813Z
2024-12-20T22:50:57.813Z
bordalix
<p>Because if he had picked 210,000,000, then a bitcoin right now would only be worth $2,600 and that is too low.</p>
<p>Before you downvote this answer, really think about it. Would $BTC be $260,000 right now if the supply had been limited to 2.1 million coins? (assume that a Satoshi is defined as 0.000000001 BTC)</p>
<p>Also note that none of the above answers actually respond to the OP except the one relating the number of BTC to the dimensions of the base of the great pyramid or something like that. What does this tell you?</p>

CC BY-SA 4.0
2023-05-18T19:07:53.657Z
2023-05-18T19:07:53.657Z
bigjosh
<p>Very simple answer:
The reward for Bitcoin Miners had been 50 Bitcoins per Block for the first 210,000 Blocks. For the next 210,000 blocks it is the half of 50 (=25).
Take the geometric series and you'll be getting the total amount of nearly 21 Million Bitcoins (nearly, because a geometric series converges to his boundary value, but never reaches it!).</p>

CC BY-SA 3.0
2014-07-17T14:36:59.717Z
You're just restating previous answers
dchapes
CC BY-SA 3.0
2014-07-17T12:21:28.920Z
2014-07-17T12:21:28.920Z
Moritz
<p>Here's a mathematical explanation:</p>

<p>Calculate the number of blocks per 4 year cycle:</p>

<pre><code>6 blocks per hour
* 24 hours per day
* 365 days per year
* 4 years per cycle
= 210,240
~= 210,000
</code></pre>

<p>Sum all the block reward sizes:</p>

<pre><code>50 + 25 + 12.5 + 6.25 + 3.125 + ... = 100
</code></pre>

<p>Multiply the two:</p>

<pre><code>210,000 * 100 = 21 million.
</code></pre>

<p>Economically, because the currency is effectively infinitely divisible, then the precise amount doesn't matter, as long as the limit remains fixed. </p>

CC BY-SA 3.0
2013-03-17T17:41:21.503Z
Good explanation, but you're just postponing the answer. Why 4 years per cycle? Why was 4 years per cycle picked as the number for years per cycle?
Andrei
CC BY-SA 3.0
2013-03-17T18:29:06.990Z
All this info makes sense, but doesn't really answer the question!
B Seven
CC BY-SA 3.0
2013-03-17T21:24:41.247Z
If you check my original answer, you'll see I ended with "but I don't know the economics behind it".  I see that has since been edited - not by me.
Chris Moore
CC BY-SA 3.0
2013-03-17T21:39:08.730Z
Interesting. Does this mean that slightly more than 21 million bitcoins will be created?
Nick ODell
CC BY-SA 3.0
2013-03-17T21:42:44.347Z
No, slightly less than 21 million, because of the rounding error on the last halvings.
Andrei
CC BY-SA 3.0
2013-03-19T20:20:16.470Z
I think we best conclude that nobody knows why Satoshi chose for 21 million or 4 years per cycle. But I would not recommend accepting this answer as correct.
In fact it only answers "What's the maximum amount of bitcoins that can exist?".
Steven Roose
CC BY-SA 3.0
2014-10-02T11:51:09.993Z
See my guess why 21 million was picked in my answer below.
Ondrej Bozek
CC BY-SA 3.0
2015-09-08T16:32:18.233Z
http://bitcoin.stackexchange.com/a/19676/3908
Ondrej Bozek
CC BY-SA 3.0
2013-03-17T16:32:20.030Z
2013-03-17T17:56:55.607Z
2013-03-17T17:56:55.607Z
Chris Moore
<p>Why did Satoshi pick 21 million as the number of bitcoins to be created? What is the significance of that number?</p>

CC BY-SA 3.0
2013-03-17T15:59:58.940Z
I thought this question had already been asked, but I can't find it.
o0'.
CC BY-SA 3.0
2013-03-18T05:31:08.973Z
I added the finance tag since this question is asking about not just the math behind how 21 million blocks was created, but the reasoning behind all that math from an economic perspective (assuming there was one)
makerofthings7
CC BY-SA 3.0
2016-05-10T16:04:39.620Z
I asked myself the same question, but I don't see any clear answer posted on this.
fduff
CC BY-SA 4.0
2022-08-25T23:54:04.330Z
Asked a different way: If Satoshi at picked a max of 11 million, would the market value of a bitcoin today be twice as high? If you don't think this is a deep question, then I think you have not thought deeply out what bitcoin really is.
bigjosh
<economic-theory><finance>
Why was 21 million picked as the number of bitcoins to be created?
osmosis
# Why was 21 million picked as the number of bitcoins to be created?

(Asked by: username_0 on 2013-03-17)

Why did Satoshi pick 21 million as the number of bitcoins to be created? What is the significance of that number?

**Tags:** economic-theory, finance

> **Comments:**
> **username_1**: I thought this question had already been asked, but I can't find it.  
> **username_2**: I added the finance tag since this question is asking about not just the math behind how 21 million blocks was created, but the reasoning behind all that math from an economic perspective (assuming there was one)  
> **username_3**: I asked myself the same question, but I don't see any clear answer posted on this.  
> **username_4**: Asked a different way: If Satoshi at picked a max of 11 million, would the market value of a bitcoin today be twice as high? If you don't think this is a deep question, then I think you have not thought deeply out what bitcoin really is.  

---

## 12 answers

### Answer by username_20 on 2013-03-17. Score: 74 | Accepted answer

Here's a mathematical explanation:

Calculate the number of blocks per 4 year cycle:

```
6 blocks per hour
* 24 hours per day
* 365 days per year
* 4 years per cycle
= 210,240
~= 210,000

```

Sum all the block reward sizes:

```
50 + 25 + 12.5 + 6.25 + 3.125 + ... = 100

```

Multiply the two:

```
210,000 * 100 = 21 million.

```

Economically, because the currency is effectively infinitely divisible, then the precise amount doesn't matter, as long as the limit remains fixed.

> **Comments:**
> **username_21**: Good explanation, but you're just postponing the answer. Why 4 years per cycle? Why was 4 years per cycle picked as the number for years per cycle?  
> **username_22**: All this info makes sense, but doesn't really answer the question!  
> **username_20**: If you check my original answer, you'll see I ended with "but I don't know the economics behind it".  I see that has since been edited - not by me.  
> **username_23**: Interesting. Does this mean that slightly more than 21 million bitcoins will be created?  
> **username_21**: No, slightly less than 21 million, because of the rounding error on the last halvings.  
> **username_24**: I think we best conclude that nobody knows why Satoshi chose for 21 million or 4 years per cycle. But I would not recommend accepting this answer as correct. In fact it only answers "What's the maximum amount of bitcoins that can exist?".  
> **username_8**: See my guess why 21 million was picked in my answer below.  
> **username_8**: http://bitcoin.stackexchange.com/a/19676/3908  

---

### Answer by username_8 on 2014-01-01. Score: 25

[Wikipedia (Gold)](http://en.wikipedia.org/wiki/Gold):

> A total of 174,100 tonnes of gold have been mined in human history,
> according to GFMS as of 2012. This is roughly equivalent to 5.6
> billion troy ounces or, in terms of volume, about 9261 m3, or a cube
> **21.0 m** on a side.

Bitcoin is often compared to gold, the total number of bitcoins matches the total amount of gold mined in human history which can be imagined as a cube **21 m** on a side.

As it is not entirely so important how many Bitcoins will exactly be mined. Satoshi could have easily chosen almost any number. He could just adjust block reward halving (210 000 blocks), reward sizes (50, 25, 12.5 ...), etc. to match some particular number.

Gold is considered **a standard for currency** and was used as such for thousands of years until recently [it was abandoned in 1971](https://wtfhappenedin1971.com/). And that could be the reason why Satoshi maybe wanted Bitcoin to resemble gold at least in this unimportant property.

## EDIT

Satoshi Nakamoto himself compared Bitcoin to gold.

[Satoshi Nakamoto @ bitcointalk.org:](https://bitcointalk.org/index.php?topic=583.msg11405&source=post_page---------------------------#msg11405)

> As a thought experiment, imagine there was a base metal as **scarce as gold** but with the following properties:
>
> * boring grey in colour
> * not a good conductor of electricity
> * not particularly strong, but not ductile or easily malleable either
> * not useful for any practical or ornamental purpose
>
> and one special, magical property:
>
> * can be transported over a communications channel

> **Comments:**
> **username_9**: M, million or meters, that is the question  
> **username_3**: And why not *5.6 billion* troy once eq. ?  
> **username_10**: Did he not account for the fact that total bitcoins in circulation will never be equal to the total bitcoins mined/ mine-able? Some of them are lost forever..  
> **username_8**: Yeah, also some gold is lying at the bottom of the sea. Some is burried etc.  

---

### Answer by username_5 on 2013-03-17. Score: 22

I don't know if this was thought up ahead of time, but it sure makes sense in hindsight.

The reason 21 million is the right number is because people don't know how to value currencies.

For instance, right now a Euro is worth $1.30 USD and a Japanese yen is worth about a U.S. penny. Ask someone which currency they would rather hold right now and most will answer Euro, because $1.30 is worth more than $0.01.

Of course, that relative unit value means nothing. Ask most currency traders which currency is better to hold and most (today) would probably say Japanese yen, because what matters is whether the value will go up or down, relative to the other being compared.

When bitcoin hit parity with the U.S. dollar in Feb 2011, it gained a sense of legitimacy that helped propel it on a tremendous pace, rising over 30X that level just four months later.

If there had been more than 6 million coins issued by then, the total dollar value of all bitoins would probably have been about the same, and thus the exchange rate would then have been lower. So let's say there were instead 60 million coins issued by Feb 2011, and each one worth a dime. That 60M X $0.10 is the exact same total dollar valuation the 6M X $1 has ($6M) for all bitcoins combined. Tthe difference is that because it had become "worth more than a dollar", and as a result people attributed greater interest and respect for it. Had it not been "worth more than a dollar" so early, it might have taken a whole lot longer to get the name recognition and attention it did that has helped attract the participants that Bitcoin has today.

Bitcoin today stands on "dollar parity"'s shoulders.

That may sound bizarre but ask that Euro / Yen question to different people yourself and then ask the reason why their answer was given.

So the number needed to be a low enough number so that while total dollar valuation of all bitcoins was still in the single-digit millions it would reach parity with the U.S. dollar at some point.

> **Comments:**
> **username_6**: So why not 16 million (3y cycle)? Why not 26 million (5y cycle)?  
> **username_6**: And why not 17 million (40 btc reward)? Why not 25 million (60 btc reward)?  
> **username_5**: Why not a 3 yr cycle? Perhaps because that would cause the first & second halvings to occur too soon (and thus too high of a concentration of coins going to those who mined or bought early).  Without knowing in 2008 what the level of traction Bitcoin would have a few years later, using four years made sure that if it took three years for Bitcoin to get any publicity (e.g., what happened with the Gawker Silk Road story in May, 2011) there was still a good year left of mining at the 50 XBT/block rate before the first halving -- thus distributing Bitcoin to a bit wider set of early adopters.  

---

### Answer by username_7 on 2013-07-18. Score: 10

The exact number of Bitcoins is not important. Whether the end result is 1 million or 100 billion makes little real difference.

The important aspect here is the **process**, not the quantity.

1. New Bitcoins enter the system in an orderly, predicable way.
2. Outside forces cannot arbitrarily flood the currency with new money.
3. An incentive is provided for people to apply their CPU power to make the currency more secure.
4. Eventually, Bitcoin has to be self-supporting through transaction fees. Hence the tapering off of blockchain rewards.

Having said all that, there are some psychological advantages to having a low limit like 21 million. It was inevitable that people would see Bitcoin as being "more valuable" if the exchange rate for a whole Bitcoin was over $1. Ensuring this "high exchange rate" but making the coins highly divisible was probably a conscious design decision.

> **Comments:**
> **username_6**: This doesn't actually answer "Why 21m?"....  
> **username_7**: And the answer is "why not?".  I was just trying to explain why the exact number doesn't matter.  

---

### Answer by username_11 on 2013-03-17. Score: 5

It is the result of a 50 bitcoin reward half life of 210,000 blocks.

Reward starts out at 50 bitcoins and halves ever 210,000 blocks. This works out to be 2.1 quadrillion monetary units of currency (satoshi). This is probably the largest number estimated to be needed for a global currency and some padding for attrition.

> **Comments:**
> **username_6**: Why 50 and not 55 or 60 or 45?  

---

### Answer by username_12 on 2017-09-07. Score: 3

* 42 is the Answer to Life, the Universe, and Everything.
* Bitcoin block rewards are cut in half every 4 years. Half of 42 is 21.
* Nearly every person can understand what x "million" means, but comprehension breaks down rapidly with larger numbers.

Therefore 21 million.

;-)

> **Comments:**
> **username_13**: Haha!! I just answered a similar question with the same answer:-)!!! And myself totally agree with the "wink" at the end ;-) ;-)  

---

### Answer by username_14 on 2013-12-03. Score: 2

This was done based on production rate mostly. They did take some things into account but the number doesn't have a real economical explanation rather than the fact that they had to stop production somewhere to maintain a value.


---

### Answer by username_15 on 2015-04-11. Score: 2

I suspect it has something to do with the M1 USD supply at the time Satoshi was developing Bitcoin. If you look at the decade prior to the white paper, inclusive of the quantitative easing period commencing with the 2008 housing bubble, the M1 money supply shows a trendline with a slope of ~58, using years as 4 digit integers. This means, that if predicting M1 over the next decade (from 2009 to 2019), there would be an expected M1 of 2.1 trillion USD, at which point 90% of all bitcoin will have been mined. Because Bitcoin is designed with an 8 decimal fraction system, the 21 million total BTC can actually be expressed as 21,000,000.00M = 21,000.00Billion = 21.00Trillion. So roughly 9x functional BTC will exist relative the USD. Using the M1 trendline to predict M1 in 2140, we see an expected ~10Trillion, which relative to the 21Trillion functional BTC, still gives us a 2x cushion on volume. And mathematically, we've seen noted elsewhere in this Q&A that the 4 year halving schedule of the coinbase gives us 21Million BTC. So, there's a convenient convergence of predicted volume needs & reward schedule @ 21Million BTC. (But realistically, this is probably just me seeing signal where I want to see signal, and not actually Satoshi's anticipation of volume.)  
  
I think another curious question is why the 4 year halving schedule? Is this based on national US elections? Is it based on a calendar Leap Year? Is it Satoshi's approximation of a reasonable re-evaluation period for BTC participation rates?  
  
![M1 Supply & Trend from St Louis Fed (FRED)](https://i.sstatic.net/QpV4t.png)


---

### Answer by username_16 on 2023-06-16. Score: 1

Here's something Nakamoto wrote on the subject

From <https://www.bitcoin.com/satoshi-archive/emails/mike-hearn/11/>

> From: Satoshi Nakamoto satoshin@gmx.com  
> Date: Mon, Jan 10, 2011 at 4:34 PM  
> To: Mike Hearn mike@plan99.net  
> Subject: Re: More BitCoin questions

...

> > I wanted something
> > that would be not too low if it was very popular and not too high if it
> > wasn't.
> >
> > It'd be interesting to see the working for this. In some sense the
> > number of coins is arbitrary as the nanocoin representation means the
> > issuance is so huge it's practically infinite.
>
> It works out to an even 10 minutes per block:
> 21000000 / (50 BTC \* 24hrs \* 365days \* 4years \* 2) = 5.99 blocks/hour
>
> I fudged it to 364.58333 days/year. The halving of 50 BTC to 25 BTC is after >210000 blocks or around 3.9954 years, which is approximate anyway based on >the retargeting mechanism's best effort.
>
> I thought about 100 BTC and 42 million, but 42 million seemed high.
>
> I wanted typical amounts to be in a familiar range. If you're tossing around >100000 units, it doesn't feel scarce. The brain is better able to work with >numbers from 0.01 to 1000.
>
> If it gets really big, the decimal can move two places and cents become the >new coins.


---

### Answer by username_17 on 2024-12-20. Score: 0

The number comes from 3 decisions:

1. Time between blocks: 10 minutes
2. Initial issuance to be 50 BTC
3. Halving every 4 years

From 1) we know each hour should have 6 blocks, so the number of blocks in 4 years is 4 \* 365 \* 24 \* 6 or 210.240

Satoshi was practical and round it to 210.000

Now, if you add all block issuances, starting with 50 and halving every 210.000 blocks, the total will be 21 million

Satoshi was pretty sure about the 10 minutes between blocks and the 4 years halving period, but he thought on using 100 BTC as initial issuance, but drop it because “42 million seemed high”


---

### Answer by username_4 on 2023-05-18. Score: 0

Because if he had picked 210,000,000, then a bitcoin right now would only be worth $2,600 and that is too low.

Before you downvote this answer, really think about it. Would $BTC be $260,000 right now if the supply had been limited to 2.1 million coins? (assume that a Satoshi is defined as 0.000000001 BTC)

Also note that none of the above answers actually respond to the OP except the one relating the number of BTC to the dimensions of the base of the great pyramid or something like that. What does this tell you?


---

### Answer by username_18 on 2014-07-17. Score: -2

Very simple answer:
The reward for Bitcoin Miners had been 50 Bitcoins per Block for the first 210,000 Blocks. For the next 210,000 blocks it is the half of 50 (=25).
Take the geometric series and you'll be getting the total amount of nearly 21 Million Bitcoins (nearly, because a geometric series converges to his boundary value, but never reaches it!).

> **Comments:**
> **username_19**: You're just restating previous answers  

---

