<p>Likely the sender didn't have an unspent output that had exactly the amount he wanted to send. So he choose an unspent output that had enough and sent the remainder back to himself.</p>

<p>Accounts do not automatically combine the funds they receive. They have a bundle of unspent transaction outputs which they can only use by completely consuming them.</p>

<p>You might ask the same question if you see a person pay with a $10 bill and get a $5 bill back. You might wonder why they didn't just pay with a $5 bill. Well, probably it's because they didn't have one, so they got a $5 bill back as change. Essentially, each of the transactions you have received remains available to you as an input to a future transaction. But using an output as an input consumes it, so if it has too much, you send the remains back to yourself as "change".</p>

CC BY-SA 3.0
2017-02-06T22:38:14.377Z
Good analogy with bills.
Zepplock
CC BY-SA 3.0
2017-02-06T17:53:28.690Z
2017-02-06T17:53:28.690Z
David Schwartz
<p>I often notice transactions such as: </p>

<p><a href="https://blockchain.info/tx/364efe5a906de5971e242f9711008e58a5ab79f136a8044b66615e8662d35f09" rel="nofollow noreferrer">364efe5a906de5971e242f9711008e58a5ab79f136a8044b66615e8662d35f09</a></p>

<p>Where funds from the address <code>3PU</code> with 1.38 BTC are split to itself (1.35 BTC) and <code>37Ek</code> (0.027BTC): </p>

<pre><code>3PUui (1.38 BTC) -&gt; 37Ekc (0.0278 BTC)
                 -&gt; 3Puui (1.3519 BTC)
</code></pre>

<p>I am wondering why the transaction doesn't look like: </p>

<pre><code>3PUui (0.0279 BTC) -&gt; 37Ekc (0.0278 BTC)
</code></pre>

<p>I read from this <a href="https://bitcoin.stackexchange.com/questions/736/how-does-change-work-in-a-bitcoin-transaction">question</a> that an input must be completely consumed in the transaction, so if I have received 1.35 BTC during the last transaction, if I refer to it, I must consume the 1.35 BTC. </p>

<p>However on <a href="https://coinb.in/#newTransaction" rel="nofollow noreferrer">https://coinb.in/#newTransaction</a> I can see the amount can be specified for each input. So why should I need to mention the input amount if it is already set by the previous transaction?</p>

CC BY-SA 3.0
2017-02-06T15:47:04.080Z
Possible duplicate of [How does change work in a bitcoin transaction?](http://bitcoin.stackexchange.com/questions/736/how-does-change-work-in-a-bitcoin-transaction)
Nate Eldredge
CC BY-SA 3.0
2017-02-08T12:38:25.000Z
Related: [What is an “unspent output”?](http://bitcoin.stackexchange.com/q/4301/5406)
Murch
<transactions><utxo><change><address-reuse>
Why do transactions send coins to the same address as they spent it from?
nowox
# Why do transactions send coins to the same address as they spent it from?

(Asked by: username_0 on 2017-02-06)

I often notice transactions such as:

[364efe5a906de5971e242f9711008e58a5ab79f136a8044b66615e8662d35f09](https://blockchain.info/tx/364efe5a906de5971e242f9711008e58a5ab79f136a8044b66615e8662d35f09)

Where funds from the address `3PU` with 1.38 BTC are split to itself (1.35 BTC) and `37Ek` (0.027BTC):

```
3PUui (1.38 BTC) -> 37Ekc (0.0278 BTC)
                 -> 3Puui (1.3519 BTC)

```

I am wondering why the transaction doesn't look like:

```
3PUui (0.0279 BTC) -> 37Ekc (0.0278 BTC)

```

I read from this [question](https://bitcoin.stackexchange.com/questions/736/how-does-change-work-in-a-bitcoin-transaction) that an input must be completely consumed in the transaction, so if I have received 1.35 BTC during the last transaction, if I refer to it, I must consume the 1.35 BTC.

However on <https://coinb.in/#newTransaction> I can see the amount can be specified for each input. So why should I need to mention the input amount if it is already set by the previous transaction?

**Tags:** transactions, utxo, change, address-reuse

> **Comments:**
> **username_1**: Possible duplicate of [How does change work in a bitcoin transaction?](http://bitcoin.stackexchange.com/questions/736/how-does-change-work-in-a-bitcoin-transaction)  
> **username_2**: Related: [What is an “unspent output”?](http://bitcoin.stackexchange.com/q/4301/5406)  

---

## 1 answers

### Answer by username_3 on 2017-02-06. Score: 4 | Accepted answer

Likely the sender didn't have an unspent output that had exactly the amount he wanted to send. So he choose an unspent output that had enough and sent the remainder back to himself.

Accounts do not automatically combine the funds they receive. They have a bundle of unspent transaction outputs which they can only use by completely consuming them.

You might ask the same question if you see a person pay with a $10 bill and get a $5 bill back. You might wonder why they didn't just pay with a $5 bill. Well, probably it's because they didn't have one, so they got a $5 bill back as change. Essentially, each of the transactions you have received remains available to you as an input to a future transaction. But using an output as an input consumes it, so if it has too much, you send the remains back to yourself as "change".

> **Comments:**
> **username_4**: Good analogy with bills.  

---

