<p>The cap on the block size excluding witness data is 1000000 bytes.</p>

<p>There is no cap on block size including witness data. Instead, there is a limit on the block weight at 4000000 weight units. Depending on the composition of the block, 4000000 WU can correspond to anything from 1000000 bytes to close to 4000000 bytes (including witness data).</p>

<p>But a block with 4000000 WU is "full" - regardless of whether it's 1 MB or 3.8 MB of data.</p>

CC BY-SA 4.0
2019-08-21T07:37:15.593Z
Why at 4000000 specifically? If that was chosen by consensus fork, why that number?
Josh
CC BY-SA 4.0
2019-08-21T15:25:17.473Z
The factor 4 for weight/ood byte was chosen as a tradeoff between encouraging UTXO consumption and enabling more network bandwidth. The limit 4000000 must be exactly the chosen factor times the old 1000000 limit, or it wouldn't be a soft fork (we must guarantee that every valid block without witnesses still stays below the limit).
Pieter Wuille
CC BY-SA 4.0
2019-08-13T23:16:18.997Z
2019-08-13T23:16:18.997Z
Pieter Wuille
<p>I am not asking about block weight. I hear many people saying that the actual Bitcon block size is 2mb. However, in any block explorer, I can see that blocks are around 1200kb. This is very confusing, where is the cap?</p>

CC BY-SA 4.0
2019-08-17T12:03:26.317Z
In actual block explorers you can also occasionally see blocks larger than 2MB.
Jannes
<block><blocksize>
What is the actual Bitcoin block size? 2mb or 1mb?
Alejandro Veintimilla
# What is the actual Bitcoin block size? 2mb or 1mb?

(Asked by: username_0 on 2019-08-13)

I am not asking about block weight. I hear many people saying that the actual Bitcon block size is 2mb. However, in any block explorer, I can see that blocks are around 1200kb. This is very confusing, where is the cap?

**Tags:** block, blocksize

> **Comments:**
> **username_1**: In actual block explorers you can also occasionally see blocks larger than 2MB.  

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## 1 answers

### Answer by username_2 on 2019-08-13. Score: 3 | Accepted answer

The cap on the block size excluding witness data is 1000000 bytes.

There is no cap on block size including witness data. Instead, there is a limit on the block weight at 4000000 weight units. Depending on the composition of the block, 4000000 WU can correspond to anything from 1000000 bytes to close to 4000000 bytes (including witness data).

But a block with 4000000 WU is "full" - regardless of whether it's 1 MB or 3.8 MB of data.

> **Comments:**
> **username_3**: Why at 4000000 specifically? If that was chosen by consensus fork, why that number?  
> **username_2**: The factor 4 for weight/ood byte was chosen as a tradeoff between encouraging UTXO consumption and enabling more network bandwidth. The limit 4000000 must be exactly the chosen factor times the old 1000000 limit, or it wouldn't be a soft fork (we must guarantee that every valid block without witnesses still stays below the limit).  

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