<p>The only valid header is 80 bytes, no more, no less. </p>

<p>The other stated length of 128 bytes is part of the internal SHA256 hashing algorithm (its input must be a fixed multiple of 64 bytes), it's not represented anywhere in this format. Likewise, the null padding added when hashing the header is not ever considered to be part of it. </p>

CC BY-SA 4.0
2020-03-17T02:16:54.140Z
Is it possible to store data in the padding, or is the form of the padding fixed?
Mark
CC BY-SA 4.0
2020-03-17T02:37:56.040Z
The padding in your diagram is imaginary, it’s not part of the block header. In SHA256 compression it is mostly null padded in a predictable manor.
Claris
CC BY-SA 4.0
2020-03-16T15:47:33.537Z
2020-03-16T15:55:59.823Z
2020-03-16T15:55:59.823Z
Claris
<p>I know that the current bitcoin block size limit is about 1,000,000 bytes (1 megabyte) of data. This includes both the block header as well as the transaction data. </p>

<p>On the other hand, the typical size of a block <em>header</em> is 80 bytes, or 128 bytes if padding is included (as depicted below):</p>

<p><a href="https://i.sstatic.net/no20k.png" rel="noreferrer"><img src="https://i.sstatic.net/no20k.png" alt="enter image description here"></a></p>

<p>Now, I have been completely unable to find any information as to whether there is a limit on the size of the block header. The block header is typically 80 bytes, but is it against protocol if a miner adds so much data that the size of the block header increases to about 192 bytes or 256 bytes? </p>

<p>For example, a goofy miner (for some pointless reason) adds a lot of irrelevant information after the nonce section, and the block header size increases to 256 bytes. Would this block header be considered valid?</p>

<mining-theory>
Is there a limit on the size of the block header?
Biology nerd
# Is there a limit on the size of the block header?

(Asked by: username_0 on 2020-03-16)

I know that the current bitcoin block size limit is about 1,000,000 bytes (1 megabyte) of data. This includes both the block header as well as the transaction data.

On the other hand, the typical size of a block *header* is 80 bytes, or 128 bytes if padding is included (as depicted below):

[![enter image description here](https://i.sstatic.net/no20k.png)](https://i.sstatic.net/no20k.png)

Now, I have been completely unable to find any information as to whether there is a limit on the size of the block header. The block header is typically 80 bytes, but is it against protocol if a miner adds so much data that the size of the block header increases to about 192 bytes or 256 bytes?

For example, a goofy miner (for some pointless reason) adds a lot of irrelevant information after the nonce section, and the block header size increases to 256 bytes. Would this block header be considered valid?

**Tags:** mining-theory


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## 1 answers

### Answer by username_1 on 2020-03-16. Score: 9

The only valid header is 80 bytes, no more, no less.

The other stated length of 128 bytes is part of the internal SHA256 hashing algorithm (its input must be a fixed multiple of 64 bytes), it's not represented anywhere in this format. Likewise, the null padding added when hashing the header is not ever considered to be part of it.

> **Comments:**
> **username_2**: Is it possible to store data in the padding, or is the form of the padding fixed?  
> **username_1**: The padding in your diagram is imaginary, it’s not part of the block header. In SHA256 compression it is mostly null padded in a predictable manor.  

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