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

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?

1 Answer 1


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.

  • Is it possible to store data in the padding, or is the form of the padding fixed?
    – Mark
    Mar 17, 2020 at 2:16
  • 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
    Mar 17, 2020 at 2:37

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