Blocks are made up mostly of transactions.

I understand that the larger the transaction output is in coins, the greater the transaction size must be because it must list all previous transaction inputs that lead up to this transaction and add up to the net output of this specific transaction.

Is there any other factor I might not have accounted for that increases transaction size or block size?


The primary driver of Block size is the number of transactions. Below is the structure of a block and the size of each part in the block.

Magic no, 4 bytes

Blocksize, 4 bytes

Blockheader, 80 bytes

Transaction counter, 1 - 9 bytes

Transactions, -many transactions

The only part of a block with a variable size is the transactions part.

See: https://en.bitcoin.it/wiki/Blocks

If you're exploring the scalability of Bitcoin, see: https://en.bitcoin.it/wiki/Scalability#Storage

  • thanks for the quick and concise answer. I have one more question: why does the blockchain store "size" metadata in each block? Can't any bitcoind client quickly add up the amount of space taken up by that particular block?
    – bvpx
    Jul 25 '13 at 9:21
  • Are you talking about the Blocksize field which is 4 bytes? If you're okay with the answer, don't forget to mark it as a valid answer ;)
    – Emre Kenci
    Jul 25 '13 at 10:52
  • Hey one more question: the "size" metadata in each block represents the size in bytes that block measures, correct? Not megabytes or bits or something else?
    – bvpx
    Jul 25 '13 at 16:04

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