<p>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.</p>

<blockquote>
  <p>Magic no, 4 bytes</p>
  
  <p>Blocksize, 4 bytes</p>
  
  <p>Blockheader, 80 bytes</p>
  
  <p>Transaction counter, 1 - 9 bytes</p>
  
  <p>Transactions, -many transactions</p>
</blockquote>

<p>The only part of a block with a variable size is the transactions part.</p>

<p>See: <a href="https://en.bitcoin.it/wiki/Blocks">https://en.bitcoin.it/wiki/Blocks</a></p>

<p>If you're exploring the scalability of Bitcoin, see: <a href="https://en.bitcoin.it/wiki/Scalability#Storage">https://en.bitcoin.it/wiki/Scalability#Storage</a> </p>

CC BY-SA 3.0
2013-07-25T09:21:36.577Z
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
CC BY-SA 3.0
2013-07-25T10:52:07.850Z
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 K.
CC BY-SA 3.0
2013-07-25T16:04:52.567Z
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
CC BY-SA 3.0
2013-07-25T08:23:36.727Z
2013-07-25T08:23:36.727Z
Emre K.
<p>Blocks are made up mostly of transactions.</p>

<p>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.</p>

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

<transactions><blocksize>
what is the primary factor responsible for the increase of block size?
bvpx
# what is the primary factor responsible for the increase of block size?

(Asked by: username_0 on 2013-07-25)

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?

**Tags:** transactions, blocksize


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## 1 answers

### Answer by username_1 on 2013-07-25. Score: 5 | Accepted answer

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>

> **Comments:**
> **username_0**: 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?  
> **username_1**: 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 ;)  
> **username_0**: 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?  

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