What is a block

Blocks, transactions and the chain: what a Bitcoin block holds, how its size is counted, and how many there are.

Bitcoin's history is a chain of blocks. Each block holds a batch of transactions. The space those transactions take up is what this wiki calls blockspace.

What a block holds #

block.space's data-types reference states a block's size exactly: an 80-byte header, then a count of its transactions, then the transactions themselves. Its size in bytes is the header plus the count plus the sum of the transactions' sizes. BTI uses that sum as a check: what it attributes inside a block must add up to the block's size.

The first transaction of every block is the coinbase transaction. The miner writes it. Its scriptSig must begin with the block's height, and the rest of it, up to 100 bytes, is free for the miner's own use.

What a transaction holds #

A transaction, in the reference's layout, is:

nVersion(4) | marker | flag | inputs | outputs | witness | nLockTime(4)

Inputs name the earlier outputs being spent.
Outputs each carry an 8-byte amount and a locking script.
The witness, in a segwit transaction, holds one stack per input: signatures, and sometimes data.
The marker and flag are two bytes of the segwit layout.

A transaction's txid hashes it without the witness; its wtxid hashes all of it. The reference says that split is what makes witness data structurally separable, and weight treats it separately. See the witness discount.

The chain #

The chain runs from the genesis block, block 0, to the newest block, the tip. BTI "walks every Bitcoin transaction, from the genesis block forward, and counts every byte exactly". Its census, the first of its three stages, had counted (as of block 972,306, 2026-10-20 11:06:05 UTC):

what	count
blocks	972,307
transactions	1,464,168,458
raw bytes	777.05 GB

The tip, as Blockspace Holdings' own node reported it, was block 972,308 (as of block 972,305, 2026-10-20 10:48:22 UTC).

Where to go next #

How large a block may be is set by weight, not bytes: see block weight and the limit. How the census counts every byte is on the byte census.

Sources #

Live figures: census as of block 972,306, 2026-10-20 11:06:05 UTC; frontier as of block 972,305, 2026-10-20 10:48:22 UTC. Refreshed hourly.