# What is a block

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

URL: https://blockspace.wiki/basics/what-is-a-block/

Bitcoin's history is a chain of [blocks](https://blockspace.wiki/glossary/block/). Each block holds a batch of [transactions](https://blockspace.wiki/glossary/transaction/). The space those transactions take up is what this wiki calls [blockspace](https://blockspace.wiki/glossary/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](https://blockspace.wiki/glossary/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](https://blockspace.wiki/glossary/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](/basics/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 972,318, 2026-10-20 12:15:32 UTC):

| what | count |
|---|---|
| blocks | 972,319 |
| transactions | 1,464,231,426 |
| raw bytes | 777.07 GB |

The tip, as [Blockspace Holdings' own node](https://blockspace.wiki/glossary/blockspace-holdings-node-and-indexers/) reported it, was block 972,320 (as of block 972,315, 2026-10-20 12:10:25 UTC).

## Where to go next

How large a block may be is set by weight, not bytes: see [block weight and the limit](/basics/block-weight-and-the-limit/). How the census counts every byte is on [the byte census](/concepts/byte-census-t1/).

Sources:

- block.space docs: data types · https://block.space/docs/datatypes
- block.space docs: BTI · https://block.space/docs/bti
- Blockspace Holdings records
- block.space meter and summary APIs · https://block.space/api/summary
- block.space frontier API (the three tier cursors and the chain tip) · https://block.space/api/frontier

Live figures: census as of block 972,318, 2026-10-20 12:15:32 UTC; frontier as of block 972,315, 2026-10-20 12:10:25 UTC

Updated: 2026-10-20