# The block header

The block header has six fields in 80 bytes: version, previous hash, merkle root, time, bits and nonce. Learn what each does and why miners hash it.

Canonical page: https://mybitcoinexplorer.com/learn/block-header
Topic: Blocks and the blockchain
Published: 2026-10-29
Updated: 2026-10-30


## Six fields
> Diagram: The 80-byte block header — Version:4,Previous block hash:32,Merkle root:32,Time:4,Bits:4,Nonce:4

| Field | What it does |
|---|---|
| Version | Signals which rules or upgrades the miner supports |
| Previous block hash | Links this block to its parent. See [What is the blockchain?](https://mybitcoinexplorer.com/learn/what-is-the-blockchain) |
| Merkle root | A single hash committing to every transaction. See [Merkle trees and the merkle root](https://mybitcoinexplorer.com/learn/merkle-root) |
| Time | When the miner says the block was made. See [Block timestamps](https://mybitcoinexplorer.com/learn/block-timestamps) |
| Bits | The compact form of the [target](https://mybitcoinexplorer.com/learn/target-and-difficulty) |
| Nonce | The number miners change while searching. See [The nonce](https://mybitcoinexplorer.com/learn/nonce) |

## Why so small?
Because the header is what miners hash, again and again, trillions of times a second. Keeping it to 80 bytes makes each attempt fast. It also lets [light clients](https://mybitcoinexplorer.com/learn/spv-light-clients) download headers only: about 4 MB per 50,000 blocks.

## The block hash
The block's ID is the double [SHA-256](https://mybitcoinexplorer.com/learn/sha-256) of these 80 bytes. For the block to be valid, that hash must be below the target set by **bits**.

### Worked example: The genesis block header
[Block 0](/block/0): bits `1d00ffff` (difficulty 1), nonce 2,083,236,893, merkle root `4a5e1e4b…`. Its hash begins with ten zeros: `000000000019d668…`.



## Hashing a header yourself
### Worked example: Verifying the genesis block hash
1. Take the 80 header bytes of [block 0](/block/0) as raw hex.
1. Hash them with SHA-256, then hash the result again.
1. Reverse the byte order of the result.
1. You get `000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f`, the genesis block hash.


## Version bits
Miners use bits in the version field to signal readiness for soft forks, and also as extra search space when the nonce runs out. See [Soft forks and hard forks](https://mybitcoinexplorer.com/learn/soft-forks-and-hard-forks).


## Key takeaways

- The header is 80 bytes and has six fields.
- It links to the previous block and commits to all transactions via the merkle root.
- The block hash must be below the target encoded in bits.

## Common questions

### Why does every block hash start with zeros?

Because it must be below the target. Small numbers written in hex start with zeros.

### Can I verify the header myself?

Yes. Hash the 80 bytes twice with SHA-256, reverse the result, and compare it with the block hash and target.

### Why is the block header only 80 bytes?

So that miners can hash it extremely fast and light clients can download every header cheaply.

### Does the header contain the transactions?

No, only the merkle root, a single hash committing to all of them.

## Related lessons

- [Merkle trees and the merkle root](https://mybitcoinexplorer.com/learn/merkle-root)
- [The nonce](https://mybitcoinexplorer.com/learn/nonce)
- [Target and difficulty](https://mybitcoinexplorer.com/learn/target-and-difficulty)
- [Proof of work](https://mybitcoinexplorer.com/learn/proof-of-work)