In one sentence
A block header is the small summary at the top of every block — exactly 80 bytes in Bitcoin — that links the block to its parent and commits to all of its transactions.
How it works
A Bitcoin block header has six fields:
- Version (4 bytes) — which set of rules the block follows, also used to coordinate upgrades.
- Previous block hash (32 bytes) — the link to the parent block.
- Merkle root (32 bytes) — one hash that commits to every transaction in the block.
- Timestamp (4 bytes) — roughly when the block was made. Nodes accept it only if it is later than the median of the previous eleven blocks and not more than two hours ahead of their own clock.
- Target (4 bytes, in a compact form called "bits") — how low the block hash must be.
- Nonce (4 bytes) — a free field miners change to get new hashes.
The block's identity is the double SHA-256 hash of these 80 bytes. Mining hashes only the header, never the whole block, which is why it can be repeated billions of times per second.
Ethereum headers carry more fields — including separate roots for the transactions, the account state and the receipts, plus gas and fee information — but serve the same purpose.
A simple example
A light client on a phone keeps only headers. Even for a chain with hundreds of thousands of blocks, all the headers together take less than 100 MB. With them, the client can check that each header points to the one before and meets its target — and then use a Merkle branch to confirm that a particular transaction sits under one of those roots. The Bitcoin whitepaper calls this approach simplified payment verification.
Common misconception
Misconception: The header contains the transactions.
In fact: The header contains only the Merkle root, a 32-byte fingerprint of the transactions. The transactions themselves are in the block body. That is why a header stays the same size whether its block holds one transaction or several thousand.
Where you meet it
- Mining, which is repeated hashing of headers.
- Light clients and simplified payment verification.
- Block explorers, which list every header field.
- Upgrade coordination, in which miners mark support for a proposed soft fork in the version field, as described in BIP-9.
Related cards
Sources
This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.