What’s in the 80 bytes?
| Field | Size | What it does |
|---|---|---|
| Version | 4 bytes | Which rules the block follows |
| Previous block hash | 32 bytes | Links this block to the one before |
| Merkle root | 32 bytes | One hash covering every transaction |
| Time | 4 bytes | When the miner says it built the block |
| Bits (target) | 4 bytes | How low the block’s hash must be |
| Nonce | 4 bytes | The number miners keep changing |
Why do miners hash only the header?
Because it stands in for the whole block. The Merkle root commits to every transaction, so changing any payment changes the header. The previous block hash commits to all history, so editing an old block breaks every header after it.
And 80 bytes is fast to hash. Miners run double SHA-256 over it, check the result against the target, change the nonce and go again, trillions of times a second.
How small is that, really?
The chain is about 969,006 blocks long (live), so every header since 2009 adds up to roughly 78 MB. That’s why light wallets can download all the headers, check the proof of work on each one, and still fit on a phone. The full blocks behind them run to hundreds of gigabytes.
Sources
- Block hashing algorithm · Bitcoin Wiki
- Block chain reference: headers, Merkle trees, target · Bitcoin developer documentation
- Bitcoin: A Peer-to-Peer Electronic Cash System · Satoshi Nakamoto, 2008
Live figures on this page come from the CloudMineCrypto API and public chain data, refreshed regularly, and are labelled where they appear. Educational only, not financial advice.