# Block Header

Source URL: https://support.dxp.blockstream.com/education/glossary/block-header
Updated: 2026-05-26T17:15:20.000Z
Letter: B

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#### Quick Definition

A block header is the compact 80-byte bundle of metadata that summarizes a Bitcoin block. By chaining and securing these headers together, miners protect the [blockchain](https://support.dxp-staging.blockstream.com/education/glossary/blockchain), and [nodes](https://support.dxp-staging.blockstream.com/education/glossary/bitcoin-node) can verify it efficiently.

#### Key Takeaways

| Field               | Purpose                                                                    | Size     |
| ------------------- | -------------------------------------------------------------------------- | -------- |
| Version             | Signals which consensus rules the miner followed when creating the block.  | 4 bytes  |
| Previous Block Hash | Cryptographically links the block to the one before it, forming the chain. | 32 bytes |
| Merkle Root         | Single 32-byte hash that commits to every transaction inside the block.    | 32 bytes |
| Timestamp           | When the miner claims the block was found.                                 | 4 bytes  |
| nBits               | Encodes the current difficulty target the header’s hash must meet.         | 4 bytes  |
| Nonce               | 32-bit counter miners iterate to search for a valid hash below the target. | 4 bytes  |

- **80 bytes total**: The fixed size is critical for bandwidth and storage efficiency, as nodes keep only headers for lightweight validation.
- **Double-SHA-256**: Each header is hashed twice; the result must be ≤ target to satisfy proof-of-work.
- **SPV (Simplified Payment Verification)**: Wallets that don’t download full blocks can verify payments by checking chains of headers plus Merkle proofs, relying on this succinct summary.

#### In-Depth Explanation

**Chaining & Immutability**  
The Previous Block Hash field forces every block to reference its parent’s header hash. A change in any historical header propagates forward, requiring new [proof-of-work](https://support.dxp-staging.blockstream.com/education/glossary/proof-of-work) for every descendant, a deterrent to attackers. This makes retroactive tampering computationally impractical, forming the backbone of Bitcoin’s security.

**Transaction Commitment**  
All transactions are hashed into a [Merkle tree](https://support.dxp-staging.blockstream.com/education/glossary/merkle-tree) whose top hash (the Merkle Root) lives in the header. This lets nodes prove a transaction’s inclusion with only log₂(n) hashes, keeping SPV proofs lightweight.

**Difficulty & Nonce Dance**

- nBits encodes the difficulty target as a compact representation.
- Miners vary the Nonce (and occasionally extra-nonce data in the coinbase transaction, an additional field adjusted when the nonce is exhausted) to find a header hash below that target.
- Roughly every two weeks (2,016 blocks), nodes recompute difficulty to ensure a valid header is found about every 10 minutes.

**Version Signaling**  
The Version field allows miners to indicate support for soft forks (e.g., SegWit, Taproot). Nodes track these bits to measure activation thresholds without disrupting consensus.

**Timestamp Constraints**  
Nodes reject headers with timestamps more than two hours into the future relative to their clocks or earlier than the median time of the last 11 blocks, guarding against denial-of-service and difficulty-manipulation attacks.

By packing these six fields into an 80-byte structure, the block header provides Bitcoin with a tamper-evident audit trail that anyone, full node or mobile wallet, can verify quickly, forming the backbone of the network’s security and [decentralization](https://support.dxp-staging.blockstream.com/education/glossary/decentralization).

Navigation: Blockstream Help Center > Education > Glossary > Block Header

## Related Terms in This Letter
- [Blind oracle](https://support.dxp.blockstream.com/education/glossary/blind-oracle)
- [Borromean ring signatures](https://support.dxp.blockstream.com/education/glossary/borromean-ring-signatures)
- [Bitcoin Script](https://support.dxp.blockstream.com/education/glossary/bitcoin-script)
- [BitVM](https://support.dxp.blockstream.com/education/glossary/bitvm)
- [BOLT12](https://support.dxp.blockstream.com/education/glossary/bolt12)
- [Bitcoin](https://support.dxp.blockstream.com/education/glossary/bitcoin)
- [Bitcoin Node](https://support.dxp.blockstream.com/education/glossary/bitcoin-node)
- [Bitcoin Core](https://support.dxp.blockstream.com/education/glossary/bitcoin-core)
- [Bulletproofs](https://support.dxp.blockstream.com/education/glossary/bulletproofs)
- [Blind signatures](https://support.dxp.blockstream.com/education/glossary/blind-signatures)
- [Blockchain](https://support.dxp.blockstream.com/education/glossary/blockchain)
- [Block](https://support.dxp.blockstream.com/education/glossary/block)
- [Block Header](https://support.dxp.blockstream.com/education/glossary/block-header) (current)
- [Block Height](https://support.dxp.blockstream.com/education/glossary/block-height)
- [Blockchain Explorer API](https://support.dxp.blockstream.com/education/glossary/blockchain-explorer-api)
- [Broadcast Transaction](https://support.dxp.blockstream.com/education/glossary/broadcast-transaction)