Block Header
A block header is the compact summary miners hash to prove work and link each cryptocurrency block to the chain.
Definition
A block header is the small data section at the start of a cryptocurrency block. In proof-of-work mining, miners repeatedly hash the block header to search for a result that satisfies the network’s mining difficulty.
The header acts like a fingerprint for the block. It summarizes key information without storing every transaction directly.
How It Works
When a miner creates a candidate block, it gathers valid transactions and builds a block header from several fields. In Bitcoin, those fields include the version, previous block hash, Merkle root, timestamp, difficulty target, and nonce.
The previous block hash links the new block to the block before it. If an older block changes, its hash changes, which also changes later block headers that point back to it.
The Merkle root summarizes all transactions in the block as one compact hash. If any transaction changes, the Merkle root changes, and the block header hash changes too.
Mining is the process of hashing this header many times. Miners change the nonce and sometimes other adjustable data to create new header variations. Each variation produces a different hash. To be valid, the hash must be below the network’s current target. Because hashes are unpredictable, miners compete by making many attempts with large amounts of hash rate.
Once a miner finds a valid header hash, it broadcasts the full block. Other nodes can hash the header once, check that it meets the target, and validate the rest of the block; block explorers such as mempool.space show these details for mined blocks.
Why It Matters
The block header makes proof-of-work efficient to verify. Creating a valid header can require trillions of attempts, but checking one valid result is fast.
It also protects the history of the blockchain. Since each header includes the previous block’s hash, changing an old transaction would force an attacker to redo the proof of work for that block and later blocks.
For miners, the block header is the main object being tested. Mining hardware does not search through entire blocks directly; it searches through block header variations until one produces a valid proof-of-work hash.
Related Terms
FAQ
What is a block header?
A block header is the small data section at the start of a cryptocurrency block that summarizes key information without storing every transaction, acting like a fingerprint for the block. In proof-of-work mining, miners repeatedly hash the block header in search of a result that satisfies the network’s mining difficulty.
What fields are in a Bitcoin block header?
A Bitcoin block header contains the version, previous block hash, Merkle root, timestamp, difficulty target, and nonce. The Merkle root summarizes every transaction in one hash, and the previous block hash links each block to its predecessor, so altering an old transaction would force an attacker to redo the proof of work for that block and later ones.
Why do miners hash the block header instead of the whole block?
A block header compactly summarizes the whole block, so mining hardware hashes header variations instead of entire blocks, changing the nonce and other adjustable data until a hash falls below the network target. Creating a valid header can take trillions of attempts, but other nodes can check the result quickly by hashing it once.