Block Header
แแแแแฎแแแแฃแแ Aug 5, 2026 What Is a Block Header?
A block header is the summary section of a
block in a blockchain, containing the blockโs core identifying data rather than its full transaction list.
The header holds the
metadata that describes a block: when it was created, how it connects to the previous block, and a single hash that represents all the transactions it contains. In essence, the block header includes everything about a block except the raw list of transactions itself.
What Does a Block Header Contain?
In Bitcoin, the block header is a fixed 80-byte structure made up of six fields:
Version: signals which consensus rules the block follows, and can be used to indicate support for protocol upgrades.
Previous block hash: the
hash of the prior blockโs header, which is what links blocks together into a chain.
Merkle root: a single hash derived from all transactions in the block, produced from a
Merkle root structure so any change to a transaction changes this value.
Timestamp: records the approximate time the block was created, expressed as a Unix timestamp.
Difficulty target (nBits): encodes the threshold that the header hash must fall below. The network difficulty is derived from this target rather than stored directly.
Nonce: a counter that miners repeatedly change through the
nonce field while searching for a valid hash.
Why Block Headers Matter
A block header is what miners hash when they attempt to make a block valid. Because the header is only 80 bytes, this is far more efficient than hashing every transaction in the block, which can number in the thousands. When a block is
mined, the header is passed through a double SHA-256 hash, and the result becomes the blockโs identifier.
Their small size also makes block headers useful for light clients. Storing the full
Bitcoin blockchain can require large amounts of storage, but a device that keeps only the block headers uses a tiny fraction of that space. Because each header commits to its transactions through the Merkle root, a light client can later check whether a specific transaction was included in a block, without having to download the entire chain. This approach still relies partly on third parties for data, but it allows some degree of verification on devices with limited resources.