Block hash

A block hash is the cryptographic hash of a block's header. It serves

as that block's unique identifier and is the value child blocks reference

as their parentHash, forming the chain itself. Paste a block hash into

tx.taxi/{hash} and you are routed to the configured explorer's block page.

How it is computed

A block hash is not a hash of every transaction byte. It is a hash of the

block header, a compact structure containing fields such as the parent

hash, the state root, the transactions root, the timestamp, and a handful

of chain-specific values. Because the header commits to the transactions

root (typically a Merkle root), any change to any transaction in the block

also changes the block hash.

• On EVM chains, the block hash is a 32-byte Keccak-256 hash of the RLP-encoded

header, shown as a 0x-prefixed 64-character hex string. This is what

ethereum and EVM L2s such as optimism

use in their /block/0x... URLs.

• On UTXO chains, the block hash is the double-SHA256 of the 80-byte

header. bitcoin block hashes are 64 hex characters

with no 0x prefix, conventionally shown with leading zeros (the

proof-of-work requirement forces those zeros).

Block hash vs block number

A block number (also called block height) is a

sequential integer assigned to each block on the canonical chain. A block

hash is a content-addressed identifier. The two are not interchangeable:

• A block number is short, human-readable, and monotonically increasing.

Explorers and casual users typically link by number.

• A block hash is long, opaque, but uniquely names a specific block even

across reorganisations.

During a reorganisation ("reorg"), two different blocks can share the

same block number but have different block hashes. Only one of those

blocks ends up on the canonical chain; the other is orphaned. This is why

trustworthy references to a block use the hash, not just the height,

particularly on chains with probabilistic finality such as

bitcoin.

Where you see block hashes

Block explorers display the block hash near the top of every block page,

alongside the block number, the parent hash, and the state root. RPC

calls such as eth_getBlockByHash accept it directly. SDKs use it to

verify that a particular transaction hash was

included in a specific, immutable block, since the receipt of a tx includes

the block hash that contained it.

Common gotchas

• A "valid-looking" hash you find by hand is not a block hash. Real block

hashes on proof-of-work chains have many leading zeros because the

network's difficulty rule forces the miner to find a header whose hash

is below a target. You cannot synthesize one.

• Two block hashes at the same height do not mean the chain is broken; it

means the network briefly saw a fork. After enough

confirmations, only one of those hashes

remains on the canonical chain.

• The block hash is committed to by the next block's parentHash. That

is what makes the chain a chain.