Transaction Hash

A transaction hash is a fixed-length string of letters and numbers that uniquely identifies a transaction on a blockchain. It is calculated by passing the transaction's data through a cryptographic hash function, so any change to the data would produce a completely different value. Wallets, exchanges, payment companies and compliance teams use the hash to find a transaction, confirm that it happened and share proof of payment.

The term is often used interchangeably with transaction ID (TXID), and on many blockchains the two are the same thing. It plays a role similar to a transaction reference number or a UETR in bank payments, with one key difference: a bank reference is assigned by an institution, while a transaction hash is derived from the transaction itself and can be checked by anyone running the network's software.

When a user signs a transaction in a wallet, the software assembles the data: inputs or sender account, recipients, amounts, fees, a counter or nonce and the digital signature. It then hashes that data. On Bitcoin the TXID is the result of applying SHA-256 twice, displayed as 64 hexadecimal characters. On Ethereum the transaction hash is computed with Keccak-256 over the encoded signed transaction and is usually shown with a 0x prefix, making 66 characters in total.

Because the hash depends on every part of the signed transaction, it is known as soon as the transaction is created, before it is broadcast or included in a block. A hash on its own therefore proves only that a transaction was constructed. Whether it actually executed must be checked on the chain.

Anyone can paste a transaction hash into a block explorer to see its status, the block that contains it, the number of confirmations, the fee paid and the addresses involved. Common uses include:

• Proof of payment. A sender shares the hash with the recipient, an exchange or a merchant, who can verify the amount and destination independently.
• Customer support. Exchanges and wallet providers ask for the hash when a deposit has not arrived, to see whether it is still pending, failed or was sent to the wrong network.
• Reconciliation. Businesses that accept stablecoins or crypto record the hash against each invoice or ledger entry, which supports on-chain settlement processes and audits.
• Compliance. Investigators and blockchain analytics tools trace funds by following transaction hashes, and firms may include them in suspicious activity reports and Travel Rule messages.

A transaction with a valid hash may sit in the mempool waiting to be included in a block. If its fee is too low it may be dropped, and on networks that allow it the sender can replace it with a new transaction carrying a higher fee, which has a different hash. On Ethereum, a transaction can be included in a block yet fail during execution, for example if a smart contract call reverts; it still has a hash and the fee is still charged, but no tokens move. Users therefore need to look at the status shown by the explorer, not just whether the hash exists.

Confirmation depth also matters. A transaction in the most recent block can in rare cases be removed by a chain reorganisation, so merchants and exchanges usually wait for several confirmations before treating a payment as final. The idea of settlement finality on probabilistic chains is discussed separately.

Early versions of Bitcoin suffered from transaction malleability: parts of the signature could be altered by a third party without invalidating it, which changed the TXID while leaving the payment itself the same. This confused services that tracked payments by TXID and complicated layer-two designs. The Segregated Witness (SegWit) upgrade, activated in 2017, moved signature data out of the part of the transaction used for the TXID and so fixed the problem for SegWit transactions.

A transaction hash also says nothing about who controls an address. It proves that value moved between two addresses, not the identity of the people behind them, and privacy tools such as mixers can break the link between hashes. For that reason it is a useful identifier and evidence of a transfer, but not a substitute for customer due diligence.