TERM_DEF // TRANSACTION / TRANSACTION_ID_TXID
TRANSACTION ID
(TXID)
(TXID)
transaction-id-txid/">Transaction ID (TXID). HASH256 of a transaction's pre-witness serialization; used to reference outputs by (txid, vout).
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Transaction section — How money moves: inputs, outputs, fees, signatures, sighash flags, and the formats that wrap them. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_TRANSACTION_ID_TXID_IS
Transaction ID (TXID) — at a glance
TRANSACTION
Transaction ID (TXID) is part of the IDENTITY sub-family of transaction-level concepts. The 32-byte SHA256(SHA256(tx)) computed over the legacy serialisation — the canonical identifier of a transaction. HASH256 of a transaction's pre-witness serialization; used to reference outputs by (txid, vout).
Why it exists
DESIGN
Every input references its parent by txid. Every block commits to its txs via the Merkle root over txids. Without a stable identifier, references would break whenever a tx was rebroadcast.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
Legacy: txid = SHA256(SHA256(version || inputs || outputs || locktime)). SegWit: same, but the marker/flag/witness fields are EXCLUDED from the hash so signature re-encoding doesn't change the txid.
1. Wallet selects UTXOs whose total value covers the spend amount + estimated fee (coin selection).
2. Wallet builds the transaction body: version, inputs (each with prev txid + vout + sequence), outputs (each with value + scriptPubKey), locktime.
3. Wallet computes the sighash for each input (which parts of the tx the signature commits to — controlled by the SIGHASH flag).
4. Wallet signs each input with the right private key. Witness/scriptSig is populated with the resulting signatures + pubkeys.
5. Tx is broadcast to peers. Mempool propagation: tens of seconds globally.
6. A miner includes it in a block. Confirmation count grows by 1 per block; after ~6 the tx is effectively final.
WORKED_EXAMPLE
Transaction ID (TXID) — IDENTITY
EXAMPLE
Pre-SegWit (legacy tx):
Anyone could re-encode the signature in scriptSig, getting a different txid
for the same payment. This was the original "tx malleability" bug.
Post-SegWit:
Signatures live in the witness, which is NOT part of the txid hash.
So the same tx always has the same txid regardless of who relays it.
Block explorers display txid byte-reversed for readability — so
the natural byte order starts with leading zeros.
KEY_PROPERTIES
ATOMIC
A transaction is either fully accepted into a block or fully rejected. There is no partial spend.
IMMUTABLE INPUTS
A UTXO can only ever be spent once. After that, it is permanently consumed.
NO BALANCES
IRREVERSIBLE
After ~6 confirmations (~1 hour), reversing the tx would require more proof-of-work than the honest network produces.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- address-reuse/">Address reuse degrades privacy — every reuse links more of your UTXOs together publicly. Modern wallets generate a fresh address per receive.
- Fee estimation matters: under-pay and your tx sits in the mempool for hours; over-pay and you tip the miner more than necessary. Use a fee estimator.
- "Change outputs" must go back to a fresh address you control. A missing output/">change output sends the difference to the miner as fee — a known footgun.
- RBF (Replace-By-Fee) lets you re-broadcast a tx with a higher fee. Useful for stuck txs but means a 0-confirmation tx is never truly final.
RELATED_CONCEPTS
Other terms from Transaction — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
Transaction ID (TXID)
HASH256 of a transaction's pre-witness serialization; used to reference outputs by (txid, vout).
Transaction (Tx)
A signed payload spending one or more UTXOs and creating new ones; every state change in Bitcoin is a tx.
Raw Transaction
The hex-serialized bytes of a transaction, ready to broadcast or analyze.
wTXID (Witness TXID)
HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
Input
A reference to a previous output being spent, plus the data (scriptSig/witness) authorizing the spend.
Output
An (amount, scriptPubKey) pair created by a transaction; spendable later by a tx whose input references it.
UTXO (Unspent Transaction Output)
An output that hasn't been spent yet; your "balance" is the sum of UTXOs you can sign for.
UTXO Set
The complete set of currently-spendable outputs maintained by every node/">full node — Bitcoin's state.