TERM_DEF // TRANSACTION / WTXID_WITNESS_TXID
WTXID (WITNESS
TXID)
TXID)
wTXID (Witness TXID). HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
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_WTXID_WITNESS_TXID_IS
wTXID (Witness TXID) — at a glance
TRANSACTION
wTXID (Witness TXID) is part of the IDENTITY sub-family of transaction-level concepts. The 32-byte hash including the witness section — used for mempool relay (BIP-339) but not for chain commitments. HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
Why it exists
DESIGN
Two txs with the same effect but different witnesses (e.g. different valid sig encodings) should be relayed as distinct objects to avoid wasted bandwidth. The wtxid distinguishes them while the txid stays stable.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
wtxid = SHA256(SHA256(full serialised tx including marker/flag/witness)). For non-SegWit txs, wtxid == txid. Mempool relay (BIP-339) keys announcements on wtxid so a tx is announced once per witness, not per (sender × peer).
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
wTXID (Witness TXID) — IDENTITY
EXAMPLE
Legacy tx: wtxid == txid (no witness data)
SegWit tx: wtxid ≠ txid (witness adds bytes to hash)
Coinbase tx wtxid is defined as all zeros for the SegWit witness
commitment to work (the coinbase commits TO the wtxid Merkle root).
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
wTXID (Witness TXID)
HASH256 of the full transaction including witness data; commits to signatures and used in the witness commitment.
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.
Transaction ID (TXID)
HASH256 of a transaction's pre-witness serialization; used to reference outputs by (txid, vout).
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.