TERM_DEF // TRANSACTION / SEQUENCE_NUMBER
SEQUENCE
NUMBER
NUMBER
Sequence Number. A per-input 32-bit field used originally for tx replacement; now signals RBF and encodes relative locktimes.
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_SEQUENCE_NUMBER_IS
Sequence Number — at a glance
TRANSACTION
Sequence Number is part of the SEQUENCE / LOCKTIME sub-family of transaction-level concepts. A 4-byte field per input that controls relative timelocks (BIP-68/112) and replace-by-fee signalling (BIP-125). A per-input 32-bit field used originally for tx replacement; now signals RBF and encodes relative locktimes.
Why it exists
DESIGN
HOW_IT_WORKS
Mechanism
HOW IT WORKS
If sequence == 0xffffffff → finalised, no locktime, no RBF. If < 0xfffffffe → BIP-125 RBF-opt-in. If bit 31 is clear AND BIP-68 active (tx version ≥ 2) → bits 0-15 are a relative timelock in either blocks (bit 22 clear) or 512-second units (bit 22 set).
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
Sequence Number — SEQUENCE / LOCKTIME
EXAMPLE
Sequence value (LE) Meaning
0xffffffff finalised; no locktime, RBF disabled
0xfffffffe nLockTime active; RBF disabled
0xfffffffd RBF enabled (BIP-125 opt-in)
0x00000010 relative timelock of 16 BLOCKS (BIP-68)
0x00400010 relative timelock of 16 × 512 SECONDS (bit 22 set)
0x80000000 bit 31 set → disables BIP-68 timelock semantics
Always check: tx version AND sequence AND bit 31 to decode timelock meaning.
KEY_PROPERTIES
TRIPLE MEANING
Sequence simultaneously controls finalisation, RBF signalling, and relative-timelocks. Don't confuse the three interpretations.
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.
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
Sequence Number
A per-input 32-bit field used originally for tx replacement; now signals RBF and encodes relative locktimes.
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)
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.