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A Bitcoin transaction’s legacy byte layout is version | input count | inputs | output count | outputs | locktime. SegWit transactions add a marker and flag after the version and place per-input witness data before locktime. These serialized bytes encode what is spent, where value goes, and the conditions for spending outputs—not a separate fee field.
Legacy Bitcoin transaction format, field by field
The Bitcoin Developer Reference describes raw format as the serialized byte format broadcast between peers. In legacy serialization, fields appear in this order:
version | input count | inputs | output count | outputs | locktime
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|---|---|---|
| 1 | version |
Four bytes. Version 1 is the baseline; version 2 indicates that BIP 68 relative-locktime rules apply, subject to those rules and transaction validity. |
| 2 | Input count | A CompactSize unsigned integer giving the number of inputs. |
| 3 | Inputs | One input record per earlier output being spent. |
| 4 | Output count | A CompactSize unsigned integer giving the number of outputs. |
| 5 | Outputs | One output record per destination or other output condition. |
| 6 | lock_time |
Four bytes, interpreted as a block height or Unix timestamp according to the locktime threshold. |
CompactSize is Bitcoin’s variable-length integer encoding. It is not a single-byte count in every case. Counts and script lengths use this encoding, so the number of bytes occupied by a count depends on its value.
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What an input contains
An ordinary input references one earlier output, called an outpoint, then provides a length-prefixed scriptSig and a four-byte sequence value. The outpoint is 36 bytes: a 32-byte transaction hash and a four-byte output index. The hash is serialized in internal byte order, which can look reversed compared with the conventional displayed TXID.
The input’s unlocking data must satisfy the conditions set by the referenced output. For witness-based spends, some or all required spending data is carried in the separate witness structure instead of scriptSig.
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What an output contains
An output consists of an eight-byte value in satoshis followed by a CompactSize script length and the scriptPubKey bytes. The script defines the conditions that must be met to spend that output later. Transactions can have multiple outputs, including a change output.
There is no explicit fee field. For a valid ordinary transaction, the fee is the total value of its inputs minus the total value of its outputs. Output value cannot exceed input value, with coinbase issuance handled as a special case.
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Sequence and locktime are different fields
sequence is stored in each input; lock_time appears once for the whole transaction. Sequence values can disable locktime in some cases and, under BIP 68’s defined conditions, express a relative-locktime constraint. Locktime is instead a transaction-wide condition interpreted using the height-or-time threshold.
How SegWit changes the serialization
A transaction with witness data uses an extended serialization:
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version | marker | flag | input count | inputs | output count | outputs | witnesses | locktime
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The marker is zero and the flag is nonzero. After all outputs, the serialization includes one witness stack for each input; the number of stacks is implicit in the input count. A stack contains length-prefixed byte arrays, and an empty stack is encoded with a zero byte. When all witness data is empty, the legacy serialization is used.
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Witness data is spending information, not another set of transaction outputs. For example, BIP 141 describes a version-zero P2WPKH spend whose witness contains a signature and public key, and a P2WSH spend whose witness can include stack data and a serialized script.
| Property | Legacy serialization | Witness-extended serialization |
|---|---|---|
| Marker and flag | Absent | Inserted after the four-byte version |
| Witness stacks | Not included | Included after outputs, one stack per input |
| When used | Used when witness data is empty | Used when witness data is present |
| Hashing relationship | Provides the serialization used for the traditional TXID | Provides the bytes used for the witness-inclusive WTXID; the traditional TXID still uses non-witness serialization |
TXID and WTXID: which bytes are hashed?
The traditional transaction identifier, or TXID, is computed from the non-witness serialization. BIP 144 states that transaction hashes used in the transaction Merkle tree and input outpoints are computed using the old, non-witness serialization. The witness-inclusive hash, or WTXID, is computed from the witness serialization.
For a transaction with no witness data, the two hashes are equal because legacy serialization is used. When witness data is present, it affects the WTXID but is excluded from the TXID used for transaction-tree and outpoint references.
Special case: the coinbase transaction
The first transaction in a block is the coinbase transaction. It has exactly one special input rather than an ordinary spend: its outpoint uses a null 32-byte hash and output index 0xffffffff. The input’s script field carries the block height as required by BIP 34 and may include additional miner-selected data within the specified limit. Coinbase output value can come from the block subsidy and collected transaction fees, so the ordinary input-minus-output explanation does not apply to it in the same way.
Raw transaction bytes are not PSBT
Raw transaction bytes are the network serialization used to relay transaction data and calculate transaction identifiers. PSBT, or Partially Signed Bitcoin Transaction, is a separate format that carries transaction data plus metadata for signing workflows; it is not the network serialization format. For version-zero witness programs, BIP 143 also defines a signature-hash construction that commits to the spent input amount and selected transaction components according to the sighash flags. That constructed signature message is not simply the complete raw transaction byte stream.
Quick Recap
Official references
- Bitcoin Developer Reference: Transactions — field layout, input and output structures, locktime, and coinbase details.
- BIP 144: Segregated Witness (Peer Services) — witness serialization and transaction-hash distinction.
- BIP 141: Segregated Witness (Consensus layer) — witness programs and spending examples.
- BIP 68: Relative lock-time using consensus-enforced sequence numbers — sequence-based relative locktime.
- BIP 143: Transaction Signature Verification for Version 0 Witness Program — signature-hash construction for version-zero witness programs.
- Bitcoin Core transaction definitions — transaction and witness data structures.
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