Below is a byte-by-byte walkthrough of a real SegWit transaction format, focusing on SegWit v0 P2WPKH, because it’s the most common and the cleanest to understand.

I’ll show:

1. Full transaction layout
2. Every field, in order, byte by byte
3. What is hashed for txid vs wtxid
4. How this differs from legacy
5. Concrete annotated hex example

No hand-waving — this is the exact wire format Bitcoin nodes parse.

1. High-level SegWit transaction structure

A SegWit transaction looks like this on the wire:

[version]
[marker]
[flag]
[inputs]
[outputs]
[witnesses]
[locktime]

The marker + flag are what tell a node:

“This transaction has witness data”

2. Concrete example (1-input, 1-output P2WPKH)

We’ll walk through a simple spend:

• 1 SegWit input (P2WPKH)
• 1 output
• Uses ECDSA (SegWit v0)
• Mainnet

I’ll show hex bytes, then explain each section.

2.1 Version (4 bytes)

01000000

• Little-endian
• Value = 0x00000001
• Transaction version = 1

📌 Same as legacy.

2.2 Marker (1 byte) — SegWit indicator

00

• Always 0x00
• Illegal as input count → legacy parsers won’t misinterpret it

2.3 Flag (1 byte)

01

• Indicates witness data is present
• Marker + flag together = 00 01

📌 These 2 bytes do NOT exist in legacy transactions

2.4 Input count (varint)

01

• 1 input

2.5 Input (UTXO being spent)

2.5.1 Previous transaction ID (32 bytes, little-endian)

e3c0...9a12   (32 bytes total, reversed)

Important:

• txids are displayed big-endian
• On wire they are little-endian

📌 This is the txid of the UTXO being spent.

2.5.2 Output index (4 bytes)

00000000

• Which output of the previous tx
• Here: output index 0

2.5.3 scriptSig length (varint)

00

🔥 Critical SegWit difference

• scriptSig is empty for native SegWit
• Length = 0

Legacy txs store signatures here — SegWit does not.

2.5.4 Sequence (4 bytes)

ffffffff

• Standard “final” sequence
• Enables RBF if < fffffffe

2.6 Output count (varint)

01

• 1 output

2.7 Output

2.7.1 Amount (8 bytes, little-endian)

00e1f50500000000

• Interpreted as satoshis
• Little-endian integer
• Example: 100000000 sats = 1 BTC

2.7.2 scriptPubKey length (varint)

16

• Decimal 22 bytes

2.7.3 scriptPubKey (22 bytes)

0014 d85c2b71d0060b09c9886aeb815e50991dda124d

Breakdown:

00        → OP_0   (SegWit version 0)
14        → push 20 bytes
<20-byte pubKeyHash>

📌 This is a P2WPKH output

3. Witness data (the “segregated” part)

⚠️ Witnesses come after outputs, but are logically tied to inputs.

Witnesses are NOT included in txid hashing.

3.1 Witness stack item count (varint)

02

Two witness items for P2WPKH:

1. Signature
2. Public key

3.2 Witness item #1: signature

Length (varint)

48

• 72 bytes

Signature bytes

3045022100...022100...01

Contents:

• DER-encoded ECDSA signature
• Last byte = sighash type (01 = SIGHASH_ALL)

3.3 Witness item #2: public key

Length

21

• 33 bytes

Public key

03a34f...c9e2

• Compressed secp256k1 public key

4. Locktime (4 bytes)

00000000

• Locktime = 0
• No timelock

5. Final raw SegWit transaction layout

Putting it all together:

01000000                      ← version
00 01                          ← marker + flag
01                              ← input count
  [32-byte prev txid]
  00000000                      ← output index
  00                            ← scriptSig length
  ffffffff                      ← sequence
01                              ← output count
  [8-byte amount]
  16
  0014[20-byte pubKeyHash]      ← scriptPubKey
02                              ← witness item count
  48 [signature]
  21 [public key]
00000000                        ← locktime

6. txid vs wtxid (EXTREMELY important)

txid is computed from:

version
inputs
outputs
locktime

❌ Excludes marker, flag, witness

wtxid is computed from:

version
marker
flag
inputs
outputs
witnesses
locktime

✔️ Includes everything

Why this matters

• txid is immutable
• Signatures can’t change txid
• Transaction malleability is solved

This is what enabled Lightning Network.

7. Compare with legacy P2PKH (side by side)

8. Block weight math (real numbers)

Example input sizes:

• Legacy P2PKH input: ~148 bytes

• SegWit P2WPKH input:

  • Non-witness: ~41 bytes
  • Witness: ~107 bytes
  • Weight = 41×4 + 107 = 271
  • Virtual size ≈ 68 vB

➡️ ~55% fee savings per input.

9. Mental model (this makes it click)

Think of SegWit as:

“Signatures are receipts — important, but they don’t define what you’re spending.”

So Bitcoin:

• Hashes what matters for ownership
• Discounts proofs used only at spend time