BA-05 · LESSON 04

Fees, weight, and vbytes

KEY DEFINITIONS

Terms used in this lesson

Virtual byte: A fee-accounting unit derived from Bitcoin transaction weight; fee rates are commonly quoted per virtual byte.
Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.

Fees equal inputs minus outputs, while feerates use virtual size derived from transaction weight.

65 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · transaction weightFee follows virtual size, not the amount sentSelect any state to inspect its exact role.

RELATIONSHIP MAPInputs, rule, and outputs

01
BASE110 bytes
Version, inputs without witness, outputs, and locktime count four weight units per byte.
02
WITNESS108 bytes
SegWit witness bytes count one weight unit each.
03
WEIGHT548 WU
Compute 110 × 4 plus witness bytes.
04
VIRTUAL SIZE137 vB
Round weight divided by four upward to a whole virtual byte.
05
FEE AT 101,370 sats
Fee rate in sats per vB multiplies virtual size, independent of payment value.

Rule shown: Weight equals non-witness bytes times four plus witness bytes; virtual size is weight divided by four and rounded up.

STAGE 1 OF 5BASE: 110 bytes

Version, inputs without witness, outputs, and locktime count four weight units per byte.

Governing relationship: Weight equals non-witness bytes times four plus witness bytes; virtual size is weight divided by four and rounded up. Complete path: 1. BASE, 110 bytes. Version, inputs without witness, outputs, and locktime count four weight units per byte. 2. WITNESS, 108 bytes. SegWit witness bytes count one weight unit each. 3. WEIGHT, 548 WU. Compute 110 × 4 plus witness bytes. 4. VIRTUAL SIZE, 137 vB. Round weight divided by four upward to a whole virtual byte. 5. FEE AT 10, 1,370 sats. Fee rate in sats per vB multiplies virtual size, independent of payment value.

1. State transitionWhat exists, what changes, and what results

BEFORE

A transaction has base size 110 bytes and total size 218 bytes. Target feerate is 12 sat/vB.

MECHANISM

Base size is the serialization without SegWit marker, flag, or witness. Total size includes the complete serialized transaction.

AFTER

Fees pay for virtual size, so witness discount affects cost without making witness bytes free.

Text equivalent: Starting from A transaction has base size 110 bytes and total size 218 bytes. Target feerate is 12 sat/vB. The mechanism begins when Base size is the serialization without SegWit marker, flag, or witness. Total size includes the complete serialized transaction. The resulting lesson is Fees pay for virtual size, so witness discount affects cost without making witness bytes free.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Calculate base and total size

Base size is the serialization without SegWit marker, flag, or witness. Total size includes the complete serialized transaction.

Complete path: 1. Calculate base and total size: Base size is the serialization without SegWit marker, flag, or witness. Total size includes the complete serialized transaction. 2. Calculate weight: An equivalent formula is weight = base size times 3 plus total size, which charges base bytes four units total and additional witness bytes one. 3. Convert to virtual size: Compute ceiling(weight / 4). The ceiling matters because fee selection uses a whole virtual-byte size. 4. Choose a target feerate: A wallet estimates the market and urgency, then chooses fee = target sat/vB times vsize, normally rounded up. 5. Check output economics: Very small change can cost more to create and later spend than it is worth and may be nonstandard dust. The wallet can add uneconomical remainder to fee instead.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Weight equals base bytes times four plus additional witness bytes, virtual size rounds upward, and integer sat/vB pricing is reconciled with input, payment, fee, and economical change.

CHANGED CONDITION

Wrong rounding, negative remainder, dust-policy change, or a change output whose future spend cost exceeds its value blocks the claimed construction.

Important distinction: A 200-byte transaction always costs exactly twice as much as a 100-byte transaction. Cost depends on weight composition and chosen feerate. Witness and non-witness bytes contribute different weight.

Text equivalent: The accepted path is enforced by Weight equals base bytes times four plus additional witness bytes, virtual size rounds upward, and integer sat/vB pricing is reconciled with input, payment, fee, and economical change. If the required condition changes, Wrong rounding, negative remainder, dust-policy change, or a change output whose future spend cost exceeds its value blocks the claimed construction.

Loading lesson visuals...

Learn the idea

A transaction fee is input value minus output value. Transaction weight charges non-witness bytes four weight units and witness bytes one weight unit. Virtual size, or vbytes, is weight divided by four and rounded up. Feerate is fee divided by virtual size, commonly measured in satoshis per vbyte.

GUIDED EXPLANATION1/5 ideas inspected

Calculate base and total size

Base size is the serialization without SegWit marker, flag, or witness. Total size includes the complete serialized transaction.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“A 200-byte transaction always costs exactly twice as much as a 100-byte transaction.”

Cost depends on weight composition and chosen feerate. Witness and non-witness bytes contribute different weight.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.