Encyclopedia How the network works · Entry 83
Bitcoin virtual size: bytes, weight and vbytes
In this article
At a glance
Key facts
The two sizes in the formula
Base size excludes witness serialization; total size includes it. The formula counts base bytes four times in total and witness bytes once. This weighting lets different spending formats consume block capacity differently.
The discount does not make signatures free. Witness data still contributes to weight, and the transaction still has non-witness fields.
Work through the rounding
For an illustrative transaction with 100 base bytes and 250 total bytes, weight is 100 × 3 + 250 = 550 weight units. Virtual size is ceiling(550 ÷ 4), or 138 vbytes. Dividing 250 raw bytes into the fee would answer a different question.
At 10 sat/vB, a 138-vbyte transaction has a target fee of 1,380 satoshis. Final size and rounding should be checked after the transaction’s actual spending data is known.
Compare wallets using the same units
A wallet may display bytes, weight units, vbytes, a total BTC fee or satoshis per virtual byte. Identify the unit before comparing estimates. Bitcoin Core’s RPC documents both sat/vB and older BTC/kvB options with different field names.
The fee rate is not based simply on the amount of bitcoin being transferred. Input and output structure determine the transaction’s size.
Direct answers
Questions people ask
Can I multiply a sat/vB quote by a raw-byte count?
Only if that raw count happens to equal the transaction’s virtual size. Use the explicitly reported virtual size or compute it from weight.
Inspect the evidence
The answer and key facts have stable claim links. These records retain the scope and qualification when reused.
Bitcoin virtual size is transaction weight divided by four, rounded up. Under the SegWit weight definition, non-witness bytes count more heavily than witness bytes. A transaction’s raw serialized byte count can therefore differ from the virtual size used when quoting fees in sat/vB.
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimWeight: Weight = base size × 3 + total size.
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimVirtual size: Virtual size = ceiling(weight ÷ 4).
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimFee rate: A sat/vB quote applies to virtual size, not necessarily raw bytes.
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimRevision history
- — Initial Bitcoin encyclopedia entry at this permanent URL.
- — Added reusable claims, explicit source locators, and matching Markdown and JSON. This publishing change does not itself establish factual verification.
Source register
Sources and references
Retrieval dates and locators are recorded individually.- Segregated Witness consensus layerBitcoin BIPs contributors
Transaction weight and virtual-size definitions in the SegWit specification.
Locator: Block size; Transaction size · Version / scope: BIP-141; immutable revision pinned in source URL · Retrieved: 2026-10-02T17:22:20.465ZOpen source - walletcreatefundedpsbt RPCBitcoin Core
Input selection, fee units, change output, selected inputs and transaction funding.
Locator: Arguments and result fields · Version / scope: Bitcoin Core 29.0.0 RPC · Retrieved: 2026-10-02T17:03:40.827ZOpen source
Research and drafting use AI assistance. A separate automated review checks claims against primary sources; no external expert or named human review is implied. Publication, substantive editing, source retrieval and verification are recorded separately. This version was independently checked by an automated reviewer on 2 October 2026.
Editorial method and correctionsDegrees of Satoshi editorial project. “Bitcoin virtual size: bytes, weight and vbytes.” Published 2026-10-02; updated 2026-10-02. https://degreesofsatoshi.com/encyclopedia/bitcoin-virtual-size/