Virtual Byte (vByte)

Overview

Virtual Byte (vByte) normalizes Bitcoin transaction weight into a familiar size-like unit. BIP 141 defines transaction weight from base size and total serialized size, then defines virtual size as weight divided by four and rounded upward.

Segregated Witness data receives a lower weight than non-witness data. As a result, a transaction’s raw serialized byte count and its vByte size can differ. Wallets use vBytes because miners generally evaluate fee rate against this weighted resource measure rather than treating every byte equally.

A fee quote such as 20 satoshis per vByte means the estimated fee is the rate multiplied by virtual size. The final size depends on selected UTXOs, script types, signature lengths, outputs, and whether change is created. Coin Selection therefore affects both transaction composition and total fee.

vByte is not a universal blockchain unit and should not be confused with an ordinary storage byte. Ethereum uses gas-based accounting, and other networks have different resource models. Cross-chain fee comparisons must convert both unit and economic context.

Payment and payout systems should estimate vSize before signing, verify actual weight after signatures are added, and record both fee rate and total fee. During Congestion, a correct vByte estimate helps choose an inclusion target without overpaying or producing a transaction too cheap to relay.

Signed transaction size can differ from a pre-signing estimate, especially for script types with variable-length signatures. Modern wallets use conservative templates and then recalculate the actual weight before broadcast. Fee bumping policy should reference the final vSize so replacement transactions meet relay and miner requirements.