What is the common unit for quoting Bitcoin transaction fees by virtual byte?

Bitcoin transaction fees are commonly quoted in satoshis per virtual byte.

A satoshi is the smallest standard unit of bitcoin, equal to 0.00000001 BTC. A virtual byte, or vbyte, is a weight-based measure used for calculating how much block space a transaction consumes under Bitcoin’s Segregated Witness rules. Combining the two gives the fee rate “satoshis per virtual byte,” often written sat/vB.

Wallets use fee rates because miners generally prioritize transactions according to the fee offered for the block space they occupy, rather than simply choosing the largest absolute fee. A transaction with a higher fee rate may therefore confirm sooner than a larger transaction paying more total satoshis.

Fee rates vary with demand for block space. They are separate from the block subsidy paid to miners and from Bitcoin’s market price. Older explanations sometimes use satoshis per byte, but virtual bytes are the more precise modern unit for transactions that account for Segregated Witness data.