What is a fee rate (sat/vB)?

Read 4 sat/vB as “4 satoshis per virtual byte”. Space in a block is limited, so the fee depends on a payment's size, not on how much it sends. In a made-up example, a 140-vB payment at 4 sat/vB pays 560 sats.

A virtual byte is a measure of block space. Since the SegWitSegWitSegregated Witness: a 2017 upgrade that moved signatures into a separate part of each payment, making room for more payments. upgrade, witness data counts at a quarter of its real size, so its contribution to virtual size is smaller than its byte size.

When lots of people are sending at once, the rate needed to get into the next block can rise. UTXOHub shows fees in dollars first, then sat/vB. Most wallets let you set the fee in sat/vB (some write sat/vbyte), so that's the number to compare with yours.

Miners can evaluate related unconfirmed transactions together, so a payment's own rate is not always the whole picture. A higher individual or package fee rate can improve the chance of selection, but cannot guarantee it.