Definition
Sats per vByte (sat/vB) is the standard way to express a Bitcoin transaction's fee rate: the number of satoshis paid per virtual byte of transaction data. Miners pack the most valuable transactions into limited block space, so fee rate, not the total fee, is what determines confirmation priority. A small transaction paying a high rate can outrank a large transaction paying a bigger total fee.
What a virtual byte is
Since the SegWit upgrade, block capacity is measured in weight units, capped at 4,000,000 per block. Most transaction data counts as 4 weight units per byte, while witness data counts as 1, a discount that reflects how older nodes validate. A transaction's virtual size is its weight divided by 4, so a 400-weight-unit transaction is 100 vBytes. Dividing the 4,000,000 weight limit by 4 yields roughly 1,000,000 vBytes per block.
How the fee is calculated
The total fee is simply fee rate multiplied by virtual size: a 140 vByte transaction at 10 sat/vB pays 1,400 satoshis. When the mempool is congested, the going rate climbs as transactions compete for space; when it is empty, even a 1 sat/vB transaction may confirm quickly. Wallets estimate rates from current mempool conditions.
Fee rate is part of the miner's reward alongside the block subsidy; see coinbase maturity for how those rewards mature, and inspect live fees in any block explorer.
In Simple Terms
Sats per vByte (sat/vB) is the standard way to express a Bitcoin transaction’s fee rate: the number of satoshis paid per virtual byte of transaction…