Satoshi per byte (sat/byte or sat/vB) is the standard unit for measuring Bitcoin transaction fee rates. One satoshi equals 0.00000001 BTC — the smallest divisible unit of Bitcoin. When you send a Bitcoin transaction, you pay a fee not based on the amount you send, but on the size of the transaction data in bytes.
Bitcoin blocks have a maximum weight of 4,000,000 weight units (roughly 1 MB). Miners must fill each block with transactions to maximize their income. Since they cannot fit every pending transaction, they sort by fee rate — highest sat/vB first. A transaction paying 50 sat/vB will be confirmed before one paying 5 sat/vB, regardless of the Bitcoin value being transferred. This creates a dynamic fee market where users compete for limited block space.
The formula is straightforward: Fee (satoshis) = fee rate (sat/vB) × transaction size (vB). A typical Bitcoin transaction ranges from 140 to 400 virtual bytes depending on the number of inputs and outputs. A simple transaction with one input and two outputs is roughly 225 vB. If the current fee rate is 10 sat/vB, you would pay 2,250 satoshis — less than a fraction of a cent at most Bitcoin prices.
