Fee Rate
Also: sat/vB, transaction fee, miner fee
network · intermediate
The price a transaction pays the miner per virtual byte (sat/vB). Higher fee rate → priority for the next block.
A transaction's fee is the difference between the sum of its inputs and the sum of its outputs. Whatever isn't paid out goes to whichever miner includes the transaction in a block.
But block space isn't sold by transaction; it's sold by size. A transaction with one big input and one output uses less space than a transaction with ten inputs and three outputs, even if the fees are identical. So users and miners both think in fee rate: fee paid divided by transaction size.
The unit is sat/vB — satoshis per virtual byte. "Virtual" because SegWit gave witness data a 75 % discount: a transaction's virtual size is (weight) / 4, where weight = 3 × base size + total size. The block weight limit is 4 million; the practical block size is therefore ~1.5–4 MB depending on witness usage.
Wallets pick fee rates by polling the mempool. Tools like mempool.space publish percentile-based estimates: "the fastest 10 % of transactions in the next block are paying ≥ X sat/vB." Underpaying leaves a transaction in the [mempool](/glossary/mempool) until either a miner gets desperate or it falls out (most nodes evict after ~2 weeks). RBF and CPFP exist to bump a stuck transaction's effective fee rate after the fact.