Transaction fee (fee rate)

In Bitcoin the fee is not an explicit field but the unspent remainder between inputs and outputs. Its size is measured in vBytes, a unit that bakes in the SegWit discount (activated in August 2017): a block admits up to 4,000,000 weight units, where one vByte equals four units. Because witness data weighs less, a SegWit transaction occupies fewer vBytes than its raw size. A fee market exists: miners prioritize transactions with a higher sat/vB to maximize revenue from a block of limited capacity.

It is the core operating cost for payment integrators and wallets. A poor estimate leaves the transaction stuck (rate too low) or overpays (rate too high). Wallets compute a recommended sat/vB from the mempool state and the confirmation target, and mechanisms such as Replace-By-Fee (RBF) or Child-Pays-For-Parent (CPFP) let a stalled transaction be sped up. Over the long run, fees sustain Bitcoin's security: as the block subsidy decays with each halving, they gain weight in miner compensation.