01Definition
Bitcoin fees are normally evaluated as a rate per virtual byte, not as a percentage of value sent. Users bid for scarce block space; miners generally select packages that maximize fee revenue within block constraints.
02A market for block space
When demand is low, inexpensive transactions may confirm quickly. During congestion, higher-fee transactions move ahead. Wallet estimates are forecasts based on recent mempool and block behavior, not guarantees.
03Size, not payment amount
A transaction spending many inputs can cost more than one moving a much larger value with a single input. SegWit introduced weight and virtual size, discounting witness data in block accounting.
⌖Reference points
01
Paid from inputs minus outputs
02
Usually quoted in sat/vB
03
Market-priced under congestion
04
Depends on data size, not value sent