Definition
A weight unit (WU) is the fundamental unit Bitcoin uses to measure how much of a block a transaction occupies. Introduced by the SegWit soft fork (BIP141), it replaced the old one-megabyte byte cap with a 4,000,000 weight-unit limit per block, while quietly increasing effective capacity by treating witness data more cheaply than base data.
The weight formula
Transaction weight is calculated as the base (non-witness) size multiplied by four, plus the witness size multiplied by one. Equivalently, every byte of the base transaction — version, inputs, outputs, locktime — costs 4 WU, while every byte of SegWit witness data (signatures, public keys) costs just 1 WU. This is the so-called witness discount, and it is the reason a block can hold meaningfully more than one megabyte of transaction data.
Why the discount exists
The rationale is that witness data does not need to be retained by every node forever the way the spendable transaction skeleton does, so it is fair to charge less block weight for it. The discount also gently incentivises SegWit and Taproot adoption, since those inputs put more of their bulk in the discounted witness.
Weight units are the raw measure; for fee estimation they are usually divided by four and expressed as vBytes. The 4,000,000 WU ceiling is what miners fill each round, selecting transactions to maximise fee revenue within that fixed budget — a constraint that directly shapes the mempool fee market every block.
In Simple Terms
A weight unit (WU) is the fundamental unit Bitcoin uses to measure how much of a block a transaction occupies. Introduced by the SegWit soft…