Weight Unit

A weight unit is the base measure of bitcoin transaction and block size introduced by Segregated Witness in 2017, with every block capped at 4 million weight units. Each non-witness byte counts as 4 weight units and each witness byte as 1, pricing signature data at a quarter of everything else.

Why it matters

The weight system replaced the old 1 megabyte block size limit without a hard fork, raising effective capacity while keeping legacy transactions valid under the same accounting. The witness discount was also an economic nudge with a purpose: witness data is consumed at validation and never enters the UTXO set, the database every node must keep forever, so the protocol charges less for bytes that impose lower permanent costs.

In practice the 4 million weight cap allows blocks well above 1 megabyte of raw data, with the exact size depending on the mix of witness and non-witness bytes in the transactions included.

How it works

A transaction's weight is its non-witness size times 4 plus its witness size times 1. Dividing weight by 4 and rounding up yields vSize in vBytes, the unit wallets quote fee rates against. A purely legacy block still maxes out near 1 megabyte because all its bytes count at weight 4, while witness-heavy blocks can serialize several times larger within the same 4 million unit budget.