vSize

vSize, or virtual size, is the measure bitcoin uses to price transactions for fees and block space after SegWit, defined as the transaction's weight divided by 4 and rounded up, expressed in vBytes. A block is limited to 4 million weight units, equivalent to 1 million vBytes of vSize capacity.

Why it matters

vSize is what a wallet is actually estimating when it quotes a fee, and what miners are actually maximizing revenue against when they fill a block. Because witness data counts at a quarter weight, a transaction's vSize can be much smaller than its raw byte count, which is why SegWit and Taproot spends are cheaper than legacy ones and why post-2017 blocks routinely serialize well above the old 1 megabyte cap.

The discount is deliberate policy: signature data is witness-only and never enters the UTXO set, so the protocol charges less for bytes that impose lower long-term costs on nodes.

How it works

Weight is calculated as non-witness bytes times 4 plus witness bytes times 1, then vSize equals weight over 4, rounded up to the next whole vByte. A legacy transaction, having no witness, has vSize equal to its raw size. A native SegWit payment of about 222 raw bytes might weigh 561 units, giving a vSize near 141 vBytes, and its fee is the chosen feerate multiplied by that figure.