Virtual byte
A virtual byte (vB) is the unit for the block space a Bitcoin transaction takes, one quarter of its weight in weight units.
How much block space a transaction takes. Signatures in SegWit transactions count for a quarter, so a virtual byte is a little less than a byte on disk. A block holds about a million.
Why not count plain bytes?
Since SegWit, signature data (the witness) counts a quarter as much as the rest, which rewards transactions that keep their signatures separate. Weight is 4 units per normal byte and 1 per witness byte; virtual bytes are weight divided by four.
How many virtual bytes fit in a block?
A block may weigh 4 million weight units, which is 1 million virtual bytes. A simple payment is about 110 to 220 vB, so a block holds a few thousand of them.
Sources
Updated 10 Oct 2026
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