Virtual Byte (vByte)

A virtual byte (vByte, or vB) is the unit used to measure a Bitcoin transaction's size for fee purposes after the SegWit upgrade. It exists because SegWit changed how transaction data counts toward the block limit: witness data is discounted, so a raw byte count no longer reflects how much block space a transaction actually consumes. The vByte gives a single, fee-relevant size number.

The conversion

One vByte equals four weight units. Since a block's ceiling is 4,000,000 weight units, that is equivalently 1,000,000 vBytes of effective capacity. To get a transaction's virtual size, you take its total weight and divide by four (rounding up). For a legacy, non-witness transaction, vBytes and raw bytes are numerically the same; for a SegWit transaction, the witness portion shrinks the vByte count below the raw byte count.

Why miners and users care

Fee rates are quoted in satoshis per vByte (sat/vB), which is the figure your wallet shows when estimating a fee. Because miners maximise fees within a fixed weight budget, the transactions that win block space are those offering the most sat/vB, not the most satoshis in total. A small high-fee-rate transaction can outbid a large low-fee-rate one.

Understanding vBytes is essential when sizing a coin selection or a consolidation: spending many UTXOs inflates your vByte count and therefore your fee, while SegWit and Taproot inputs keep the witness discount working in your favour. In our day-to-day work, sat/vB is the number we watch to decide whether a broadcast confirms quickly or sits in the mempool.