Virtual Byte (vByte)

Overview

What it means and why it exists

A virtual byte, abbreviated vB, is the fee-sizing unit commonly used for Bitcoin transactions after SegWit. It converts transaction weight into a byte-like number while accounting for witness data differently.

How it works

Compute transaction weight in weight units and divide by four, rounding up to a whole virtual byte for a BIP-141 vsize. Wallet fee estimators commonly quote sat/vB. Multiply the target feerate by vsize to estimate the fee; the blockchain does not charge an independent fixed price for each ordinary byte displayed by an explorer.

Technical details worth understanding

The difference matters when comparing a legacy transaction with a SegWit or Taproot transaction. Their raw serialized lengths can be similar while their virtual sizes differ. In Bitcoin Core's mempool policy, a sigops-adjusted size can affect relay and mining policy, so a tooling label called vsize should be interpreted in context.

What can go wrong

Fee-rate recommendations fluctuate and are not guarantees of inclusion by a deadline. The actual transaction can grow after coin selection or signing. Estimating from the payment value, such as 100 versus 1,000 dollars, misses how many inputs and outputs the transaction contains.

Practical checks

A useful fee quote includes both the proposed feerate and the transaction's estimated vsize. A simple payment with several small UTXOs can be larger and more expensive than a greater-value payment using one input. Verify whether your tool's 'vsize' is the BIP-141 calculation or a policy-adjusted estimate.

Related concepts

Blockchain, Bitcoin (BTC), Transaction Hash.

Further reading

Bitcoin BIP-141 — Virtual Size.