Transaction Weight

Overview

What it means and why it exists

Transaction weight is Bitcoin's resource-accounting measure that assigns different costs to transaction data with and without SegWit witness information. It is measured in weight units, not ordinary bytes.

How it works

Under BIP 141, transaction weight equals three times the base serialized size plus the total serialized size. Equivalently, it is four times non-witness bytes plus witness bytes. The rule encourages moving signature-related data into witness fields without making that data free. Nodes use weight for block limits.

Technical details worth understanding

A block's maximum consensus weight is four million weight units. For a transaction, virtual size is calculated by rounding its weight divided by four upward. A feerate expressed in satoshis per virtual byte therefore uses virtual size, not raw total serialized bytes. Bitcoin Core policy has additional sigop-adjusted considerations in some contexts.

What can go wrong

A 400-byte network message does not necessarily cost 400 virtual bytes. Fee estimators that multiply feerate by the wrong size can overpay or underpay, particularly with SegWit inputs. Fee amounts remain dependent on input selection, scripts and current demand.

Practical checks

When budgeting miner fees, record weight units and the wallet's calculated virtual size separately. Compare alternative coin selections using the same feerate. If a tool reports wire size, do not substitute it without checking the SegWit rule. For multi-input refunds, fee estimation should be performed after recipients and inputs are known.

Related concepts

Blockchain, Bitcoin (BTC), Transaction Hash.

Further reading

Bitcoin BIP-141 — SegWit Weight.