Segregated Witness
Learn what Segregated Witness is and how it affects Bitcoin transactions, block capacity, fees, and mining.
Definition
Segregated Witness, often called SegWit, is a Bitcoin protocol upgrade that separates transaction signature data from the main transaction data. It changes how transaction size is counted, allowing more transactions to fit into each block without simply raising the old block size limit. For miners, SegWit affects block construction, fee selection, and fee revenue.
How It Works
Every Bitcoin transaction includes details about the coins being spent, destination addresses, and digital signatures that prove the spender can move the coins. Before SegWit, all of this data was counted together under the traditional block size limit.
SegWit moves the signature portion, called witness data, into a separate structure. The transaction still contains the signatures, and nodes still verify them, but the network counts the data using a system called block weight. This gives witness data a lower weight than the core transaction data.
Because of this weighting system, a SegWit transaction usually uses fewer virtual bytes than a similar non-SegWit transaction. Fees are commonly measured in satoshis per virtual byte, so SegWit users can often pay less for the same priority. Miners still choose transactions from the mempool by fee rate, but SegWit changes the math used to compare block space.
SegWit also fixed transaction malleability, a problem where certain transaction identifiers could be changed before confirmation without changing the actual payment. That fix made it easier to build second-layer systems, such as payment channels, on top of Bitcoin.
Why It Matters
SegWit matters because it improved Bitcoin’s capacity while keeping full-node requirements more conservative than a simple large block increase. Better use of block space can reduce fee pressure when many users compete for confirmations.
For miners, SegWit can increase the number of fee-paying transactions that fit into a valid block. Mining pools can earn more total fees by selecting transactions with strong fee rates and favorable weight.
For users, SegWit can mean lower fees, faster confirmation for the same fee, and better compatibility with modern Bitcoin wallet features. For the network, it helped prepare Bitcoin for scaling tools that rely on stable transaction identifiers.