What Is Block Weight? How SegWit Changed Block Size Limits

From bytes to weight

Early Bitcoin limited a block to 1,000,000 bytes. Segregated Witness, specified in BIP-141, moved signature data into a separate witness section and replaced the plain byte limit with a measure called weight. Non-witness bytes count as four weight units each, while witness bytes count as one. The consensus limit is 4,000,000 weight units, which allows blocks larger than a megabyte when they contain much witness data, while a block with no witness data still tops out near the old 1,000,000 bytes.

What the source code says

Bitcoin Core's consensus header defines the maximum block weight as 4,000,000 and the witness scale factor as 4. Litecoin's source defines the same 4,000,000 weight limit and adds a separate allowance for MWEB data on top of the standard serialised size. Dogecoin's consensus constants list a 4,000,000 weight figure and also keep a 1,000,000-byte base size limit, so each chain's capacity depends on its own rules.

Why it matters

Block capacity sets how many transactions fit in each block and therefore how fees behave when demand is high. Weight also gives a discount to transactions that use witness data, which makes some spending types cheaper than others in fee terms. Wallets quote fees by virtual size, which is weight divided by four.

Reading it as a user

You rarely see weight itself. You see a fee rate, such as coins per virtual byte, and an estimated size for your transaction. Knowing that the size depends on what kind of inputs you spend explains why the same payment can cost different fees from different wallets or address types.