Weight unit

Old nodes count a block's stripped bytes and reject anything over 1,000,000, while new nodes count weight and reject anything over 4,000,000. Those two rules are satisfied by the same blocks, which is the trick that made Segregated Witness a soft fork when it activated at block 481,824 in August 2017. Everything you pay to move bitcoin is priced in this unit or in a quarter of it.

How it works

The weight unit exists because nodes running different software have to agree on when a block is full, even when they are not looking at the same bytes.

A node that never upgraded is not sent witness data at all. It sees the stripped block and applies the ceiling it has always applied, 1,000,000 bytes. An upgraded node reassembles the whole thing and applies a ceiling of 4,000,000 weight. Charge four units for every non-witness byte and one unit for every witness byte, and the two tests can never disagree: any block inside the weight budget also strips to under a megabyte. A rule that only ever tightens what old software already accepts is exactly what a soft fork is, and this pricing is what made the capacity increase fit inside one.

The choice of a quarter was not arbitrary. Witness bytes are signatures, and signatures are the part of a transaction a node can discard once the spend is buried; they never enter the UTXO set, the one structure every validating machine has to keep on hand forever. Pricing them low steered wallets toward a format that grows the expensive set more slowly, and it did that by making the responsible choice also the cheap one.

A second budget runs on the same scale and gets far less attention. BIP141 caps a block at 80,000 signature operation units, with legacy operations counted four times over, so a block stuffed with old-style multisig can run out of signature checks before it runs out of room.

Capacity is therefore a range rather than a number. A block of purely legacy payments still cannot exceed 1,000,000 bytes, while a witness-heavy one can approach four megabytes, and most land between one and two because most blocks are a mix.

Where you see it

Weight units are printed next to a block on every explorer and are the number a mining pool actually optimizes against.

The clearest place they show up is in a phrase that stopped being true in August 2017. Bitcoin does not have a one megabyte block limit; it has a four million unit budget, and the fullness bar you see on an explorer is measured against that. Pools assemble a template by filling the budget with the best-paying candidates and keeping a slice back for the coinbase transaction, so a mined block usually lands just under the cap rather than exactly on it.

Weight units are also the hidden subject of the loudest argument in bitcoin. Storing data inside witness space costs a quarter of what the same bytes cost anywhere else, which is precisely why inscriptions live there. The debate about whether that data belongs on the chain is, underneath the noise, a debate about a ratio chosen when BIP141 was written in 2015.

For someone buying and holding, the unit is met indirectly. You will never type a weight into a wallet, but it is the reason spending an old-format coin costs more than spending a modern one.

Weight unit vs block weight

A weight unit is the ruler; block weight is one measurement taken with it. Transactions have weights too, and a block's figure is the sum of its transactions plus the header and counters, which is why the two words are not interchangeable. The confusion becomes expensive when somebody reads the 4,000,000 figure as a per-transaction allowance. It is not: relay policy turns away a single standard transaction long before it approaches the block ceiling, and consensus measures only the block.