Maximum block size

A block cannot grow to fit demand, and that is deliberate. Bitcoin Core caps a block at 4,000,000 weight units, a budget that discounts witness data and works out near 4 MB for a witness heavy block and 1 MB for old style transactions. That fixed supply is one half of every fee you pay to move coins.

How it works

Status: the flat 1,000,000 byte rule stopped being the binding constraint on August 24, 2017, when SegWit activated at block 481,824. It survives underneath as a cap on the non witness portion of a block, but the limit that decides what fits is the 4,000,000 weight unit budget defined by BIP141.

Maximum block size began as an emergency brake rather than an economic policy. Satoshi Nakamoto added the 1,000,000 byte constant in 2010, without documentation or discussion, so that a hostile miner could not flood a young network with one gigantic block that every participant would then have to store forever. Nobody wrote down what should happen when ordinary demand caught up with it. That silence is a large part of why arguing about the number later turned so bitter.

BIP141 changed the accounting instead of the constant. Weight counts base data three times plus the whole serialization once, which comes to four units per byte outside the witness and one unit per witness byte. Cap that at 4,000,000 and two ceilings fall out together: base data can still never exceed 1,000,000 bytes, and roughly 3 MB of signature data can ride along on top of it. Software written before 2017 only ever measured the stripped serialization, so it saw a legal block and raised no objection, which is how capacity grew through a soft fork rather than a chain split.

Weight is not the only ceiling in the rulebook. A block may also spend no more than 80,000 signature operations, a separate budget that keeps a small block from taking a disproportionate time to verify.

Where you see it

Real blocks sit well below the theoretical maximum, because ordinary payments carry very little witness data. A busy block usually serializes to somewhere above 1.5 megabytes, and only transactions stuffed with inscription style data push toward the 4 MB extreme. Explorers print size and weight side by side and the two disagree by design: divide the weight by four and you get virtual bytes, the unit your wallet uses when it quotes a fee rate in sat/vB.

The number is also the reason two other coins exist. Bitcoin Cash split away on August 1, 2017 at block 478,558 with an 8 MB limit, and the SegWit2x proposal to double the base cap to 2 MB was called off on November 8, 2017 for lack of support. Both events belong to the block size war, a three year argument about whether a bigger block or a second layer should absorb growth. The side that won left the base limit alone and moved the pressure to Lightning.

Maximum block size vs block weight

Maximum block size is the question; block weight is the unit the answer is now given in. Asking how many megabytes fit produces a range rather than a figure, because the answer depends on how much of the data sits in witnesses. Asking how many weight units fit produces exactly one number, 4,000,000, for every block since SegWit activated. This is why protocol documentation stopped quoting a size at all, and why any source still telling you the limit is 1 MB is describing bitcoin as it was before 2017.