Block

Blocks turn a stream of payments into an ordered, permanent record. Each one carries a header, a transaction count, and the transactions themselves, and the whole package is capped at four million weight units, which in practice means a few thousand payments. When people say a payment has confirmed, they mean a miner put it in a block.

How it works

A block has three parts, and only the first has a fixed size.

The header is 80 bytes and never varies. After it comes the number of transactions, written in bitcoin's variable length compactSize format. Then the transactions themselves, in order, with one hard requirement: the first must be the coinbase transaction, the one that pays the miner. A block whose coinbase sits in second place is rejected as firmly as one containing a forged signature.

Size is no longer counted in plain bytes. Since SegWit activated in August 2017, the limit is 4,000,000 weight units, where a byte outside the witness costs four units and a witness byte costs one. That produces a ceiling close to 4 MB for witness heavy blocks and about 1 MB for a block full of old style transactions, with typical real blocks landing between 1.5 MB and 2 MB. The original 1,000,000 byte rule survives underneath as a cap on the non witness portion, which is why the honest answer to "how big can a block be" is a range rather than a number.

Everything else is checked by every node independently. The header hash must be at or below the target, the merkle root must match the transactions actually present, no input may be spent twice, and the coinbase output must not claim more than the subsidy plus the fees of the transactions included. The protocol also imposes a waiting period on the miner: coins created in a block cannot be spent until 100 further blocks have been built on top, so a miner whose block is later dropped in a reorg cannot have already spent the proceeds.

Why this matters when you buy bitcoin

Block space is an auction, and the fee on your withdrawal is a bid in it.

Blocks are finite and demand is not, so when the mempool is busy the transactions that pay the most per virtual byte are taken first and the rest wait. This is the mechanism behind every "why is my withdrawal stuck" support ticket. Checking a fee estimator before you move coins, and moving them at a quiet hour when you can, costs nothing and routinely saves more than the difference in exchange fees you spent an afternoon comparing.

The ten minute figure is an average, not a timetable. Block discovery behaves like a memoryless random process, so intervals scatter widely: more than a third of blocks take longer than ten minutes, and about one in twenty takes longer than half an hour. An exchange that credits deposits after several confirmations is therefore quoting you a distribution, not a delivery time, and a deposit that has not landed in twenty minutes is usually normal rather than lost.

There is also a way to sidestep block space altogether. Lightning payments settle off chain and only touch a block when a channel opens or closes, which is why an app like Strike can move value in seconds while an on chain withdrawal waits for the next block. That trade is worth understanding before you assume every bitcoin transfer has to cost a fee to a miner.

Inside block 774,628, the largest ever mined

Block 774,628, mined by the Luxor pool on February 1, 2023, is the biggest block bitcoin has produced: 3.96 MB.

It contained 63 transactions, and one of them accounted for 3.94 MB of the total, an inscription of the Taproot Wizard image stuffed into a Taproot witness. The pool coordinated the transaction directly rather than picking it out of the mempool, and it paid the block barely anything: about 0.009 BTC in fees against a subsidy that was still 6.25 BTC at the time. The previous record had been 2.77 MB.

The block is worth looking up because it shows the weight rule in action. The old 1 MB limit would have made that transaction impossible, and the 4,000,000 weight unit budget made it possible only because witness data is discounted. It also shows that a single participant can occupy an entire block, which is exactly what the block size war was fought over.

Block vs block header

A block is the whole object; the header is the 80 byte summary at the front of it. The distinction is practical rather than pedantic. Miners hash only the header, light wallets download only headers, and a node syncing for the first time fetches the header chain before it asks for a single transaction. If a document says "the block hash", it means the hash of those 80 bytes, not a hash of the payments inside.

Block vs stale block

A block is on the chain the network agrees on; a stale block is a perfectly valid block that lost the race to be that chain. Two miners occasionally solve at nearly the same moment, the network briefly holds two candidates, and the branch that stops growing is abandoned. Nothing was wrong with the discarded block, and its transactions almost always reappear in the next one. This is also why a single confirmation is weaker than six: the deeper a block sits, the less plausible it becomes that a competing branch overtakes it.