Block height

Height is bitcoin's native way of saying when. It is an integer that rises by exactly one per block, it depends on nobody's clock, and the consensus rules that need a schedule are written against it rather than against dates. Halving countdowns, confirmation counts, and activation announcements are all reading the same counter.

How it works

Block height is a position in a sequence, not a name, and that difference matters more than it sounds.

Two blocks can briefly claim the same height when miners solve at nearly the same moment, and after a reorganization a height can point at a different block than it did an hour ago. The permanent identifier is the block hash. Height is how far along the chain you are, and it is only unambiguous once you say which chain you mean, which in practice is the one with the most accumulated work.

Confirmations come straight out of the arithmetic. A transaction in a block at height 900,000, on a chain whose tip is at 900,005, has six confirmations, because the count includes the block it is in. That single formula, tip minus height plus one, is behind every "5 of 6 confirmations" progress bar you have watched.

Consensus prefers heights to timestamps for anything scheduled, because timestamps are supplied by miners and only loosely policed, while heights cannot be fabricated without doing the work. Locktime is the exception that proves the rule: a locktime below 500,000,000 is interpreted as a block height, and anything at or above that number is read as a Unix timestamp, which is why a transaction can be told to become valid at a specific height.

Height was not always inside the block. BIP34 made it mandatory to write the height into the coinbase transaction, and the rule became binding at block 227,835 in March 2013. The reason was concrete: in November 2010 two coinbase transactions were created that duplicated earlier ones exactly, at heights 91,842 and 91,880, because nothing forced their contents to differ. Committing the height guarantees every coinbase transaction is unique.

Why this matters when you buy bitcoin

Block height is the number that lets you check what an exchange or a wallet is telling you, without asking anyone.

When you withdraw, you get a transaction ID. A block explorer will show the height of the block that included it and the height of the current tip; the difference plus one is your confirmation count. If a support agent says the withdrawal was sent an hour ago and the explorer shows no block containing it, the coins have not moved, whatever the account page says.

Heights also let you read exchange policies precisely. Platforms credit deposits after a set number of confirmations, which is a statement about heights, not minutes. Since blocks arrive on an average rather than a schedule, the same three confirmation policy can take twenty minutes on one day and an hour on another, and neither is a fault.

Then there is the marketing. Every halving countdown is a projection: the event is defined by a height, and the date attached to it is a guess based on assumed block times. Anyone offering you a product because "the halving is on" a specific date is selling a forecast. The same goes for screenshots showing a payment "confirmed at height" something. Heights and transaction IDs are trivial to fake in an image and impossible to fake on a chain you check yourself, which is the habit our security guide keeps coming back to.

Heights that changed the rules

A handful of heights are worth knowing because bitcoin behaves differently before and after them.

Height 0 is the genesis block, mined on January 3, 2009, whose coins can never be spent. Height 210,000, reached on November 28, 2012, was the first halving. Height 227,835 made BIP34 binding in March 2013. Height 481,824, on August 24, 2017, is where SegWit took effect and blocks began carrying witness data. Height 709,632, on November 14, 2021, activated Taproot. Height 840,000, on April 20, 2024, was the fourth halving. Height 1,050,000 will be the fifth, projected for some time in 2028 depending on how fast blocks actually arrive.

Block height vs confirmations

Block height is an absolute position; a confirmation count is a relative distance. A block's height never changes while it stays on the chain, but its confirmation count grows by one roughly every ten minutes. Wallets show confirmations because what you care about is depth, and exchanges quote thresholds in confirmations for the same reason. Saying "my transaction is at height 900,000" tells you nothing about whether it is safe yet.

Block height vs coinbase block height

Block height is a property of the block; the coinbase block height is a copy of that number written inside the block's first transaction. They should always agree, and a node rejects the block if they do not. The distinction matters because the coinbase copy is data that a miner chooses to include, checked against the height the chain already implies, and it is the mechanism that stopped duplicate coinbase transactions for good.