Block height is the position of a block in a blockchain, counted as the number of blocks that come before it. The first block, the genesis block, has a height of 0; the block built on it has height 1, and so on. Wallets, exchanges and developers use block height to locate blocks, count confirmations and schedule protocol changes.
Height is the chain's own clock. Because blocks arrive at a roughly predictable pace, a height can be translated into an approximate date, and many rules of the Bitcoin blockchain are written in terms of height rather than calendar time. On the Ethereum blockchain the same idea is usually called the block number. Height should not be confused with the block's hash, which is its unique fingerprint.
Block height in plain terms
Picture a tower of numbered bricks. The bottom brick is number 0, and each new brick laid on top gets the next number. If you want to describe where a particular brick sits, you simply give its number; the higher the number, the more recent the brick and the more bricks are pressing down on it.
Block height works the same way. A payment recorded at height 100 lies under every block added since. The difference between the current top of the tower and your brick tells you how buried, and therefore how settled, your payment is. And because a new brick arrives at a fairly steady rhythm, the number also acts like a calendar that the whole network agrees on.
How height is used
The most common everyday use is counting confirmations. The number of confirmations a transaction has is:
- is the number of confirmations.
- is the height of the newest block on the chain (the "tip").
- is the height of the block that contains the transaction.
For example, if a payment was included in block 900,000 and the tip is now block 900,005, the payment has confirmations, a level many exchanges treat as final for bitcoin deposits. A transaction still in the mempool has no height and zero confirmations, which is the situation described as a zero-confirmation transaction.
Height also anchors several protocol mechanisms:
- Coinbase marking. Since BIP 34, every Bitcoin block must state its own height inside the coinbase transaction, which keeps coinbase transactions in different blocks from being identical.
- Time locks. A Bitcoin transaction's lock-time field is read as a block height when its value is below 500,000,000 and as a Unix timestamp otherwise, so a payment can be made unspendable until a given height.
- Reward maturity. Newly created coins in the coinbase cannot be spent until 100 more blocks have been added.
Height as a schedule
Bitcoin's monetary policy is written in blocks, not dates. The block reward subsidy is cut in half every 210,000 blocks, an event known as the halving. The fourth halving took place at height 840,000 in April 2024, and the next is set for height 1,050,000, expected around 2028. Mining difficulty is recalculated every 2,016 blocks, roughly every two weeks.
Upgrades are scheduled the same way. The SegWit soft fork became active at height 481,824 in August 2017. Because Bitcoin targets one block every ten minutes, about 144 blocks are added per day and about 52,560 per year, so a future height can be converted into an estimated date, though the true date drifts with changes in hashrate.
Ethereum moved from height-based to time-based scheduling. Under proof of stake the network counts 12-second slots and 32-slot epochs, and upgrades are activated at a chosen epoch or timestamp. Block numbers still increase by one with each produced block, but because slots can be empty, the block number and the slot number are different counters.
Height, forks and reorganisations
Height identifies a position, not a single block. When two miners find valid blocks at almost the same moment, both blocks briefly share the same height. Nodes follow the chain with the most cumulative work, and the losing block becomes a stale block. During this short fork, an explorer may show a different block at a given height from one minute to the next. For that reason software that must be exact refers to blocks by hash and treats very recent heights as provisional.
When a hard fork splits a network into two chains, both keep the shared history and the same heights up to the split, then continue counting separately. Bitcoin Cash, for example, shares Bitcoin's blocks up to height 478,558 and diverges from height 478,559.
The current height of any chain, and the block at each height, can be looked up in a block explorer or by asking a full node directly. Each block header does not store its own height in Bitcoin; nodes work it out by counting back to genesis, which is one reason the height is written into the coinbase.