Block height
Block height is a block’s place in line: how many blocks came before it, starting from zero.
Photo: “measuring tape” by Sean MacEntee, CC BY 2.0, via Flickr (edited: resized).
Block height is the number of blocks preceding a particular block on a blockchain, so the first block, the genesis block, has height 0 [1]. Bitcoin uses heights to schedule rules such as halvings every 210,000 blocks [2].
Key points
- 1Height counts blocks from the genesis block, which is height 0.
- 2Protocol events like halvings and difficulty adjustments are set by height, not by date.
- 3During a short fork two blocks can share a height, so blocks are identified uniquely by their hash.
How does block height work?#
Every time a valid block is added, the chain gets one block taller. The developer guide defines a block’s height as the number of blocks between it and the first Bitcoin block, the genesis block (block 0) [3]. Miners even write the height of the block they are building into the first bytes of its coinbase field [1]. Ethereum blocks record a block number in the same spirit [4].
| Step | Value |
|---|---|
| Genesis block | Height 0 |
| First point where difficulty could adjust | Height 2,016 |
| Halvings | Every 210,000 blocks: 210,000, 420,000, 630,000, 840,000 |
| Your confirmations | Current height − your block’s height + 1 |
| Locktime values below 500 million | Read as a block height, not a time |
Height 2,016 comes from the developer guide [3], the halving interval from Bitcoin Core [2] and the locktime rule from the transaction guide [5].
Why is block height not a unique ID?#
Because two miners can find a block at almost the same moment. For a short time there are two different blocks at the same height, until one branch grows longer. The developer guide therefore warns that height should not be used as a globally unique identifier; blocks are referenced by the hash of their header instead [3].
Which words are related?#
- Confirmation — counted from the height of your transaction’s block.
- Halving — triggered at fixed heights.
- Blocks and confirmations — the full guide.
Frequently asked questions#
Why does the count start at zero?
Height counts the blocks before a block. Nothing precedes the genesis block, so its height is 0 [1].
Where can I see the current block height?
Any block explorer shows the latest height. Because every node counts independently, different explorers may differ by a block for a few seconds.
Is block height the same on Bitcoin and Ethereum?
The idea is the same, but each chain counts its own blocks at its own pace, so the numbers are unrelated.
Sources#
Grade A = primary source (regulator, protocol specification, client code, original author). Grade B = expert secondary source used for explanation only.
- Abitcoin.org developer documentation. Bitcoin Developer Glossary, 2026.
- ABitcoin Core (GitHub). src/kernel/chainparams.cpp (nPowTargetSpacing, nPowTargetTimespan), 2026.
- Abitcoin.org developer documentation. Developer Guide: Block Chain, 2026.
- Aethereum.org. Blocks, 2026.
- Abitcoin.org developer documentation. Developer Guide: Transactions, 2026.