Confirmation
Also: confirmations, block depth
network · beginner
The number of blocks built on top of the one containing a transaction. More confirmations → exponentially harder to reverse.
A transaction is confirmed the moment a miner includes it in a block. But because every block can in principle be replaced by a longer competing chain, finality isn't binary — it accrues over time as more blocks are stacked on top.
Common heuristics:
- 0 confirmations — in the mempool, not in a block. Acceptable for a coffee, not for a car.
- 1 confirmation (~10 minutes) — included once. Could still be reorged by a one-deeper chain, but the probability drops rapidly with each block.
- 3–6 confirmations — what most exchanges require for deposits. Reversing a 6-deep transaction means out-mining the entire network for an hour straight.
- 100 confirmations — required to spend a [coinbase](/glossary/coinbase) output. The protocol bakes in the assumption that 100-deep blocks are not getting reorganized.
Reorgs do happen — most are one block deep, almost always from natural network propagation lag rather than malice. Deeper reorgs are vanishingly rare on Bitcoin; the last reorg longer than 2 blocks on mainnet was in 2013.
The practical takeaway: more value → wait for more confirmations. There's no point waiting for 6 confirmations on a $10 payment, and one confirmation on a $10M payment is reckless.