Confirmation

A confirmation is a block that includes your transaction, or is mined on top of the block that does. The moment a transaction lands in a block it has one confirmation — it is said to be mined at a depth of one. Every subsequent block found by the network adds another confirmation, burying the transaction deeper under accumulated proof-of-work and making any attempt to reverse it progressively more expensive. Confirmations are how Bitcoin turns raw hashrate into settlement assurance you can count on your fingers.

Why depth equals security

An unconfirmed transaction sitting in the mempool is only a promise: it can still be displaced by a conflicting transaction through a race, a fee-bump of a competing spend, or a deliberate double-spend attempt. Once mined, reversing it requires rewriting history — an attacker must re-mine the block containing the transaction and every block above it, then outpace the honest network to present a longer chain. Each additional confirmation adds another block header's worth of work to that bill, which is why reversal probability falls off a cliff with depth. A one-confirmation transaction can be undone by an ordinary short reorg that happens naturally when two miners find blocks near-simultaneously; a six-confirmation transaction requires a sustained, deliberate attack with a major fraction of global hashrate.

The six-confirmation convention

The classic Bitcoin client treated six confirmations as practically settled — a threshold derived from the whitepaper's own probability math under the assumption that an attacker controls well under 10% of hashrate. Six blocks take roughly an hour at the ten-minute target that difficulty adjustment maintains. But six is a convention, not a protocol rule: the network never declares anything final. Real-world practice scales the wait to the stakes — a coffee merchant may accept one confirmation or even zero, an exchange crediting a large deposit often waits more, and 144 blocks (about a day) is a common conservative benchmark for high-value settlement. Coinbase rewards are the extreme case: consensus itself forbids miners from spending a block reward until it has matured 100 blocks, precisely because a reorged reward vanishes entirely.

What a confirmation is not

Confirmations measure burial, not validity — an invalid transaction never gets one, because every node rejects blocks containing it regardless of the work attached. And the count itself carries no timestamp guarantee: blocks arrive by Poisson process, so an "hour" of six confirmations is an average that routinely stretches or compresses. Wallets that display "confirmed" after one block and "final" after six are drawing sensible but arbitrary lines on a continuous curve.

The practical face of probabilistic finality

Confirmations are how probabilistic finality shows up in daily use: rather than a settled/unsettled flag, Bitcoin offers a dial of confidence that every user reads according to their own risk. For a node runner, watching a payment climb from zero to six is watching the global hashrate vote, block by block, that your transaction is part of history — and for a miner, every block you find is someone else's confirmation.

Operationally, confirmation policy is worth writing down anywhere money changes hands automatically. A payment processor, a self-hosted checkout, or a script sweeping pool payouts should encode explicit thresholds — how many confirmations before goods ship, before a balance is spendable, before funds move to cold storage — scaled to the amounts involved. The protocol will never make that judgment for you, and the failure mode of not making it yourself is discovering your policy during a reorg, which is the one moment it cannot be changed calmly. Exchanges publish their deposit thresholds for the same reason: a stated policy, chosen in advance, is the only kind that holds up when the chain does something unusual.