Block Confirmation
A block confirmation is reached when a transaction has been included in a valid block. Each additional block added on top counts as another confirmation and makes the transaction harder to reverse.
Counting confirmations
While a transaction waits in the mempool it has zero confirmations. Once a miner includes it in a block that the network accepts, it has one. When the next block is built on top, it has two, and so on. Wallets and block explorers show this count so you can see how deeply a transaction is buried in the chain.
Why more confirmations mean more security
Occasionally two miners find a block at almost the same time, and the network briefly has two competing versions of the chain. One version soon pulls ahead and the other is abandoned; transactions only in the abandoned block go back to the mempool. A transaction with one confirmation can therefore, rarely, be undone by accident.
To reverse a transaction on purpose, an attacker would need to rebuild the chain from that block onward faster than everyone else. On a proof-of-work chain, each extra confirmation adds the cost of another block's worth of work to such an attack. That is why the risk of reversal drops quickly as confirmations grow.
How many are enough?
For Bitcoin, six confirmations, about an hour on average, is a long-standing convention for large amounts. Exchanges set their own requirements per network, often lower for large chains and higher for smaller ones that are cheaper to attack. Some proof-of-stake networks such as Ethereum also offer finality: after a certain point, typically within minutes, reversing a block would require destroying a large share of all staked ETH.
Common mistakes
Treating zero-confirmation transactions as final for large payments is risky, because they can still be replaced. Another mistake is panicking when a deposit is not credited immediately: exchanges usually wait for their required number of confirmations first.
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Frequently asked questions
How long does one Bitcoin confirmation take?
About ten minutes on average, but individual blocks can take anywhere from seconds to over an hour, and your transaction must first be picked from the mempool.
Why does my exchange need more confirmations than my wallet shows?
Each exchange sets its own safety threshold per network to protect against reorganizations and double-spend attacks.
Can a confirmed transaction be reversed?
With many confirmations on a large network it is practically impossible. With very few confirmations or on a small network, a chain reorganization or attack is still possible.
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