Confirmation
In simple terms
A confirmation counts how many blocks have been added since your transaction was included. More confirmations means it is more firmly settled, which is why an exchange makes you wait for several before crediting a deposit.
Definition
Each block added on top of the block containing your transaction, making it progressively harder to reverse.
In depth
Confirmation depth measures how much work or stake would have to be redone to reorganise the chain past a transaction. On proof-of-work chains reversal requires rebuilding every block after it faster than the honest network extends the tip, so the cost of reversal grows with depth. Chains with explicit finality gadgets replace this probabilistic model with a checkpoint after which reversal is not economically undone but protocol-prohibited, which is why confirmation counts vary so much between networks.
How does Confirmation work?
When a transaction is included in a block it has one confirmation. Each subsequent block adds one more. Services choose a threshold based on how much value is at risk and how quickly the chain reorganises in practice, then credit the deposit once that threshold is met. Until then the transaction is real but not yet treated as settled, which is why a balance can show as pending on an exchange while an explorer already shows it on-chain.
An example
An exchange requires an illustrative three confirmations for a Bitcoin deposit. The transaction is included, and with blocks arriving roughly every ten minutes the deposit credits somewhere around half an hour later. A different exchange requiring six would take roughly twice as long for the same transaction.
Figures are illustrative only.
What beginners get wrong
- Contacting support at zero confirmations. Nothing is wrong yet, and the chain has not had the chance to include it.
- Assuming every chain needs the same number. Requirements differ enormously by network, and copying a figure across is meaningless.
- Treating one confirmation as final for a large amount. Shallow reorganisations do happen, which is exactly what the threshold exists to absorb.
Related terms
Part of
How does a blockchain transaction actually work? — the subject page for blockchain mechanics, with all 15 of its definitions in one place.
Educational only — not financial advice.