Overview
A confirmation occurs when a Bitcoin transaction is included in a valid block that is appended to the blockchain. The first confirmation happens when the transaction appears in a mined block. Each subsequent block added on top of that block counts as an additional confirmation, further securing the transaction against reversal.
Confirmation Depth and Security
Confirmations Time (approx.) Security Level
──────────────────────────────────────────────────
0 0 min Unconfirmed (in mempool)
1 ~10 min Included in a block
2 ~20 min Moderately secure
3 ~30 min Secure for moderate amounts
6 ~60 min Highly secure (traditional standard)
100 ~16.7 hours Required for coinbase maturity
Why More Confirmations = More Security
To reverse a confirmed transaction, an attacker would need to mine an alternative chain starting from the block before the transaction's block and make it longer than the current chain. Each additional confirmation makes this exponentially harder:
Current chain: [A]──[B]──[C]──[D]──[E]──[F]
↑
TX included here (5 confirmations)
To reverse TX, attacker must:
1. Start from block before [B]
2. Mine [B']──[C']──[D']──[E']──[F']──[G']
(6 blocks, faster than the honest network)
3. Probability decreases exponentially with depth
Recommended Confirmations by Amount
- Small payments (coffee, small purchases): 0-1 confirmations may be acceptable, especially on the Lightning Network
- Medium payments: 1-3 confirmations
- Large payments: 6+ confirmations
- Very large or irreversible exchanges: Some services wait for even more
Common Misconceptions
Zero-confirmation (0-conf) transactions are not worthless. For small, in-person transactions where the risk of a double spend is low (the merchant can see the customer), 0-conf provides practical instant payments. However, for high-value or remote transactions, waiting for confirmations is essential.