Why "6 confirmations" became the gold standard, and when you need fewer (or more)
"My transaction is 'confirmed' but my exchange says to wait for 6 confirmations. Is my money safe or not?"
In Bitcoin, finality is probabilistic, not absolute. Each confirmation makes reversal exponentially harder, but technically never impossible.
When your transaction is included in a block, it has 1 confirmation. Every subsequent block adds another confirmation.
Each block is like a floor of a building, with your transaction inside. To "undo" your transaction, an attacker would need to demolish your floor AND rebuild every floor above it faster than honest builders keep adding new floors.
1 floor above? Maybe possible. 6 floors above? Virtually impossible without controlling most of the world's mining power.
Sometimes two miners find valid blocks simultaneously. The network temporarily has two competing chains. Eventually, one chain gets more work and "wins" — the other is abandoned, and its transactions return to the mempool.
Satoshi's whitepaper calculated the probability of an attacker catching up to the honest chain based on their hashrate percentage:
| Attacker Hashrate | 1 Conf | 3 Conf | 6 Conf |
|---|---|---|---|
| 10% | 20.5% | 1.3% | 0.02% |
| 30% | 45.0% | 13.0% | 2.1% |
| 45% | 72.0% | 47.0% | 26.0% |
| Situation | Recommended | Reasoning |
|---|---|---|
| Coffee purchase | 0 (unconfirmed) | Low value, trusted relationship, instant |
| Online purchase | 1-2 | Moderate value, some risk acceptable |
| Larger payments | 3 | Good balance of speed and security |
| Exchange deposits | 3-6 | Exchanges are common attack targets |
| Large transactions | 6+ | Standard "final" threshold |
| Life-changing amounts | 60+ | Maximum paranoia for maximum value |
Unconfirmed transactions are visible in the mempool but not yet in a block. They're useful for speed but come with risks: