Transaction Confirmations

When a Bitcoin transaction receives its first confirmation, it means a miner has included it in a block and that block has been added to the blockchain. The transaction is now part of the permanent record.

But “one confirmation” isn’t quite the same as “completely final.”

Here’s why. Each new block added after the one containing your transaction makes it progressively harder to reverse. To reverse a transaction with one confirmation, an attacker would need to remine that block and all blocks since — which requires outpacing the entire network from that point. With two confirmations, they’d need to outpace from two blocks back. With six, from six blocks back.

The probability of a successful rewrite drops exponentially with each new confirmation. With one confirmation, reversal is theoretically possible but difficult. With six confirmations — roughly one hour — it’s considered practically impossible for any realistic attacker.

This is why Bitcoin exchanges and merchants set confirmation thresholds. For small purchases, one or two confirmations is typically considered sufficient. For large transactions — significant amounts moving between exchanges or institutions — six or more confirmations is the standard.

For most day-to-day purposes, the practical experience is simple: send a transaction, wait ten to twenty minutes for the first confirmation, and the money has arrived. For larger amounts, waiting an hour adds a level of finality that’s essentially absolute.

Finality in Bitcoin isn’t binary — it’s probabilistic, growing stronger with every subsequent block. In practice, after six confirmations, it’s as final as anything in finance.

Tomorrow: the week in review — mining, nodes, and the transaction lifecycle.

— The Daily Bit