Bitcoin treats a payment as confirmed once a miner has built a new block on top of the block that holds it. Each block added afterward is one more confirmation. Blocks arrive about every ten minutes on average, so three confirmations take roughly half an hour and six take about an hour.
Confirmed is a scale, not a switch. One block means the payment is on the chain, but a short reorganization could still, in principle, replace that block. Each later block makes that outcome harder, because anyone who wants to rewrite your payment must redo the mining work for every block above it.
Before the first block, the transaction waits in the mempool, the queue of unconfirmed payments that miners have not yet included. In that state it can still be dropped or replaced, which is why a wallet shows it as pending even after you press send.
How long a confirmation takes
Ten minutes is an average, not a schedule. Mining is a race of random guesses, so an individual block can land in under a minute or stretch toward an hour. Every 2,016 blocks the network adjusts difficulty to drag the average back toward ten minutes.
What exchanges require
Exchanges choose their own numbers because a reversed deposit is their loss, not the sender’s. Kraken credits a bitcoin deposit after three confirmations, about 45 minutes. Several other large platforms use similar figures, usually two or three blocks. For a payment you control, six confirmations is the cautious default.
Why six became the number
The six-block rule comes from the Bitcoin white paper. In section 11, Satoshi Nakamoto worked out how likely an attacker is to rewrite a payment after a given number of blocks. Against an attacker holding 10% of mining power, the chance falls to 0.024% after six blocks. That is the math behind the convention, and it explains why fewer blocks are fine for small amounts: the risk drops sharply with each block but never reaches zero. An attacker with more than half of mining power can eventually overtake the chain no matter how long you wait, which is the 51% attack.
When a transaction is stuck
A payment that sits at zero confirmations usually means the fee is too low for current demand. Look it up on a block explorer to see the rate you paid. If your wallet supports replace-by-fee, resend the same payment with a higher fee. If you are the receiver, child-pays-for-parent spends the unconfirmed coins with a high fee, pulling both transactions into a block. Ignore any accelerator service that asks you to send bitcoin to a stranger; those are scams.
Not every payment needs a confirmation wait. The Lightning Network settles in seconds by moving payments off-chain; only opening and closing a channel touch the main chain. At the other extreme, newly mined coins cannot be spent until 100 more blocks have been added, a rule baked into Bitcoin’s code.
For a small payment, one confirmation is usually enough. For a deposit you want to trade, expect two or three. For a large transfer, wait for six. The number is a trade-off between speed and the cost of undoing a payment, not a rule set in stone.
Adapted from How Many Bitcoin Confirmations Do You Need?.