Why depth equals safety
Bitcoin blocks are chained: each one references the previous. Rewriting history means rebuilding every block from the target onwards and outpacing the rest of the network while doing it.
One block deep, that is expensive. Six blocks deep, it costs more than almost any transaction is worth, which is the entire reasoning behind the number. Six is not a technical requirement anywhere in the protocol; it is a risk threshold that became convention because it made the cost of reversal absurd.
How long confirmations take
Blocks arrive roughly every ten minutes on average, so six confirmations take about an hour.
The word average is doing real work in that sentence. Block times follow a random distribution, so intervals of two minutes and intervals of forty both happen regularly. A merchant quoting “one hour” to a customer will be wrong in both directions often enough to matter, and the honest phrasing is a range.
What thresholds to set
There is a trade-off, and it is straightforward: more confirmations means less risk and a longer wait at checkout.
One confirmation
for ordinary retail amounts. Reversing a single block requires resources that make it pointless against a small order.
Three
for mid-sized transactions, particularly where goods ship immediately.
Six
for large amounts, exchange deposits and anything irreversible on delivery.
Most gateways let the merchant set this per amount band rather than globally, which is the arrangement worth using: a twenty-dollar order and a twenty-thousand-dollar order do not need the same caution.
When a transaction stays unconfirmed
Almost always one cause: the fee attached was too low for current demand, so miners are working through more profitable transactions first.
The transaction sits in the mempool, the pool of broadcast transactions waiting for inclusion. It has a hash and it is visible in any explorer, and it has not happened yet in any sense that matters.
Two ways out. Replace-by-fee lets the sender rebroadcast the same transaction with a higher fee, provided the original was flagged as replaceable. Child-pays-for-parent lets the recipient spend the incoming output with a high fee, giving miners an incentive to confirm both together.
If neither is used, the transaction eventually drops out of the mempool and the funds are simply still in the sender’s wallet.
How other networks compare
Confirmation counts do not transfer between chains, and the numbers look different because the block times do.
Litecoin produces a block every 2,5 minutes, so a comparable depth arrives four times sooner. TRON confirms in three to ten seconds. Ethereum takes one to three minutes and longer when busy. Solana settles in under a second.
The underlying question stays the same everywhere: how expensive would it be to undo this, and is that more than the payment is worth.