One block built on top of the block containing your transaction; more confirmations mean history is harder to reverse.
A transaction included in a block has one confirmation. Each block added after it adds another. Confirmations are a probabilistic measure of safety on chains without instant finality, because reversing your transaction means out-building every block since.
Exchanges publish confirmation requirements per asset, and they differ widely: a chain with fast blocks and low hash-rate may need dozens, while a high-security chain needs a handful.
Example: an exchange requiring 6 confirmations on a ten-minute block-time credits your deposit roughly an hour after you send. If you are racing a margin call, that hour is the real constraint, not the exchange's interface.
Original diagrams for the ideas on this page. Illustrative, not real market data.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
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