A MOD 10 check validates that a credit card number is in the correct range and format to actually be a valid credit card number — and confirms the card brand matches what the consumer claims to be using.
What it's actually for
- Stops obviously invalid card numbers from ever being processed
- Catches customer input errors — transposed digits, missing digits, the wrong card type selected
- Cleans up order flow by catching mistakes before submission, not after
- Cuts down on manual intervention and authorization call-backs
What it doesn't tell you
A MOD 10 check does not confirm a card is currently active — only that the number follows the correct formatting convention for a card number. A perfectly formatted number can still belong to a closed account or a stolen card; that's a different check entirely.
How it runs
It's implemented as client-side code directly on the purchase page, running the moment a customer submits payment details — orders only continue on to the merchant after passing. Setup cost is minimal, and free code snippets implementing the algorithm are widely available.
This is genuinely one of the lowest-cost, easiest things to implement, and it's worth having in place before almost anything else on a site that accepts credit cards.