Eli5: how does platforms like Steam or Amazon know if you’re using a correct visa card or not if you didn’t buy anything with it?

Question

When you enter your card details to add it to your account and you write 1 number wrong, it instantly tells you that the card is wrong. How do they know?

In: Technology

Answer 1

The last digit is a checksum: for any given combination of the first 15 digits, there’s only one possible value for the last digit of the card. The website can perform that same calculation to verify that you didn’t make a mistake entering in the card number.