Check number in cc numbers and bank account numbers

Credit card numbers are often typed in, input, transferred and quoted. All of this transmission can cause errors, especially considering that humans are involved. To minimize this,credit card numbers contain a check digit.

In a typical sixteen digit credit card number, the first fifteen digits are determined by the issuing bank, but the last digit, called the check digit, is mathematically determined based on all the other digits.

You don’t select this last digit, it is deterministic. The exact mathematic formula for its generation was invented by Hans Peter Luhn, an engineer at IBM in 1954.

The Luhn algorithm is based around the principle of modulo arithmetic and digital roots.

The check-digit is what number needs to be added to this total to make the next multiple of 10. In our case, we’d need to add 3 to make 70. So the check-digit for this fictitious number is 3.