When did credit cards go from 13 digits to 16 digits?

Social Security numbers have always had nine digits, which is sufficient to distinguish every person in America, but credit cards are international. The first four numbers identify the credit card issuer, leaving twelve digits to identify users. Twelve digits provide for 999 billion, 999 million, 999 thousand, 999 individuals to have a unique credit card number. The current population of the planet is less than eight billion, so it seems unlikely that more digits will be necessary.

Part of the credit card number is the built in, error detecting check digits that can be one or two digits long. With the check digit built inside of the account number the checking process can be done immediately at the point of sale without the need to communicate with the home office.
The algorithm is some thing like “Take every even numbered digit and double it. Add all the odd numbered digits and add them to the doubled even digits. Add the digits in the sum to get a number. If it is more than one digit add the number again. That becomes the check digit. Include that digit as part of the account number.”
(The above is not the right method I am only using it as an example.)
The check digits catch all kinds of errors, transposed digits, dropped digits, false numbers, etc. If you have only one check digit the chances are you will catch 90% of mistakes, 2 digits will catch 99%, and 3 digits will catch 99.9% of mistakes without the need to communicate with the bank. Not perfect but damn good – and fast.
With 4 digits for the bank and 2 for check digits that leaves 10 digits for individual which is enough to handle just shy of 10 billion people.
When all credit cards are finally equipped with the Chip and PIN system, check digits will become a quaint relic of the past.

@LuckyGuy is correct. However, using two digits for error correction still leaves 10 billion minus one individual numbers, which should be enough till 2040 (see), and using the PIN system (which is increasingly common) credit cards could handle the16 billion people estimated to be the population in 2100 (although one would imagine that by then a better economic/monetary system will have been created).

Apparently, VISA stopped issuing cards with 13 digits in 2005. I don’t have a good reference for you on that. But I’m not sure all credit cards started out with 13… I think the switch to 16 digits was meant to standardize length as well as improve security.

@LuckyGuy As someone who has entered more ISBNs into databases than you can possibly imagine, I am a big fan of the check digit. :)

@glacial It is shocking to me it is as late as 2005. I worked retail and don’t remember visa having only 13 numbers any time recently.

@LuckyGuy We used to figure out the check digit for SKU/UPC’s back in the day because so many people would leave the last number off when they would write by hand because it is smaller or separated from the rest of the numbers usually. So stupid to make it small or separated. Who thought of that?