Credit Card Numbers

So Many Credit Card Digits

Credit Card NumbersA Social Security card has nine digits.  So do two-part Zip codes.  A domestic phone number, including area code, has ten digits.  Yet a complete MasterCard number has twenty digits.

Why so many?

It is not mathematically necessary for any credit-card account number to have more than eight digits.  Each cardholder must, of course, have a unique number.  Visa and MasterCard are estimated to have about sixty-five million cardholders each.  Thus their numbering systems must have at least sixty-five million available numbers.

There are one hundred million possible combinations of eight digits- 00000000, 00000001, 00000002, 00000003, all the way up to 99999999.  So eight digits would be enough.  To allow for future growth, an issuer the size of Visa or MasterCard could opt for nine digits- enough for a billion different numbers.

From: Big Secrets by William Poundstone

A Social Security card has nine digits.  So do two-part…