A standard credit card number is 16 digits long
Most credit cards issued in the United States have 16 digits. American Express is the main exception — AmEx cards carry 15 digits. Discover cards also use 16 digits, as do Visa and Mastercard. The length is fixed for each card network, so you will always see the same digit count from the same issuer.
The digits are not random. Each one serves a purpose, and the pattern tells you information about the card before you even swipe it or enter it online. Understanding this structure helps you spot errors when you are entering your number, and it explains why a card number alone is not enough to make a purchase.
Key Takeaways
- Visa, Mastercard, and Discover cards have 16 digits; American Express has 15 digits.
- The first digit identifies the card network (4 for Visa, 5 for Mastercard, 3 for American Express, 6 for Discover).
- Digits 2 through 6 identify the bank or issuer that issued the card.
- The last digit is a check digit that validates the entire number using a mathematical formula.
- A card number alone cannot be used to make a purchase — you also need the expiration date and CVV security code.
What the first digit tells you
The first digit of your card number is the Major Industry Identifier (MII). It narrows down which type of financial institution issued the card. For credit cards, the first digit is almost always 3, 4, 5, or 6.
A card starting with 4 is a Visa card. A card starting with 5 is a Mastercard. American Express cards start with 3 (specifically 34 or 37). Discover cards start with 6. This is why you can often tell which network a card belongs to just by looking at the first digit — the card networks have owned these numbers for decades.
Digits 2 through 6: The issuer identification number
The next five digits, combined with the first digit, make up the Issuer Identification Number (IIN), also called the Bank Identification Number (BIN). This six-digit code tells you which bank or credit union issued the card. Chase, Bank of America, Citi, and every other card issuer has its own set of IINs assigned to it.
When you swipe your card at a store or enter it online, the payment processor uses the IIN to route the transaction to the correct bank. The IIN is not secret — it is printed on your card and visible to merchants. Knowing the IIN does not let someone use your card, because the remaining digits and security codes are still needed.
Digits 7 through 15: Your account number
The middle section of your card number is your account number. This is the unique identifier for your specific account at that bank. Two people with cards from the same bank will have the same IIN but different account numbers in this section.
The account number is what ties the card to your credit line, your payment history, and your account balance. It is the part of the number that is truly sensitive, because it identifies you specifically rather than just your bank.
The last digit: The check digit
The final digit is called the check digit or checksum. It is calculated using a mathematical formula called the Luhn algorithm, which runs through all the previous digits and produces a single digit that validates the entire number. This digit catches typos.
When you enter your card number online or at a terminal, the system runs the Luhn algorithm on the digits you entered. If the check digit does not match what the formula produces, the system knows you made a mistake — either you mistyped a digit, or the card number is not real. This is why you cannot just guess a credit card number and have it work; the check digit will fail the validation.
Why card number length varies
American Express uses 15 digits instead of 16 because it was designed differently from Visa and Mastercard. AmEx has always issued its own cards and processed its own transactions, rather than licensing its network to banks the way Visa and Mastercard do. The 15-digit format has worked for AmEx for decades, and there is no reason for them to change it.
Discover also uses 16 digits, matching Visa and Mastercard. Some smaller card networks or international cards may use different lengths, but in the United States, 15 or 16 digits is standard. If you see a card number that is significantly shorter or longer, it is likely not a credit card.
What you need beyond the card number to make a purchase
The card number alone is not enough to charge a purchase to your account. You also need the expiration date (month and year) and the CVV or CVC security code — the three or four digit number on the back of the card (or front, for American Express).
Online purchases require all three pieces of information. In-person purchases at a physical store may only require the card number and expiration date if you swipe or insert the card, because the terminal reads the full card data from the chip or magnetic stripe. But for phone orders or online transactions, the CVV is mandatory on most merchant sites. This three-part requirement is why stolen card numbers alone are less useful to fraudsters than a complete set of card data.
Frequently Asked Questions
Why do some cards have 16 digits and others have 15?
American Express designed its system with 15 digits, while Visa, Mastercard, and Discover use 16. The difference comes from how each network was built and how they assign account numbers. Both lengths are standard and valid within their respective networks.
Can someone use my card number if they only have the number itself?
Not for most online purchases. They would also need your expiration date and CVV security code. In-person fraud is harder because the card itself must be present or the card data must be read from the chip. A card number alone is incomplete.
What does the check digit actually do?
The check digit validates that the card number is mathematically correct using the Luhn algorithm. If you mistype even one digit, the check digit will not match the formula, and the transaction will be rejected. It catches typos but does not prevent fraud if all the information is correct.
Are the first six digits of my card number secret?
No. The first six digits (the IIN) are printed on your card and visible to any merchant you give it to. They identify your bank and card type, not your specific account. The account number and CVV are the sensitive parts.
Do all Visa cards have exactly 16 digits?
Yes, all Visa cards have 16 digits. All Mastercard and Discover cards also have 16 digits. American Express is the only major network using 15. This consistency makes it straightforward to spot which network a card belongs to just by counting the digits.