A credit card has 16 digits, split into four groups of four

The 16 numbers printed on the front of your card are not all the same thing. They break into four distinct parts, each serving a different purpose. The first six digits identify your bank and card type. The next nine digits are your unique account number. The last digit is a check digit that validates the whole number. On the back, you'll find a three- or four-digit security code that serves a separate function.

Understanding what each number does helps you know what information is safe to share and what to guard closely. It also helps you spot when something is wrong with a transaction or when you're being asked for more information than necessary.

Key Takeaways

  • The 16 digits on your card break into four groups: the bank identifier (first 6), your account number (next 9), a check digit (last 1), and a separate security code on the back (3 or 4 digits).
  • The first digit tells you the card type: 4 for Visa, 5 for Mastercard, 3 for American Express, and 6 for Discover.
  • The security code on the back is never printed on receipts and should never be stored in a database, making it the most sensitive number after the full card number itself.
  • You can safely share your card number and expiration date for online purchases, but the security code should only be entered directly into a payment form, never typed into an email or given over the phone unless you initiated the call.

The first six digits: Bank and card type

The opening digit of your card number identifies the card network. Visa cards start with 4, Mastercard with 5, American Express with 3, and Discover with 6. This first digit is called the Major Industry Identifier, or MII.

The next five digits (digits 2 through 6) identify your specific bank or card issuer. Chase, Bank of America, Citi, and every other issuer has its own code in this range. Together, these six digits are called the Issuer Identification Number, or IIN. When you swipe or insert your card at a store, the payment terminal reads these six digits first to route the transaction to the right network and bank.

Digits 7 through 15: Your account number

These nine digits are your unique account identifier within your bank's system. No two cardholders at the same bank will have the same nine-digit sequence here, even if they hold the same card product. This is the core number that links the physical card to your actual account.

Your bank uses this number to pull up your balance, transaction history, and credit limit when you make a purchase. If you lose your card and get a replacement, your bank will issue a new card with a different account number in this section, even though it's the same account. The old card number becomes invalid immediately.

The 16th digit: The check digit

The last digit on your card is calculated from the first 15 using a mathematical formula called the Luhn algorithm. This digit exists so that payment systems can instantly verify that a card number is mathematically valid before sending it for processing.

If you type your card number wrong during an online purchase, the payment form often catches the error before it even reaches your bank, because the check digit won't match. This saves time and reduces failed transactions. You don't need to understand the math—just know that this digit is a built-in error detector.

The security code on the back: CVV or CVC

The three- or four-digit code on the back of your card is called the CVV (Card Verification Value) on Visa and Discover cards, or the CVC (Card Verification Code) on Mastercard. American Express calls it the CID and places it on the front. This code is never printed on receipts and is not stored in most merchant databases, which makes it more sensitive than your card number itself.

The security code proves you physically have the card in your hand. Online retailers ask for it to reduce fraud, because a thief who has only your card number and expiration date cannot complete a purchase without also knowing this code. Never enter your security code into an email, text message, or phone call, and never give it to someone who contacted you—legitimate companies will never ask for it that way.

Why some cards have fewer visible numbers

American Express cards have 15 digits instead of 16. The first digit is 3, followed by a four-digit type code, then a six-digit issuer code, then a nine-digit account number. There is no separate check digit printed on the card itself, though the number still follows the Luhn algorithm internally.

Some newer cards, particularly those issued by online banks or fintech companies, print fewer numbers on the physical card for security reasons. The full 16-digit number still exists in the bank's system, but the card itself may show only the last four digits. You can find your full number in your online banking portal or by calling the bank.

What you should and should not share

Your card number and expiration date are safe to enter into a legitimate online payment form. Merchants need these to process your purchase. However, never type them into an email, text message, or form that you did not navigate to yourself—always go directly to the merchant's website or app.

Your security code should only be entered into a payment form on a website or app, never shared verbally or in writing. If a company calls you asking for your security code, hang up and call the number on the back of your card to verify the request. Your PIN (the code you use at an ATM or in-store) is even more sensitive and should never be shared with anyone, including bank employees.

Frequently Asked Questions

Why do some cards show all 16 numbers and others show only the last four?

Banks choose how many digits to print based on their security preferences. Showing fewer numbers reduces the risk if your physical card is lost or photographed. The full number always exists in the bank's system and appears in your online banking portal or statements.

Is it safe to give my card number and expiration date to an online store?

Yes, if you navigated to the store's website yourself and the payment page is secure (look for a padlock icon and "https" in the address bar). Never enter this information into a link sent to you via email or text, even if it looks like it came from a company you trust.

What should I do if I notice my card number has been used fraudulently?

Call the number on the back of your card immediately. Your bank can freeze the card, dispute the charges, and issue you a replacement with a new number. Federal law limits your liability for fraudulent charges to $50, and most banks waive this entirely if you report it promptly.

Can someone make a purchase with just my card number and expiration date?

Online, no—they would also need your security code. In person or over the phone, it depends on the merchant's security practices, but most require additional verification. The security code exists specifically to prevent this scenario.

Do I need to memorize my card number?

No. Keep your card in a safe place and use your online banking portal or app to view your number when you need it. Memorizing it increases the risk that you'll accidentally share it or type it into an unsafe form.