6 Min Read | January 12, 2023
What Is a Credit Card Number?
Credit card numbers may seem random, but many digits serve a specific purpose, like identifying card issuers or ensuring legitimacy through a special algorithm.
At-A-Glance
Credit card numbers act as identifiers that point to an industry, a lending institution, and more.
Each card number ends with a “check digit” that, for example, allows an online merchant to quickly tell if a card number was mistyped.
Security measures, such as restrictions on how merchants can print your card information, can help keep credit card transactions safe.
“They’re so old, their credit card number is 1!” A joke like that might make you think credit card numbers are so lengthy because that many accounts have been created since the first credit cards were issued in the 1950s. But in reality, the long string of numbers is deliberate and purposeful, with each number or group of numbers playing a specific role.
Since a typical credit card number contains 15 or 16 digits, the number of possible permutations can reach a quadrillion, or 1,000,000,000,000,000. With so many possibilities, it’s easy to see why a specific digit structure was implemented to make sense of the numbers we see on our credit cards every time we pull one out to swipe, insert, or tap. Let’s take a look at the role each section of a credit card number plays.
The First Credit Card Digit: The Major Industry Identifier
If you look at the credit card in your wallet, it will likely start with a 3, 4, 5, or 6. This first number is called the major industry identifier (MII) number, and it shows the primary industry associated with the card or card issuer. The 3 through 6 range represents MIIs most commonly used by the majority of personal credit card issuers – namely, the MIIs for the travel and entertainment, merchandising, or banking and financial services industries.
Some other MII examples include 1, designating an airline issuer, and 7, which is primarily reserved for cards issued in the petroleum industry.
The Second Section: The Issuer Identification Number
The next five to seven digits help make up the issuer identification number (IIN) or the bank identification number (BIN). This section denotes the financial institution issuing the credit card. Card issuers may vary which numbers specifically relate to the type of card, and not every issuer has its own specific code.
It’s worth noting that, technically, the IIN also includes the MII, so this section actually comprises the first six to eight digits of a card number. But why six to eight? Prior to a 2017 change issued by the International Organization for Standardization (ISO) – the standards body that sets card-numbering conventions – the IIN was defined as the first six card digits.1 Due to an expected shortage of future possible issuer identification numbers, this section was increased to a fixed eight digits.
All the Rest but One: The Individual Account Number
This section immediately follows the IIN. The individual account number is assigned by the card issuer and is unique to each credit card account. This section can vary in length, depending on the card issuer and industry, but is commonly around six or seven digits long for typical personal credit cards and ends just before the last digit. Some card issuers will issue different individual account numbers for an authorized user’s card, even though the secondary card is linked to the same account as the primary account holder.
While the MII and IIN let merchants know what institution the money is coming from, this section tells them which accounts will be charged.
The Last Digit: The Check Digit
The individual account number is immediately followed by the “check digit.” If you’ve ever wondered why a minor typo doesn’t accidentally charge someone else’s card when making an online purchase, it’s because of this check digit. This last number serves as a quick check to determine whether a credit card number is legitimate. If incorrect, the card will be declined, instead of being misdirected to someone else’s account.
Most credit cards use a public domain formula, called the Luhn algorithm, to verify accounts. It’s a fairly simple calculation that, put simply, involves adding up each digit of the card number in a particular manner. If the sum ends in a zero, the card passes the algorithm and is a potential credit card number.
It’s important to note that the Luhn algorithm isn’t designed to protect anyone from fraud or theft. Instead, it gives most merchants, especially automated ones like online retailers, a quick way to protect against accidental errors, such as accidentally typing a 5 instead of a 6 or entering an extra number.
The Other Numbers on a Credit Card
When a credit card is used over the phone or online, the buyer is often required to put in the “CVV” or “CV2” code alongside the card number. This card verification value acts as an added security measure and usually appears as a three- or four-digit number on the front or back of the card, depending on the specific card and card issuer. For more on card verification values and how they help keep your card information secure, read “What Is a CVV Number?”
Other common numbers found on credit cards include the card’s expiration date, as well as the card issuer’s customer service phone number.
How Credit Card Numbers Are Protected
Given that 84% of American adults have a credit card,2 added security measures can go a long way toward keeping these individuals’ card information secure. Many modern credit cards have advanced encryption capabilities that prevent sellers from obtaining full credit card numbers or account numbers when purchases are made. For example, in-person transactions using an EMV chip are more secure than the old “swipe” method because they create a unique code for the transaction. For online purchases, many digital wallets and mobile payment apps can create temporary “virtual credit card numbers” that create a bridge between the seller’s and buyer’s full account information.
There are also laws and regulations to ensure that individuals’ sensitive information is protected, such as the Fair and Accurate Credit Transaction Act (FACTA), which regulates what credit card digits are allowed to appear on an electronically printed receipt, among other things.3 It’s why credit card numbers appear as mostly asterisks on a receipt, as the law states that no more than the last five digits can be printed – and never any part of an expiration date.
The Takeaway
Understanding your credit card isn’t just about reading the terms and conditions. It’s also about understanding the big numbers right on the front – or back, depending on the card. Credit card numbers contain unique identifiers that can inform a merchant about what industry the card is from, what financial institution issued the card, and even the validity of the card itself.
1 “Changes to the Issuer Identification Number (IIN) Standard,” ISO
2 “Economic Well-Being of U.S. Households in 2021-May-2022,” Federal Reserve
3 “FACTA Subchapter III – Credit Reporting Agencies,” House.gov
Ryan Lynch is a freelance writer, educator, and musician whose work concentrates on finance, STEM, and the arts.
All Credit Intel content is written by freelance authors and commissioned and paid for by American Express.
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