Credit Card Number Structure: IIN, BIN, Check Digit and Examples

The Parts of a Card Number

4 5 3 9 5 7 8 1 | 2 3 4 5 6 7 8 | 9
|-------------|   |-----------|   check digit (Luhn)
  IIN / BIN       account identifier
^ Major Industry Identifier (first digit)

(The number above is a made-up example, not a real card.)

Major Industry Identifier

The first digit indicates the industry category under ISO/IEC 7812:

Issuer Identification Number (IIN / BIN)

The leading digits identify the network and the issuing bank or organisation. Historically the IIN was six digits; ISO/IEC 7812-1 was revised to an eight-digit IIN, and card networks moved to eight-digit BINs around 2022. Systems that route or analyse transactions by BIN had to be updated to read eight digits.

Account Identifier

The digits between the IIN and the check digit identify the individual account. They're assigned by the issuer and have no public meaning.

Check Digit

The last digit makes the whole number pass the Luhn check, which catches most typing mistakes. The Luhn algorithm explained shows how it's calculated.

Network Prefixes and Lengths

Ranges change over time as networks add new ranges, so production code should rely on a maintained library or the payment processor rather than a hard-coded table. Braintree's open-source credit-card-type library is one widely used reference.

Credit Card Number Examples

The sample numbers below are public test numbers that Stripe documents for its sandbox. They follow the real format of each network and pass the Luhn check, but they belong to no account and are declined outside test mode.

Taking the sample Visa number apart:

• 4 is the Major Industry Identifier (banking and financial) and the Visa prefix.
• 4242 4242 is the eight-digit IIN/BIN.
• 4242 424 is the account identifier.
• 2, the last digit, is the Luhn check digit that makes the whole number valid as a checksum.

Use these only in a provider's test mode. Each provider publishes its own list, so check their documentation for sandbox behaviour such as declines or 3-D Secure prompts.

Why This Matters for Developers

Understanding the structure helps with:

• Card type detection: showing the right logo and security-code length as the user types.
• Formatting: spacing numbers as 4-4-4-4 for most cards, 4-6-5 for American Express.
• Front-end validation: checking length and Luhn before submitting, so users catch typos early.
• Not over-validating: rejecting a 19-digit card or a new BIN range because your rules are outdated is a common bug.

How to test payment forms turns these into a test checklist.

What the Number Doesn't Tell You

The structure says nothing about:

• Whether the account exists or is open
• The cardholder's name or address
• Credit limit or balance
• Whether it's credit, debit or prepaid (this needs a BIN database lookup and isn't always reliable)

Only the issuer, through the payment network's authorisation process, can confirm a card is usable. Why generated card numbers don't work explains this.

Privacy and Security Notes

• PCI DSS limits how much of a card number may be displayed; showing the first six and last four digits is a common maximum, and many systems show only the last four.
• Never log full card numbers in application logs or analytics.
• BIN information is semi-public, but combined with other data it can be sensitive; treat it carefully.

Seeing the Structure With Test Data

The test card generator creates synthetic numbers that follow these rules (the right prefix, length and Luhn check digit) for Visa, Mastercard, American Express, Discover, JCB, Diners Club and UnionPay. It runs in your browser and is meant for testing form formatting and validation, not for payments. For end-to-end tests with a payment provider, use their official sandbox numbers, described in test credit card numbers for developers.

Summary

A card number is an IIN (network and issuer), an account identifier and a Luhn check digit. Use the structure for formatting and early validation, rely on the processor for everything else, and never log full numbers.