Luhn Algorithm – How Credit & Debit Card Validation Really Works

Learn how banks and payment systems check whether a credit or debit card number is valid using the Luhn Algorithm (Mod-10 check), explained in the simplest way possible.

What Is the Luhn Algorithm?

The Luhn Algorithm, also called the Mod-10 algorithm, is a simple mathematical formula used to validate credit card and debit card numbers. It helps detect:

• Typing mistakes
• Fake card numbers
• Randomly generated invalid numbers

It is used by:

• Visa
• MasterCard
• American Express
• Discover
• RuPay and most global card networks

Important:
The Luhn check does not tell you if a card has money, is active, or is stolen. It only verifies whether the number format is mathematically correct.

Why Is the Luhn Check Important?

The Luhn algorithm helps to:

• Prevent payment errors
• Reduce fraud attempts
• Avoid failed transactions
• Improve user experience in payment forms
• Validate card numbers before sending them to a payment gateway

Every modern payment gateway, banking system, and e-commerce platform uses this check as the first validation layer.

Luhn Algorithm Explained Like a Child (Super Easy)

Let’s pretend the card number is:

4 5 3 9

Step 1: Start from the Right

Always count from the right side.

4   5   3   9
↑   ↑   ↑   ↑

Step 2: Double Every Second Number

Double every second digit from the right:

4 → 8  
5 → 5  
3 → 6  
9 → 9

Now it becomes:

8 5 6 9

Step 3: Fix Big Numbers

If any number becomes bigger than 9, add its digits.

Example:

• 12 → 1 + 2 = 3

(Here, all values are already small.)

Step 4: Add Everything

8 + 5 + 6 + 9 = 28

Step 5: The Magic Rule

If the total is divisible by 10, the number is ✅ VALID
If not, it is ❌ INVALID

28 ÷ 10 ≠ whole number → INVALID

Example of a Valid Luhn Number

Card number:

4 5 3 9 1

After applying all steps:

8 + 5 + 6 + 9 + 2 = 30
30 ÷ 10 = valid ✅

So this number passes the Luhn check.

Luhn Algorithm in Developer Terms

1. Remove spaces and special characters from the card number
2. Starting from the right:
   • Double every second digit
3. If result > 9 → subtract 9
4. Add all digits
5. If total % 10 == 0 → Valid card number

Sample Luhn Algorithm Code (JavaScript)

function luhnCheck(cardNumber) {
  let sum = 0;
  let shouldDouble = false;

  for (let i = cardNumber.length - 1; i >= 0; i--) {
    let digit = parseInt(cardNumber[i]);

    if (shouldDouble) {
      digit *= 2;
      if (digit > 9) digit -= 9;
    }

    sum += digit;
    shouldDouble = !shouldDouble;
  }

  return sum % 10 === 0;
}

Does Passing the Luhn Check Mean the Card Is Real?

❌ No.
It only means:

• The number is mathematically correct
• It could still be:
  • Expired
  • Blocked
  • Stolen
  • Empty of funds

Only banks and payment gateways can confirm real validity through authorization.

Security & PCI Compliance Note

• Never store full card numbers
• Never store CVV
• Always use tokenization via payment gateways (Stripe, Razorpay, PayPal, etc.)
• Mask card numbers (e.g., **** **** **** 1234)