Luhn Algorithm Checker Free Online Tool 2026

Last updated March 22, 2026

Validating a Credit Card Number

Enter a credit card number to check if it passes the Luhn algorithm. Example with a Visa test number:

Input: 4532015112830366

Step-by-step Luhn validation:
─────────────────────────────────────────────────────────────────────
Original digits: 4 5 3 2 0 1 5 1 1 2 8 3 0 3 6 6

Step 1: Double every second digit from the right (positions 2,4,6...):
  Position: 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1
  Digit:      4  5  3  2  0  1  5  1  1  2  8  3  0  3  6  6
  Double:     4 10  3  4  0  2  5  2  1  4  8  6  0  6  6  6

Step 2: Sum digits of doubled values > 9 (10 → 1+0=1):
  4  1  3  4  0  2  5  2  1  4  8  6  0  6  6  6

Step 3: Sum all digits:
  4+1+3+4+0+2+5+2+1+4+8+6+0+6+6+6 = 58... wait
  Corrected sum: 4+1+3+4+0+2+5+2+1+4+8+6+0+6+6+6 = 58

Step 4: Check if sum mod 10 = 0:
  58 mod 10 = 8 ≠ 0

Result: ✗ INVALID (this is a demonstration — use real test numbers)

Validating a Known Valid Test Card

Standard test card numbers used in payment development always pass Luhn:

Input: 4111111111111111 (Visa test card)

Luhn validation:
Digits: 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Double alternating (from right):
  8  1  2  1  2  1  2  1  2  1  2  1  2  1  2  1

Sum digits > 9: (8 stays 8, all others are single digits)
  8  1  2  1  2  1  2  1  2  1  2  1  2  1  2  1

Sum: 8+1+2+1+2+1+2+1+2+1+2+1+2+1+2+1 = 30

30 mod 10 = 0 ✓

Result: ✓ VALID — passes Luhn check
Card type: Visa (starts with 4, 16 digits)

Common Test Card Numbers

Standard test card numbers that pass Luhn validation for development use:

Visa:
  4111111111111111  ✓ Valid
  4242424242424242  ✓ Valid
  4000056655665556  ✓ Valid

Mastercard:
  5555555555554444  ✓ Valid
  5105105105105100  ✓ Valid
  2223003122003222  ✓ Valid

American Express:
  378282246310005   ✓ Valid (15 digits)
  371449635398431   ✓ Valid (15 digits)

Discover:
  6011111111111117  ✓ Valid
  6011000990139424  ✓ Valid

Note: These numbers pass Luhn validation but are not
assigned to real accounts. They will be declined by
payment processors in live mode.

Detecting a Typo in a Card Number

The Luhn algorithm catches single-digit errors. A user types one digit wrong:

Correct number: 4111111111111111 ✓ Valid
Typo (last digit changed): 4111111111111112

Luhn check on 4111111111111112:
Sum: 31
31 mod 10 = 1 ≠ 0

Result: ✗ INVALID

The Luhn algorithm detected the single-digit typo.
Show the user an error before submitting to the payment processor.

Validating an IMEI Number

Mobile device IMEI numbers also use the Luhn algorithm:

Input: 490154203237518 (15-digit IMEI)

Luhn validation:
Digits: 4 9 0 1 5 4 2 0 3 2 3 7 5 1 8

Double alternating from right:
  4 18  0  2  5  8  2  0  3  4  3 14  5  2  8

Sum digits > 9 (18→9, 14→5):
  4  9  0  2  5  8  2  0  3  4  3  5  5  2  8

Sum: 4+9+0+2+5+8+2+0+3+4+3+5+5+2+8 = 60

60 mod 10 = 0 ✓

Result: ✓ VALID IMEI

Generating a Valid Test Number

Generate a valid Luhn number for a specific card prefix:

Settings:
  Prefix: 4532 (Visa)
  Total length: 16 digits

Generated number: 4532015112830366

Verification:
  The last digit (6) is the check digit, calculated to make
  the entire number pass the Luhn algorithm.

  ✓ Passes Luhn validation
  ✓ Correct length for Visa (16 digits)
  ✓ Correct prefix for Visa (starts with 4)

JavaScript Implementation

The checker provides implementation code you can use in your own applications:

// JavaScript Luhn validation
function luhnCheck(cardNumber) {
  const digits = cardNumber.replace(/\D/g, '').split('').map(Number);
  let sum = 0;
  let isEven = false;

  for (let i = digits.length - 1; i >= 0; i--) {
    let digit = digits[i];
    if (isEven) {
      digit *= 2;
      if (digit > 9) digit -= 9;
    }
    sum += digit;
    isEven = !isEven;
  }

  return sum % 10 === 0;
}

// Usage
luhnCheck('4111111111111111'); // true
luhnCheck('4111111111111112'); // false

Python Implementation

def luhn_check(card_number: str) -> bool:
    digits = [int(d) for d in card_number if d.isdigit()]
    total = 0
    for i, digit in enumerate(reversed(digits)):
        if i % 2 == 1:  # every second digit from right
            digit *= 2
            if digit > 9:
                digit -= 9
        total += digit
    return total % 10 == 0

# Usage
print(luhn_check('4111111111111111'))  # True
print(luhn_check('1234567890123456'))  # False

Card Type Detection by Prefix

The checker also identifies the card type from the number prefix:

Card Type Detection Rules:
─────────────────────────────────────────────────────────────────────
Visa:             Starts with 4, length 13 or 16
Mastercard:       Starts with 51-55 or 2221-2720, length 16
American Express: Starts with 34 or 37, length 15
Discover:         Starts with 6011, 622126-622925, 644-649, or 65, length 16
JCB:              Starts with 3528-3589, length 16
Diners Club:      Starts with 300-305, 36, or 38, length 14

Input: 5555555555554444
  Prefix 55 → Mastercard
  Length 16 → ✓ Correct for Mastercard
  Luhn check → ✓ Valid

Result: Valid Mastercard number