Luhn Check Digit Calculator

The calculator operates in two modes:

Validate mode: Enter a complete number (e.g., a full credit card number) and the tool checks whether it passes the Luhn algorithm. If valid, you'll see a green checkmark; if invalid, the tool shows which digit is incorrect.

Generate mode: Enter a number missing its final check digit, and the tool calculates what that digit should be. This is useful for understanding how check digits are assigned.

In both modes, the tool shows a step-by-step breakdown:

1. Each digit is highlighted in the grid
2. Every second digit from the right is doubled
3. If the doubled value exceeds 9, its digits are summed (e.g., 16 → 1+6 = 7)
4. All resulting values are summed
5. If the total modulo 10 equals 0, the number is valid

The Luhn algorithm protects many types of identification numbers:

• Credit & debit cards: Visa, Mastercard, American Express, Discover, and virtually all payment cards worldwide
• IMEI numbers: The 15-digit identifier on every mobile phone
• Canadian Social Insurance Numbers (SIN)
• National Provider Identifiers (NPI): US healthcare provider IDs
• Israel ID numbers
• South African ID numbers

The algorithm catches all single-digit errors and most transpositions of adjacent digits — making it effective against the most common data entry mistakes.

Understanding the Luhn algorithm's limitations is important:

• It's not a security check: Luhn only catches typos. A number that passes Luhn is not necessarily a real, active credit card.
• It won't detect all errors: Transposing '09' ↔ '90' is not caught by Luhn, as both produce the same checksum.
• It runs entirely locally: This tool processes everything in your browser. No numbers are sent to any server — your data stays completely private.