Use this calculator to verify numbers with the Luhn (Mod 10) algorithm or generate a missing check digit. You can paste values with spaces or dashes—the tool will clean them automatically.
Validate a full number
Generate a check digit
What is the Luhn formula?
The Luhn formula, often called the Luhn algorithm or Mod 10 algorithm, is a checksum method used to detect simple input errors in identification numbers. It appears in many places, including credit card numbers, IMEI numbers, and a variety of membership or account identifiers.
The key idea is simple: the final digit is not random. It is a check digit calculated from the digits before it. When someone types the full number, a system can quickly test whether the number is structurally valid.
Why this algorithm is useful
- Catches common mistakes: single-digit typos and many adjacent transpositions.
- Fast to compute: works in real-time on forms, payment pages, and APIs.
- Easy to implement: no heavy cryptography or special libraries required.
- Improves data quality: prevents bad entries before they hit your database.
Important note: passing a Luhn check does not prove a number is active, issued, or authorized. It only means the format and checksum are mathematically consistent.
How the Luhn check works
Validation steps
To validate a full number:
- Start from the rightmost digit and move left.
- Double every second digit (right-to-left pattern).
- If doubling gives a number over 9, subtract 9 from that result.
- Add all digits together.
- If the total is divisible by 10, the number is Luhn-valid.
Generating a check digit
For a base number missing the last digit, you can compute the final digit so that the full result passes Luhn validation. This is exactly what the generator above does.
Example walkthrough
Suppose the base number is 7992739871. The calculator computes the check digit as 3,
resulting in 79927398713. If you run that full number through the validation tool, it returns valid.
This kind of workflow is useful in testing environments when you need realistic but non-sensitive sample data for forms.
Best practices for developers and analysts
Front-end form validation
Run Luhn checks before submission to provide immediate user feedback. Pair it with input formatting and card-type checks (length/prefix) for better UX.
Back-end verification
Always validate again on the server. Client-side checks improve convenience, but server-side checks protect integrity.
Security reminder
Luhn is a checksum, not encryption. Never store sensitive payment data in plain text, and follow PCI-DSS or your applicable security standards.
Quick FAQ
Does Luhn detect all errors?
No. It detects many common data-entry errors, but not every possible manipulation.
Is every valid credit card number a real card?
No. A number can pass Luhn and still be unissued, expired, blocked, or otherwise invalid for transactions.
Can this calculator handle spaces and dashes?
Yes. It strips non-digit characters automatically before processing.
Final thoughts
The Luhn formula is one of those tiny tools with outsized practical value. Whether you are building checkout forms, cleaning datasets, or teaching checksum concepts, this calculator gives you a fast and accurate way to validate numbers and generate check digits in seconds.