Luhn Calculator Tool
Use this tool to validate a full number (like a card, IMEI, or ID value) or generate the correct final check digit for a partial number.
Spaces and hyphens are allowed; only digits are processed.
Enter all digits except the final check digit.
What is a Luhn calculator?
A Luhn calculator applies the Luhn algorithm, a checksum formula used to quickly detect typing mistakes in identification numbers. You will commonly see it used with payment card numbers, SIM-related identifiers, and IMEI values. The algorithm is simple, fast, and extremely practical.
Important note: passing a Luhn check does not prove a number is real, active, or authorized. It only means the digit pattern is mathematically consistent with the checksum rule.
How to use this tool
1) Validate an existing number
Paste a full number into the first field and click Validate Number. The tool will return whether the number satisfies the Luhn checksum. If invalid, it also shows what the expected final check digit should be based on the preceding digits.
2) Create a valid check digit
Enter a partial number (everything except the final digit) into the second field and click Calculate Check Digit. The tool computes the correct check digit and returns the completed valid number.
How the Luhn algorithm works
- Start from the right and move left.
- Double every second digit.
- If doubling creates a number greater than 9, subtract 9 from it.
- Add all digits together.
- If the total is divisible by 10, the number is valid.
This catches most accidental errors, especially single-digit mistakes and many adjacent transpositions.
Manual example
Validate 79927398713
Applying the digit-doubling pattern from right to left results in a total that is divisible by 10, so the number is valid. This is a classic example often used when teaching checksum logic.
Where Luhn checks are useful
- Front-end form validation: catch obvious input mistakes early.
- Data cleaning pipelines: flag malformed identifiers before storage.
- Test data generation: create structurally valid sample IDs for QA.
- Mobile device checks: validate IMEI formatting patterns.
Common mistakes to avoid
- Assuming a valid checksum means the number is genuine or billable.
- Running validation on incomplete input and expecting a pass.
- Forgetting to remove non-digits before processing (this tool handles that for you).
- Using Luhn as a security mechanism instead of basic error detection.
FAQ
Is this tool storing my numbers?
No. This page runs entirely in your browser with client-side JavaScript.
Can I use spaces or hyphens?
Yes. The calculator strips non-digit characters and uses only numeric digits for computation.
Does every ID system use Luhn?
No. Many do, but some use different checksum schemes or no checksum at all.