Check digits / Luhn
Luhn Algorithm Checker (Mod 10) with Working
Check any number protected by the Luhn mod-10 checksum, such as a payment card number or an IMEI, and see the working digit by digit. It only checks numbers you already have; it does not generate them.
Luhn Algorithm Checker (Mod 10) with Working: Starting from the right, every second digit is doubled, and 9 is taken off any result above 9. All the digits are then added up: 79927398713 adds up to 70, a multiple of 10, so it passes. The leading digits and the length suggest the card family, such as 4 for Visa or 34 and 37 for American Express. Runs 100% locally in your browser with zero server file uploads.
- Category
- Business & admin tools
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Runs entirely in your browser
From the rightmost digit, every second digit is doubled, and 9 is subtracted when doubling gives more than 9; the number passes when all the results add up to a multiple of 10. The check catches any single mistyped digit and most swaps of neighbouring digits, which is why card numbers, IMEIs, and many ID numbers use it. Passing says nothing about whether a card or phone exists: this tool only checks numbers, it does not make them.
Example
For 4111 1111 1111 1111, the 4 at the front and seven of the 1s are doubled, to 8 and 2, and the other eight 1s stay: 8 + 7 × 2 + 8 × 1 = 30, a multiple of 10, so it passes. It is a well-known test number that no bank issues.
History
Hans Peter Luhn, an IBM engineer, patented the method in 1960. It is a mod-10 check, simple enough to work out by hand; the IBAN check uses the stronger mod 97, and ISBN-10 mod 11.
How to use it
- Paste the number; spaces and dashes are ignored.
- Read whether it passes, and the card family if it looks like a card number.
- Follow the doubled digits to see where the sum comes from.
Privacy & limitations
The number is checked in your browser and never sent anywhere.
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Frequently asked questions
Does passing mean the card is real?
No: about one in ten random numbers passes. The check only catches typing mistakes; whether a card or account exists is known only to its bank.
What mistakes does it catch?
Every single wrong digit, and almost every swap of two neighbouring digits; the one swap it misses is 09 and 90.
Where is it used?
Payment card numbers, IMEI numbers on phones, Canadian social insurance numbers, and many loyalty and ID numbers.
Free tool · runs in your browser · no account required