Check Digit Validator & Generator
Validate or calculate check digits with Luhn (mod 10), Verhoeff, Damm, ISBN-10, ISBN-13, EAN-13 (JAN), UPC-A, Modulus 10 and Modulus 11 (weights 2-7). When a number is wrong, the tool shows the expected check digit and the corrected number. Hyphens and spaces are ignored.
What is a check digit?
A check digit is an extra digit, usually at the end of an identification number, calculated from the other digits. If someone mistypes a digit or swaps two neighboring digits, the check digit no longer matches and the error is caught. Credit card numbers, ISBNs and product barcodes all use one. This tool validates a complete number or generates the check digit for a number without one, using nine algorithms.
How to use
- Choose Validate or Generate with the switch at the top.
- Pick an Algorithm from the list.
- In Validate mode, enter the full number in Number with check digit. A green message confirms a valid number; otherwise the expected check digit and the corrected number are shown.
- In Generate mode, enter Number without check digit. The complete number appears under Generated Number:; click the copy icon to copy it.
Hyphens and spaces are ignored, and other characters are removed as you type. For ISBN-10 validation, a final X (meaning 10) is accepted.
Supported algorithms
| Algorithm | Digits | Typical use | Example |
|---|---|---|---|
| Luhn (Modulus 10) | any | Credit cards, IMEI | 4532015112830366 |
| Verhoeff | any | Aadhaar and other ID numbers | 1428570 |
| Damm | any | Custom IDs | 5724 |
| ISBN-10 | 10 | Books (before 2007) | 0-306-40615-2 |
| ISBN-13 | 13 | Books (since 2007) | 978-0-306-40615-7 |
| EAN-13 | 13 | Retail barcodes (JAN in Japan) | 4901777018686 |
| UPC-A | 12 | Retail barcodes (North America) | 012345678905 |
| Modulus 10 (Weight 1-2-1) | any | Custom numbering | 12344 |
| Modulus 11 (Weight 2-7) | any | Japanese My Number (個人番号) | 123456789018 |
For algorithms with a fixed length, the tool reports a wrong number of digits separately from a wrong check digit.
How the main algorithms work
- Luhn: from the right, double every second digit (subtract 9 if the result is over 9), add everything, and choose the check digit that makes the total a multiple of 10. Example: 7992739871 → check digit 3.
- ISBN-13 / EAN-13 / UPC-A: weight the digits alternately 1 and 3 (UPC-A starts with 3), sum them, and take (10 − sum mod 10) mod 10.
- ISBN-10: weights 10 down to 2; the check digit is (11 − sum mod 11) mod 11, written X when it is 10.
- Modulus 11 (2-7): weights 2, 3, 4, 5, 6, 7, 2, 3 ... from the right; the check digit is 11 − (sum mod 11), or 0 when the remainder is 0 or 1.
- Modulus 10 (1-2-1): weights 1, 2, 1, 2 ... from the left, with digit sums for two-digit products.
- Verhoeff and Damm use lookup tables (the dihedral group D5 and a quasigroup) and catch every single-digit error and every adjacent transposition.
Features
- Validate and generate modes for nine algorithms
- Expected check digit and corrected number shown for invalid input
- Digit-count checks for ISBN, EAN-13 and UPC-A
- Copy button for generated numbers and a quick reference table of examples
- Runs entirely in the browser; mode, algorithm and input are remembered locally
FAQ
Does a valid Luhn result mean a credit card is real?
No. It only means the number is well-formed. A check digit catches typing mistakes; it does not prove that a card, book or product exists, and it is not a security feature.
Which errors does each algorithm catch?
Luhn, EAN/UPC and similar mod 10 schemes catch all single-digit errors and most swaps of neighboring digits (Luhn misses 09 ↔ 90). ISBN-10, Verhoeff and Damm catch every single-digit error and every adjacent swap.
Is the number I enter sent anywhere?
No. Calculations happen locally and nothing is uploaded.