The Check Digit Calculator computes or verifies a check digit using common algorithms used in payment cards, ISBNs, EAN barcodes, and UPC product codes. Spaces and dashes are stripped automatically so codes can be pasted directly. Choose an algorithm, pick “compute” or “verify”, and the tool will return the expected digit or whether the supplied number is valid.
How to Use Check Digit Calculator
- Choose an algorithm — Luhn, Mod 11, ISBN-10, ISBN-13, EAN-13, or UPC-A.
- Pick the mode: compute a new check digit, or verify a complete number.
- Paste or type the code; separators are removed automatically.
- Read the resulting check digit or validation status.
Formula & Theory - Check Digit Calculator
Luhn: double every 2nd digit from the right; sum digits; check digit = (10 - sum mod 10) mod 10
Mod 11: weights 2..7 cyclic; sum × weight; check = (11 - sum mod 11) mod 11 (10 → "X")
ISBN-13: weight 1 and 3 alternating; check = (10 - sum mod 10) mod 10
EAN-13: weights 1 and 3 alternating from the right; check = (10 - sum mod 10) mod 10
UPC-A: weights 3 and 1 alternating; check = (10 - sum mod 10) mod 10Use Cases for Check Digit Calculator
- Verify a credit-card number was entered correctly.
- Generate ISBN-10 or ISBN-13 check digits when publishing a book.
- Validate scanned EAN-13 or UPC-A barcodes.
- Demonstrate check-digit algorithms in a classroom setting.