acalculator

What is the check digit?

Pick the method, then type the number without its check digit to make one, or with it to check it. The check digit calculator shows each step’s result as one digit and the full number.

Your numbers

I want to
Answer
Check digit 3: 79927398713

7992739871: Check digit 3: 79927398713.

Check digit
3
Full number
79927398713

Answer: Check digit 3: 79927398713. 7992739871: Check digit 3: 79927398713.

How to calculate

Works out the check digit of a number with the Luhn, GS1 (UPC, EAN, ISBN-13), ISBN-10 or ISO 7064 MOD 97-10 method, or checks whether a full number has the right one.

Example with the default inputs (Method Luhn (mod 10): cards, IMEI, I want to Make the check digit, Number 7992739871): 7992739871: Check digit 3: 79927398713.

Method: Luhn: double every second digit from the right of the payload (subtract 9 from results over 9), sum, check = (10 − sum mod 10) mod 10. GS1: weights 3, 1, 3, … from the right of the payload, check = (10 − sum mod 10) mod 10. ISBN-10: weights 2 to 10 from the right, check = (11 − sum mod 11) mod 11, 10 written X. MOD 97-10: check = 98 − (payload × 100 mod 97), two digits.

  • Spaces and hyphens are ignored; every other character must be a digit (X only as the last ISBN-10 character).
  • In check mode the last digit (the last two for MOD 97-10) is the check digit.
  • A valid check digit catches typing slips; it does not show that a card, product or book exists.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Method Luhn (mod 10): cards, IMEI, I want to Make the check digit, Number 7992739871 gives Check digit 3, Full number 79927398713, Answer Check digit 3: 79927398713.Source: Wikipedia, "Luhn algorithm" (example 7992739871, check digit 3), https://en.wikipedia.org/wiki/Luhn_algorithm (retrieved 2026-10-05)
  2. Method GS1: UPC, EAN, ISBN-13, GTIN, SSCC, I want to Make the check digit, Number 03600024145 gives Check digit 7, Full number 036000241457.Source: Wikipedia, "Check digit", UPC example 036000241457, https://en.wikipedia.org/wiki/Check_digit (retrieved 2026-10-05)
  3. Method GS1: UPC, EAN, ISBN-13, GTIN, SSCC, I want to Check a number, Number 978-0-7475-3269-9 gives Valid Yes, Check digit 9, Answer Valid.Source: Wikipedia, "Check digit", ISBN-13 example 978-0-7475-3269-9, https://en.wikipedia.org/wiki/Check_digit (retrieved 2026-10-05)
  4. Method ISBN-10 (mod 11), I want to Check a number, Number 0-201-53082-1 gives Valid Yes, Check digit 1.Source: Wikipedia, "Check digit", ISBN-10 example 0-201-53082-1 (weighted sum 99 = 9 × 11), https://en.wikipedia.org/wiki/Check_digit (retrieved 2026-10-05)
  5. Method ISBN-10 (mod 11), I want to Make the check digit, Number 080442957 gives Check digit X, Full number 080442957X.
  6. Method ISO 7064 MOD 97-10, I want to Check a number, Number 3214282912345698765432161182 gives Valid Yes, Check digit 82, Answer Valid.Source: Wikipedia, "International Bank Account Number" (GB82 WEST 1234 5698 7654 32; W = 32, E = 14, S = 28, T = 29, G = 16, B = 11), https://en.wikipedia.org/wiki/International_Bank_Account_Number (retrieved 2026-10-05)

How it works

The payload is the number without its check digit. Count its digits from the right: position 1 is the digit next to the check digit.

  • Luhn (mod 10). Double the digits in odd positions (1, 3, 5, …); a doubled value over 9 counts 9 less. Add these and the digits in even positions. Check digit = (10 − sum mod 10) mod 10.
  • GS1 (UPC, EAN, ISBN-13, GTIN, SSCC). Multiply the digits in odd positions by 3 and those in even positions by 1, and add. Check digit = (10 − sum mod 10) mod 10.
  • ISBN-10 (mod 11). The payload is 9 digits. Multiply the digit in position k by k + 1 (so the first digit of the ISBN has weight 10 and the ninth weight 2), and add. Check digit = (11 − sum mod 11) mod 11, written X when it is 10.
  • ISO 7064 MOD 97-10. Write 00 after the payload and read it as one whole number N. Check digits = 98 − (N mod 97), written as two digits (02 to 98).

Rules

  • Spaces and hyphens are ignored. Every other character must be a digit; in check mode an ISBN-10 may end in X.
  • Payload lengths: Luhn and MOD 97-10, 1 to 100 digits; GS1, 1 to 17 digits (GTIN-8 has 7, UPC-A 11, EAN-13 and ISBN-13 12, GTIN-14 13, SSCC 17); ISBN-10, exactly 9.
  • In check mode, the last digit (the last two for MOD 97-10) is taken as the check digit, and the rest as the payload.

What the calculator shows

  • Make: the check digit and the full number (payload then check digit).
  • Check: "Valid" when the given check digit equals the one worked out, or "Not valid" with the right check digit.

Worked examples by hand

Luhn, 7992739871. From the right: 1, 7, 8, 9, 3, 7, 2, 9, 9, 7. Doubled odd positions: 1 → 2, 8 → 16 → 7, 3 → 6, 2 → 4, 9 → 18 → 9. Sum = 2 + 7 + 7 + 9 + 6 + 7 + 4 + 9 + 9 + 7 = 67. Check digit = (10 − 7) mod 10 = 3, full number 79927398713.

GS1, UPC 03600024145. From the right with weights 3, 1, …: 5×3 + 4 + 1×3 + 4 + 2×3 + 0 + 0×3 + 0 + 6×3 + 3 + 0×3 = 15 + 4 + 3 + 4 + 6 + 18 + 3 = 53. Check digit = (10 − 3) = 7, full number 036000241457.

GS1, ISBN-13 978-0-7475-3269-9. Payload 978074753269. Weights from the left are 1, 3, 1, 3, …: 9 + 21 + 8 + 0 + 7 + 12 + 7 + 15 + 3 + 6 + 6 + 27 = 121. Check digit = 10 − 1 = 9, which matches: valid.

ISBN-10 0-201-53082-1. Payload 020153082. 0×10 + 2×9 + 0×8 + 1×7 + 5×6 + 3×5 + 0×4 + 8×3 + 2×2 = 98; 98 mod 11 = 10; check digit = 11 − 10 = 1, which matches: valid (with the check digit, weight 1, the sum is 99 = 9 × 11).

ISBN-10 080442957. 0×10 + 8×9 + 0×8 + 4×7 + 4×6 + 2×5 + 9×4 + 5×3 + 7×2 = 199 = 18 × 11 + 1. Check digit = 11 − 1 = 10, written X: 080442957X.

MOD 97-10, 32142829123456987654321611. This is the IBAN GB82 WEST 1234 5698 7654 32 with the country code and check digits moved to the end and letters made numbers (W = 32, E = 14, S = 28, T = 29, G = 16, B = 11). N = 3214282912345698765432161100; N mod 97 = 16; check digits = 98 − 16 = 82, which matches: valid.

Other questions people ask

What is a check digit?

A check digit is an extra digit worked out from the other digits of a number and put at its end. When someone types the number wrong, the check digit usually no longer fits, so the slip is caught before it does harm.

How do I calculate a Luhn check digit?

From the right of the number (without the check digit), double every second digit, starting with the last one. If a doubled digit is over 9, subtract 9. Add all the digits. The check digit is what you add to reach the next multiple of 10. For 7992739871 the sum is 67, so the check digit is 3.

How is a UPC or EAN check digit worked out?

GS1 numbers (UPC, EAN, ISBN-13, GTIN and SSCC) weight the digits 3, 1, 3, 1 and so on from the right of the number without the check digit. Add the products; the check digit takes the sum up to the next multiple of 10. For the UPC 03600024145 the sum is 53, so the check digit is 7.

Why does an ISBN-10 sometimes end in X?

The ISBN-10 check digit works modulo 11, so it can be 10. One digit cannot show 10, so the Roman numeral X is used. ISBN-13 uses the GS1 method instead, whose check digit is always 0 to 9.

Does a valid check digit mean a card number is real?

No. A check digit only shows the number is well formed. A card, product, or book can have a valid check digit and still not exist, and a check digit cannot tell you who owns an account.

What is MOD 97-10?

It is a method from ISO 7064 that gives two check digits: append 00 to the number, take the remainder by 97, and subtract it from 98. IBANs use it after moving the country code and check digits to the end and turning letters into numbers.