Checksum Digit Calculator

This checksum digit calculator works out a simple modulo 10 check digit for any number, the kind of extra digit added to account numbers, reference codes, barcodes and other identifiers so mistakes can be caught automatically. Enter a number into the single field and the calculator adds together its digits to get the digit sum, then works out the check digit as whatever amount needs to be added to that sum to make it a round multiple of ten. The results show the check digit itself, the digit sum it was calculated from, and the full number with the check digit appended on the end, ready to use or compare. This is handy whenever you are building or testing a simple validation scheme: if a digit later gets mistyped or transposed, recalculating the checksum on the altered number will usually produce a different check digit, flagging the error before it causes problems further down the line. Note that this tool uses the plain digit sum method rather than a weighted scheme; more sophisticated systems such as the Luhn algorithm, used on credit cards and some ID numbers, double alternating digits before summing, so use the dedicated Luhn calculator if that is what you need. This one suits simpler internal reference numbers, student exercises, or anywhere a lightweight, easy to explain check digit scheme is enough.

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Number

Digit sum

Number with check digit

The formula

Add the digits of the number to get the digit sum. The check digit is (10 minus (digit sum mod 10)) mod 10, chosen so that the digit sum plus the check digit is a multiple of ten.

Worked example

For 123 the digit sum is 1 + 2 + 3 = 6, so the check digit is (10 minus 6) mod 10 = 4, giving 1234. Enter 123 to confirm.

Frequently asked questions

What is a check digit?

An extra digit added to a number so that a simple calculation over all the digits comes out to a fixed value, catching typos.

How does this scheme work?

It sums the digits and adds whatever makes the total a multiple of ten.

Is this the same as Luhn?

No. Luhn weights alternate digits by doubling them. This is the simpler plain digit sum check; see the Luhn calculator for cards.

Who this calculator is for

This calculator is for students, programmers and anyone needing a quick, reliable result.

What this calculator assumes

You enter valid input.
The standard algorithm is applied.
Results are rounded for display.

Formula and sources

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Scientific Calculator: general scientific maths.
Percentage Calculator: percentages and changes.
Prime Factorisation Calculator: factor a number into primes.

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