Digit Sum Calculator – Add the Digits of Any Number
Add the digits of a number, with the digital root for repeated sums.
About the Digit Sum Calculator
What is the Digit Sum Calculator?
A digit sum calculator adds the digits of a number and reduces it to a single "digital root" by repeating the process.
Why use this calculator?
Digit sums underlie casting out nines, divisibility checks by 3 and 9, and quick sanity checks in arithmetic.
How to use it
- Enter any whole number (sign and separators allowed).
- Read the digit sum, product and digital root.
Formula
Digital root = sum of digits, repeated until one digit (mod 9).
A number is divisible by 9 exactly when its digital root is 9.
Frequently Asked Questions
What is casting out nines?
A short-cut check: the digital root of a sum/product equals the digital root of the result, so mismatches reveal mistakes.
Is digital root the same as mod 9?
Almost: it equals n mod 9 except that 0 maps to 9 for positive numbers above zero.
What order of operations does this use?
It follows standard PEMDAS/BODMAS rules: parentheses, exponents, multiplication and division, then addition and subtraction.
How precise are the answers?
The calculator uses double-precision floating-point math (about 15 significant digits) and rounds the display. For exact symbolic results use dedicated algebra software.
Can I enter negative numbers or decimals?
Yes. Negative values, decimals and very large or small numbers are all supported unless a field says otherwise.
What does it mean if I get an error?
Errors usually mean an invalid input, such as division by zero or a negative value where only positives make sense. Check your entries and try again.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.
How accurate are the results?
We use standard, widely accepted formulas and calculate in your browser using double-precision arithmetic. Results are rounded for readability, so treat them as reliable estimates rather than a guarantee. Real-world factors can vary the true figure.
Can I add this calculator to my own website?
Yes. Open this calculator page and use the "Add to your website" box in the sidebar to copy a ready-made iframe snippet, then paste it into any HTML page.
Does it work on a phone or tablet?
Yes. The layout is fully responsive, so it works on desktops, laptops, tablets and mobile phones, and every input adapts to touch screens.