Digit Sum Calculator

Work out digit sum instantly with clear inputs, formula shown and shareable results.

About Digit Sum Calculator

The digit sum adds every digit of a number. Because 10 ≡ 1 (mod 9), the digit sum is congruent to the original number modulo 9, which is the basis of the divisibility test for 9 and of casting out nines.

Formula

Digit sum

S(v) = sum of the decimal digits of v, S(v) ≡ v (mod 9)

Worked example

With the default values below, Digit Sum Calculator returns Sum of the digits of 35. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number: 98,765
Repeat the summing this many timesaccepts 1–10: 1

Results

Sum of the digits: 35
After repeating the summing: 35
Number of digits: 5
Product of the digits: 15,120

Step by step

1Values used
2Digit sum
3Sum of the digits= 35
4After repeating the summing= 35
5Number of digits= 5
6Product of the digits= 15,120

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Digit sum of 98765?

9 + 8 + 7 + 6 + 5 = 35, and repeating gives 8.

How does casting out nines check arithmetic?

Digit sums of the operands must combine the same way as the operands themselves modulo 9, so a mismatch reveals an error.