Digit Sum & Digital Root Calculator

How it is calculated

The tool keeps only the digits 0 to 9 of what you type, adds them to get the digit sum, and then repeats the process on the sum until a single digit remains, which is the digital root. For a whole number, the digital root equals the remainder after division by 9 (with 9 instead of 0 for nonzero multiples of 9), so root = 1 + (n − 1) mod 9 for n above zero. All other characters, including a minus sign, a decimal point or separators, are discarded, so 3.14 is treated as the digits 3, 1 and 4. Empty input or input with no digits gives no result. The digit sum is the basis of the divisibility check by 3 and 9.

Common mistakes

• Typing a decimal like 3.14 and expecting the dot to be respected. It is stripped, and 3, 1 and 4 are added.
• Entering a negative number: the sign is dropped, so -57 gives the same result as 57.
• Confusing the digit sum (35) with the digital root (8) when only one digit is wanted.
• Using the digital root as proof that a calculation is correct. A matching root can still hide an error such as swapped digits.