Sum of digits of a number in Python

The sum of the digits of 9875 is 9 + 8 + 7 + 5 = 29. This page shows the arithmetic way (with % and //) and the string way.

The program

n = int(input("Enter a number: "))

number = abs(n)
total = 0
while number > 0:
    total += number % 10
    number //= 10

print(f"Sum of digits of {n} = {total}")

Typed in this example: 9875. Press Run to type your own.

Enter a number: 9875
Sum of digits of 9875 = 29

How it works

• number % 10 gives the last digit: 9875 % 10 is 5.
• number //= 10 removes the last digit: 9875 // 10 is 987.
• The loop repeats until no digits are left (number becomes 0). The digits are added in the order 5, 7, 8, 9.
• abs(n) removes the minus sign, so -9875 also gives 29. Without it, the loop would not run for a negative number, because number > 0 would be false.
• For 0, the loop does not run and the sum is 0, which is correct.

Another way: with a string

str(n) turns the number into text, and a loop over a string gives one character at a time:

n = -9875

total = sum(int(digit) for digit in str(abs(n)))
print(total)

29

int(digit) turns each character, such as "9", back into a number before it is added. The string way is shorter. The arithmetic way is worth knowing, because the same % 10 and // 10 steps are used to reverse a number and to count digits.