The sum of digits of a number is the total you get when you add all its digits. For example, the digits of 1234 are 1, 2, 3 and 4, and their sum is 10. In this lesson you will find the sum of digits with a while loop, with a string, and with recursion.

Run the code in our online Python compiler. The compiler does not take keyboard input, so the examples use fixed values.

 

Method 1: using a while loop

Take the last digit with n % 10, add it to a running total, and remove it with n // 10. Repeat until no digits are left.

def sum_of_digits(n):
    total = 0
    while n > 0:
        total += n % 10
        n = n // 10
    return total

print(sum_of_digits(1234))
print(sum_of_digits(999))
print(sum_of_digits(0))
print(sum_of_digits(1000))

Output:

10
27
0
1

How it works

Here is the loop for n = 1234:

Step

n % 10 (last digit)

total

n after the step

1

4

4

123

2

3

7

12

3

2

9

1

4

1

10

0

When n becomes 0, the loop stops, and the total is the sum of the digits.

 

Method 2: using a string

Convert the number to a string, turn each character back into a number, and add them up with sum().

n = 1234
print(sum(int(digit) for digit in str(n)))

Output:

10

This is the shortest solution. It is easy to read, but it works with text instead of the arithmetic idea of the first method.

 

Method 3: using recursion

The sum of digits of n is the last digit plus the sum of digits of the rest of the number.

def sum_of_digits(n):
    if n == 0:
        return 0
    return n % 10 + sum_of_digits(n // 10)

print(sum_of_digits(1234))

Output:

10

The base case is n == 0, where the sum is 0. Each call handles one digit.

 

Which method should you use?

Method

Best for

while loop

Exams and interviews

String and sum()

The shortest code

Recursion

Learning how recursion works

 

Take the number from the user (on your computer)

n = int(input('Enter a number: '))
print('Sum of digits:', sum_of_digits(n))

Our online compiler does not support input() yet, so use a fixed value there.

 

Common mistakes

  • Forgetting n = n // 10. The loop never ends.
  • Using / instead of //. Normal division gives a decimal number.
  • Starting total at 1. It must start at 0.
  • Adding the characters of a string. Without int(), digit is text, and sum() raises a TypeError.

 

Practice questions

  1. Find the sum of the digits of 98765.
  2. Check whether the sum of the digits of a number is even or odd.
  3. Keep adding the digits until you get a single digit (for example, 987 gives 24, then 6).
  4. Find the product of the digits of a number.
  5. Find the largest digit of a number.

 

FAQs

What is the sum of the digits of 1234?

1 + 2 + 3 + 4 = 10.

How do I find the sum of digits without converting to a string?

Use a while loop with n % 10 and n // 10.

Does this work for negative numbers?

The loop above returns 0 for a negative number. Use abs(n) to find the sum of the digits of its absolute value.

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