Python Program to Find the Sum of Natural Numbers – 2025

Hey guys, in this blog we will see a Python Program to Find the Sum of Natural Numbers.

Example 1: Using While Loop

# Python Program to Find the Sum of Natural Numbers

num = int(input("Enter a positive number -> "))

if num < 0:
   print("Enter a positive number")
else:
   sum = 0
   # use while loop to iterate until zero
   while(num > 0):
       sum += num
       num -= 1
   print("The sum is ->", sum)

Output

Enter a positive number -> 4
The sum is -> 10

Enter a positive number -> 5
The sum is -> 15

Enter a positive number -> 6
The sum is -> 21

• In this way, we have simply declared a sum variable and we keep on adding the number to this variable.
• Also simultaneously we are reducing our original number in every iteration.
• Suppose the user inputs 4, our program will add 4 to sum which is currently 0, and reduce our org number 4 to 3, now in the next iteration our program will add 3 to sum, now sum becomes 7 and num becomes 2 and in this way when num becomes 0, our loop will break.

Example 2: Using the formula

sum = n*(n+1)/2

# Sum of natural numbers up to num

num = int(input("Enter a positive number -> "))

if num < 0:
    print("Enter a positive number")
else:
    sum = (num*(num+1))//2
    print("The sum is ->", sum)

Output

Enter a positive number -> 8
The sum is -> 36

Enter a positive number -> 9
The sum is -> 45

Enter a positive number -> 10
The sum is -> 55

• This is the easiest way of calculating the sum of all natural numbers till a positive number.
• Suppose n=8, so the sum of all natural numbers till 8 will be (8*9)/2 which equals 36.

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