Write a Python Program to Find the Sum of Natural Numbers

Last Updated on 16 March 2025

Natural numbers are positive integers starting from 1, 2, 3, 4, 5, … and so on. The sum of the first N natural numbers is calculated using the formula:

\[S_N = \sum_{i=1}^{N} i = \frac{N (N + 1)}{2}\]

For example, the sum of the first 5 natural numbers is:

1+2+3+4+5=15

In this tutorial, we will write a Python program to find the sum of natural numbers using loops, recursion, and a mathematical formula.

Method 1: Using a Loop (Iterative Approach)
python

# Taking user input
n = int(input("Enter a positive integer: "))

# Initialize sum variable
sum_natural = 0

# Using a loop to calculate sum
for i in range(1, n + 1):
    sum_natural += i

# Printing the result
print("Sum of first", n, "natural numbers is:", sum_natural)

Output

Enter a positive integer: 8
Sum of first 8 natural numbers is: 36

Method 2: Using a Mathematical Formula (Fastest Approach)

# Taking user input
n = int(input("Enter a positive integer: "))

# Using the formula S = (n * (n + 1)) // 2
sum_natural = (n * (n + 1)) // 2

# Printing the result
print("Sum of first", n, "natural numbers is:", sum_natural)

Output

Enter a positive integer: 10
Sum of first 10 natural numbers is: 55

Method 3: Using Recursion

def sum_natural(n):
    if n == 1:
        return 1
    else:
        return n + sum_natural(n - 1)

# Taking user input
n = int(input("Enter a positive integer: "))

# Printing the result
print("Sum of first", n, "natural numbers is:", sum_natural(n))

Output

Enter a positive integer: 4
Sum of first 4 natural numbers is: 10