C Program to Find Simple Interest

Problem Description

Write a Simple Interest Program in C using the SI formula.

What is Simple Interest in C?

Simple Interest is the amount of money that a borrower pays to a lender as a percentage of the principal amount borrowed.

Common terms in the calculation of Simple interest:

  • Principal: The amount of money borrowed.
  • Period: he period of time for which the money is borrowed.
  • Rate of interest: The percentage of the principal amount that is paid as interest.
Formula to Calculate Simple Interest:
Simple Interest = (P × R × T)/100

Here, P = Principal Amount, R = Rate of Interest and T = Time.

Find the Simple Interest on ₹7000 for 2 years at 50% per annum.



Given, P = 7000, R = 50 and T = 2.

Simple Interest (SI) = (P * R * T)/100

SI = (7000 * 50 * 2)/100
SI = 700000/100
SI = 7000

Therefore, the simple interest on ₹7000 at 50% per annum for 2 years is ₹7000.

Amount = Principal + Simple Interest = 7000 + 7000 = 14000.

1. Enter the principal amount, rate of interest, and time period.
2. Calculate the simple interest using the formula SI = (P * R * T)/100.
3. Print the Simple Interest.

Program/Source Code

Here is source code of the C program to calculate the simple interest. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

 * C program to find the simple interest
int main()
    float principal, rate, time;
    // Get user input
    printf("Enter the principal amount: ");
    printf("Enter the rate of interest: ");
    printf("Enter the time: ");
    float  simple_interest, amount;
    // Calculate simple interest and amount
    simple_interest = ((principal*rate*time)/100);
    amount = simple_interest + principal;
    // Print the output
    printf("Simple Interest = %f \nAmount = %f",simple_interest, amount);
    return 0;
Program Explanation

1. Declare variables to store the values of the principal amount, rate of interest, time period, simple interest, and amount.
2. Get user input for the principal amount, rate of interest, and time period using the scanf() function in C and calculate the simple interest using the formula
simple_interest = ((principal*rate*time)/100);
3. Calculate the amount using the formula amount = principal + simple_interest and store the value in the variable amount.
4. Print the amount and simple interest using the standard printf() functions.


Time complexity: O(1)
The time complexity of the simple interest program is O(1), since it only performs a constant number of operations, regardless of the size of the input.

Space complexity: O(1)
The space complexity of the simple interest program is also O(1), since it only requires a constant amount of memory to store the input variables and the output results.

Runtime Test Cases

Testcase 1: In this case, enter P=7000, R=50, and T=2 as the parameters to calculate simple interest.

Enter the principal amount: 7000
Enter the rate of interest: 50
Enter the time: 2
Simple Interest = 7000.000000 
Amount = 14000.000000

Testcase 2: In this case, enter P=4000, R=20, and T=3 as the parameters to calculate simple interest.

Enter the principal amount: 4000
Enter the rate of interest: 20
Enter the time: 3
Simple Interest = 2400.000000 
Amount = 6400.000000

To practice programs on every topic in C, please visit “Programming Examples in C”, “Data Structures in C” and “Algorithms in C”.

Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & discussions at Telegram SanfoundryClasses.