Calculate Simple Interest in C++

Simple interest in C++: P × R × T / 100. Rate 5 means 5%, not 0.05.

BeginnerBasic ProgramsExample 10 of 14

calculate-simple-interest.cpp

1#include <iostream>
2#include <iomanip>
3using namespace std;
4
5int main() {
6    float principal, rate, time, interest;
7    
8    cout << "Enter principal amount: ";
9    cin >> principal;
10    
11    cout << "Enter rate of interest (per year): ";
12    cin >> rate;
13    
14    cout << "Enter time (in years): ";
15    cin >> time;
16    
17    // Simple Interest = (P * R * T) / 100
18    interest = (principal * rate * time) / 100;
19    
20    cout << fixed << setprecision(2);
21    cout << "Simple Interest = " << interest << endl;
22    cout << "Total Amount = " << principal + interest << endl;
23    
24    return 0;
25}

Output

Enter principal amount: 10000
Enter rate of interest (per year): 5
Enter time (in years): 2
Simple Interest = 1000.00
Total Amount = 11000.00

What's going on

Simple interest in C++ is P * R * T / 100. Rate 5 means five percent, not 0.05.

float principal, rate, time, interest;

float (or double) so 10000 * 5 * 2 / 100 keeps the tenths. The same formula in int truncates.

cin >> principal; then rate, then time.

Sample: 10000, 5, 2.

interest = (principal * rate * time) / 100;

10000 * 5 * 2 / 100 = 1000. That's the sample. Total amount is principal + interest = 11000.

Enter 0.05 for the rate and the result is 100× too small (10 instead of 1000). The prompt says "per year" as a percent.

cout << fixed << setprecision(2);

Two digits after the dot, same as the float-multiplication program. Needs <iomanip>.

Sanity check: 1000 at 5% for 2 years is 100. If that isn't what you get, the rate interpretation is wrong.

This is not compound interest. Compound uses a power — that's the next program.

PreviousConvert Fahrenheit to Celsius NextCalculate Compound Interest