Simple Interest Calculator

What Is Simple Interest?

Simple interest is a method of calculating interest where the charge is applied only to the original principal amount. Unlike compound interest, simple interest does not factor in any previously accumulated interest — it stays flat and predictable throughout the entire duration.

This makes simple interest easy to understand and calculate, especially for short-term financial instruments where the holding period is relatively brief.

How the Calculator Works

This calculator uses the standard simple interest formula to compute the interest earned and the total amount you receive at the end of the period.

If you enter the time in months, the calculator automatically converts it to years before applying the formula.

Example Calculation

Suppose you invest ₹1,00,000 at an annual interest rate of 7.5% for 3 years using simple interest.

You earn ₹7,500 each year, and that amount remains the same regardless of how long you stay invested — because interest is only calculated on the original ₹1,00,000.

Simple Interest vs Compound Interest

The key difference between simple and compound interest lies in what the interest is calculated on. With simple interest, you earn interest only on the original principal. With compound interest, you earn interest on the principal plus any interest that has already been added — often described as "interest on interest."

Here is a side-by-side comparison using the same example — ₹1,00,000 at 7.5% for 3 years:

Over 3 years, compound interest earns ₹1,730 more than simple interest. The gap widens significantly over longer periods and higher rates.

Where Is Simple Interest Used?

Simple interest is commonly applied in situations where the duration is short or where transparency and predictability are prioritised:

• Treasury bills (T-bills) — short-term government securities typically use simple interest calculations.
• Short-term fixed deposits — some banks calculate interest on short-term FDs using the simple interest method.
• Personal loans — certain personal and consumer loans use simple interest, especially for shorter tenures.
• Savings accounts — in some countries, basic savings accounts still use simple interest.
• Car loans — some auto loan structures calculate interest on the original loan amount only.

How to Use This Calculator

1. Enter the principal amount — the initial sum you are investing or borrowing.
2. Enter the annual interest rate — the yearly rate of interest as a percentage.
3. Enter the time period — the duration for which the principal is invested or borrowed.
4. Toggle between years and months — use the toggle to switch the time unit depending on your need.

Click "Calculate" to see the interest earned, total amount, and a visual breakdown of principal vs interest.