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Simple Interest Calculator: Interest, Total, Rate and Time

This free simple interest calculator finds the interest earned or owed on a principal at a fixed rate over time. It can also solve for the missing rate or the time.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

Simple interest is interest on the original amount only. It grows by the same amount every year, so it is easy to work out and easy to compare. It is common in school maths, short loans and some fixed-rate agreements.

Choose what you want to find, fill the other boxes and read the answer with the formula.

How to Use This Tool

  1. Pick what to find: interest, rate, time or principal.
  2. Fill in the boxes that the choice needs.
  3. Press Calculate.
  4. Check the working in the table.

How the Calculation Works

The core formula is I = P × R × T ÷ 100. Rearranged: R = I × 100 ÷ (P × T), T = I × 100 ÷ (P × R) and P = I × 100 ÷ (R × T). The total amount is P + I.

Worked Example

50,000 at 6% for 3 years gives interest of 50,000 × 6 × 3 ÷ 100 = 9,000, so the total is 59,000. If you know the interest is 9,000, the principal is 50,000 and the time is 3 years, the rate is 6%.

Common Uses

  • School and exam questions.
  • Short loans between people.
  • Quick checks of fixed-rate deals.
  • Comparing with compound interest.

Tips and Common Mistakes

  • Time must be in years.
  • The rate is a yearly rate.
  • For more than one year at compound interest, use the compound interest calculator.

Next, you may need the compound interest calculator, loan EMI calculator, fixed deposit calculator, ROI calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

Interest = P × R × T ÷ 100, where P is the principal, R the yearly rate in percent and T the time in years. The total amount is P plus the interest.
Simple interest is always worked out on the original principal. Compound interest is worked out on the principal plus interest already earned.
Short-term loans, some bonds and school problems. Most bank savings and long loans use compound interest.
Yes. Convert months to years by dividing by 12. For example 6 months is 0.5 years.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.