What is a Simple Interest Calculator?
A simple interest calculator works out the interest on a principal amount at a fixed yearly rate for a period of time, where interest is charged only on the original principal and not on past interest.
Simple Interest Calculator applies the classic P × R × T ÷ 100 formula. Simple interest is used for many short-term loans between individuals, some gold and agricultural loans, and as a teaching baseline for compound interest.
How does the Simple Interest Calculator work?
Simple interest is charged only on the original principal, so the interest is the same every year.
SI = P × R × T ÷ 100
Total = P + SI
(T in years; months ÷ 12, days ÷ 365)Worked example using the starting values on this page:
- Principal amount
- ₹1,00,000
- Rate of interest (p.a.)
- 8.0%
- Time period
- 3
- Time in
- Years
- Principal
- ₹1,00,000
- Simple interest
- ₹24,000
- Total amount
- ₹1,24,000
- Compound interest (yearly) for comparison
- ₹25,971
How to use the Simple Interest Calculator
- Enter the principal amount (anywhere from ₹100 to ₹10,00,00,000). Type it in the box or drag the slider.
- Enter the rate of interest (p.a.) (anywhere from 0.1% to 50.0%). Type it in the box or drag the slider.
- Enter the time period (anywhere from 1 to 3,650). Type it in the box or drag the slider.
- Choose time in: Years or Months or Days.
- Read the simple interest at the top of the results, then check the breakdown below it.
- Change one input at a time to compare scenarios, and note how much each change moves the result.
Benefits of using the Simple Interest Calculator
- Shows the maturity value and interest earned before you lock money in.
- Lets you compare rates, tenures and compounding frequencies side by side.
- Helps you plan around a goal date by testing different tenures.
- Free and instant, with the formula shown, so you can match it against your bank’s figures.
Mistakes to avoid with the Simple Interest Calculator
The common mistake is applying a yearly rate to a time entered in months without dividing by 12.
Simple interest stays the same every year; compare it with compound interest to see the difference time makes.
Helpful checks before using the result
These checks make a simple interest more useful in practice.
- Start with a realistic base case rather than the first value that fits the range slider.
- Change one major assumption at a time so the effect stays easy to interpret.
- Use the result to compare scenarios, not as a final provider quote or guaranteed outcome.
- Verify rates, rules, and product terms separately before acting on the estimate.
Simple vs compound interest
The difference grows with time.
- Simple interest is calculated only on the original principal, so it is the same amount every year.
- Compound interest is calculated on principal plus past interest, so it grows faster each year.
- Over one year with yearly compounding the two are equal; over 10 years at 8%, compound interest on ₹1 lakh is about ₹1.16 lakh against ₹80,000 simple.
Converting the time period
The rate is yearly, so time must be in years.
Months
Divide by 12: 18 months is 1.5 years.
Days
Divide by 365: 90 days is about 0.247 years.
Monthly rate
A rate quoted per month (for example 2% a month) is 24% a year.
Simple Interest Calculator FAQs
What is the formula for simple interest?
SI = P × R × T ÷ 100, where P is the principal, R is the yearly rate in percent and T is the time in years. Total amount = P + SI.
What is the simple interest on ₹1 lakh at 8% for 3 years?
₹1,00,000 × 8 × 3 ÷ 100 = ₹24,000, so the total is ₹1,24,000.
How do I calculate simple interest for months?
Convert months to years by dividing by 12. For 6 months at 12% on ₹50,000: 50,000 × 12 × 0.5 ÷ 100 = ₹3,000.
How do I calculate simple interest by date?
Count the days between the two dates, choose Days, and enter that number. The calculator uses a 365-day year.
