Simple Interest Calculator - Interest, Principal, Rate or Time
Pick what you want to find, fill in the other three values and read the answer with the working shown. Time can be in years, months or days.
Maintained by Hirantha Sandaruwan Lakshan. Updated 13 October 2026.
About this tool
The calculation runs in your browser and nothing is sent anywhere.
How it works
Simple interest is charged only on the original amount, never on interest already earned. The formula is I = P x r x t, where I is the interest, P the principal, r the yearly rate as a fraction (8% is 0.08) and t the time in years. The total you end up with is P + I.
Rearranged, the same formula finds any missing value: P = I / (r x t) for the principal, r = I / (P x t) for the rate and t = I / (P x r) for the time. The calculator does the rearranging for you and shows the numbers it used.
Time is counted in years. Months are a twelfth of a year and days are a 365th, a common day count (your bank or lender says which one it uses). So 90 days is 90 / 365 of a year, and 18 months is 1.5 years.
Simple interest is the right model for interest paid at maturity on a fixed deposit, for a Treasury bill's yield and for a flat-rate loan. If interest is added to the balance and earns interest itself, use the compound interest calculator instead.
Worked examples
Find the interest: Rs. 50,000 at 8% for 3 years
I = 50,000 x 0.08 x 3 = Rs. 12,000. The total is Rs. 62,000.
Find the rate: Rs. 12,000 interest on Rs. 50,000 over 3 years
r = 12,000 / (50,000 x 3) = 0.08, which is 8% a year.
Find the time: Rs. 12,000 interest on Rs. 50,000 at 8%
t = 12,000 / (50,000 x 0.08) = 3 years, which is 36 months or 1,095 days.
Interest for a number of days: Rs. 200,000 at 9% for 90 days
I = 200,000 x 0.09 x 90 / 365 = Rs. 4,438.36, so the total is Rs. 204,438.36.
The four forms of the simple interest formula
| To find | Formula | Worked example |
|---|---|---|
| Interest (I) | I = P x r x t | 50,000 x 0.08 x 3 = 12,000 |
| Principal (P) | P = I / (r x t) | 12,000 / (0.08 x 3) = 50,000 |
| Rate (r) | r = I / (P x t) | 12,000 / (50,000 x 3) = 0.08, or 8% a year |
| Time (t) | t = I / (P x r) | 12,000 / (50,000 x 0.08) = 3 years |
Frequently asked questions
What is the simple interest formula?
I = P x r x t. Multiply the principal by the yearly rate (as a fraction) and by the time in years. For Rs. 50,000 at 8% for 3 years that is 50,000 x 0.08 x 3 = Rs. 12,000.
How is simple interest different from compound interest?
Simple interest is the same every year because it is worked out on the original amount only. Compound interest adds each year's interest to the balance, so later years earn more. Over 3 years at 8%, Rs. 50,000 earns Rs. 12,000 of simple interest but about Rs. 12,986 compounded yearly.
How do I use months or days for the time?
Choose months or days in the box. The calculator divides months by 12 and days by 365 to get years. A fixed deposit for 18 months is 1.5 years, and 90 days is 90 / 365, about 0.2466 of a year.
Why does the time answer show years, months and days?
When you find the time, the result is shown in the unit you picked, plus the same time in years and rounded days, so you can read it whichever way is handy. Days are counted on a 365-day year.
Is the interest on a Sri Lankan fixed deposit simple?
In the bank rate tables we checked on 13 October 2026, the rate for a deposit paid at maturity works as a simple yearly rate, so the interest is deposit x rate x months / 12. The fixed deposit calculator on this site adds tax and the effective annual rate (AER).
Can the answer be a rate above 100%?
Yes, if the interest is bigger than the principal for the time given, for example Rs. 15,000 interest on Rs. 10,000 in a year is 150% a year. Check the figures if that is not what you expected.
Sources & references
Updated Oct 13, 2026