Simple Interest Calculator
Enter the principal, the yearly rate and the time. We give you the interest earned or owed and the total amount.
This calculator gives estimates for information only. It is not financial, tax or legal advice. Real results depend on lender terms, fees and local rules.
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The formula
Simple interest = P x R x T (P = principal, R = annual rate as a decimal, T = time in years). Total = P + interest.
How simple interest works
The interest is the same every year, because it is always based on the original amount. 1,000 at 5 percent earns 50 a year. After three years that is 150, and the total is 1,150.
Where it is used
Short-term personal loans, some car loans, bonds that pay a fixed coupon and many school maths problems. Most savings accounts and credit cards use compound interest instead.
Months instead of years
Divide the months by 12 to get years. 9 months is 0.75 years. We handle that for you with the months field.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Principal ($): 1000; Annual interest rate (%): 5; Years: 3 | Simple interest: $150.00 |
| Principal ($): 10000; Annual interest rate (%): 6; Years: 10 | Simple interest: $6,000.00 |
| Principal ($): 2500; Annual interest rate (%): 4.5; Months: 18 | Simple interest: $168.75 |
Simple Interest Calculator: frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated on the original amount only. Compound interest is calculated on the amount plus the interest already earned, so it grows faster over time.
Is this financial advice?
No. It gives an estimate from the numbers you enter. Real offers depend on fees, tax and the lender or provider, so check the terms before you commit.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026