Simple Interest Calculator
100% private — runs on your device, never uploaded. Works offline once loaded.
Enter a principal amount, annual interest rate and time in years to calculate the simple interest and the final amount using the classic P x r x t formula. All calculations run locally in your browser.
The simple interest formula
Simple interest is one of the most fundamental calculations in finance. It uses the formula I = P x r x t, where P is the principal (the starting amount), r is the annual interest rate expressed as a decimal, and t is the time in years. This calculator takes your rate as a percentage and divides it by one hundred for you, then multiplies the three values together.
The final amount is simply the principal plus the interest earned or owed: A = P + I. Because interest is always based on the original principal, the amount grows in a straight line over time rather than curving upward the way compound interest does.
When simple interest applies
Simple interest shows up in many short-term and fixed arrangements. Some personal loans, car loans, and short promotional financing use it, as do certain bonds and treasury instruments that pay a flat coupon. It is also common in classroom examples because it isolates the core relationship between principal, rate and time.
Knowing whether a product uses simple or compound interest matters. For a borrower, simple interest is usually cheaper over the same term because it never charges interest on accumulated interest. For a saver, compound interest is generally more rewarding for the same reason.
- Principal: the original amount invested or borrowed
- Rate: the annual interest percentage
- Time: the length of the loan or investment in years
Tips for accurate results
Keep your units consistent. The time input is in years, so a six-month period is 0.5 and eighteen months is 1.5. If your rate is quoted monthly, multiply it by twelve first to get the annual rate this tool expects.
Use the calculator to test scenarios quickly. Because it updates as you type, you can see exactly how a higher rate or a longer term changes both the interest and the final total, which makes it a handy teaching and planning aid.
Frequently asked questions
How do I enter a period shorter than a year?
Use a decimal. Six months is 0.5 years, three months is 0.25, and eighteen months is 1.5. The time field accepts fractions.
What does interest per year mean?
It shows the flat interest charged for a single year at your principal and rate, which stays constant every year under simple interest.
Can I use it for savings and loans?
Yes. The same formula applies whether you are earning interest on savings or paying it on a loan, as long as the product uses simple rather than compound interest.
Why is my result different from my bank statement?
Most banks compound interest daily or monthly. This tool uses pure simple interest, so it will differ from any account that compounds.
Do I enter the rate as a percentage or a decimal?
Enter it as a percentage, for example 5 for five percent. The calculator converts it to a decimal internally.
Is my data stored anywhere?
No. All values remain in your browser and nothing is transmitted or saved on a server.
Advertisement