Free Simple Interest Calculator — SI Formula & Results
Calculate simple interest, total amount, and monthly payment for any principal, rate, and time period — free, in your browser.
Simple Interest
—
Total Amount (P + I)
—
Monthly Payment
—
How it works
Enter principal amount
Type the starting loan or investment amount.
Set the interest rate
Enter the annual interest rate as a percentage.
Choose the time period
Enter the duration in years or months.
Read the results
Interest, total amount, and monthly payment all show instantly.
Common use cases
Savings interest
Estimate how much interest a savings account earns over a fixed period.
Short-term loans
Calculate interest on a personal or payday loan with a simple interest structure.
Student finance
Understand how interest accrues on a student loan over time.
Financial education
Teach the concept of simple interest with real-number examples.
Frequently asked questions
What is the simple interest formula?
SI = (Principal × Rate × Time) / 100, where Rate is annual % and Time is in years. Total Amount = Principal + SI.
How is this different from compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on principal plus accumulated interest, so it grows faster.
Can I enter time in months?
Yes — toggle to months and the calculator converts automatically (months ÷ 12 = years).
Does it handle fractional years?
Yes — entering 18 months, for example, uses 1.5 years in the formula.
Is this free?
Yes, completely free with no account needed.
Is my data stored?
No — all calculations run in your browser. Nothing is transmitted.