Everyday & industry
Simple Interest Calculator
Calculate simple interest from the original principal, annual interest rate and time, or solve for one missing variable.
Use this calculator to calculate simple interest from principal, annual rate and time, or solve for one missing variable.
Simple interest is calculated only on the original principal and does not include compounding.
Enter an annual percentage rate and time in years, months or days. Months divide by 12; day-based calculations use a 365-day year. Use one currency for all amounts; the dollar symbol is generic and no currency conversion is performed.
Calculations run locally in your browser. Your financial inputs are not saved.
Choose a simple interest calculation.
Example Simple Interest Calculation
Principal $10,000, annual rate 5%, time 3 years. I = P × r × t = 10,000 × 0.05 × 3 = $1,500.00. Total amount = $10,000.00 + $1,500.00 = $11,500.00. This is a static mathematical example, not an expected return or product recommendation.
About this calculator
Understanding Simple Interest
Simple interest is calculated only on the original principal. Unlike compound interest, previously earned or charged interest does not itself earn or incur additional interest. Simple interest grows linearly when principal and rate remain constant.
- I = P × r × t, where I is interest, P is principal, r is the annual rate as a decimal, and t is time in years.
- Total amount A = P + I.
- Under simple interest, doubling the time doubles the interest when principal and rate remain unchanged.
Month Example
For $12,000 at 6% for 6 months, time is 6 ÷ 12 = 0.5 years. Interest is $12,000 × 0.06 × 0.5 = $360.00, and the total amount is $12,360.00.
Day Example
For $10,000 at 5% for 365 days, time is 365 ÷ 365 = 1 year. Interest is $10,000 × 0.05 × 1 = $500.00. Day-based calculations use a 365-day year.
Frequently asked questions
What is simple interest?
Simple interest is interest calculated only on the original principal. It does not compound.
What is the formula for simple interest?
I = P × r × t. P is principal, r is the annual rate as a decimal, and t is time in years. The total amount is P + I.
How is simple interest different from compound interest?
Simple interest uses only the original principal. Compound interest also applies interest to previously accumulated interest.
How do I calculate simple interest for months?
Divide the number of months by 12 to obtain years, then use I = P × r × t.
How do I calculate simple interest for days?
Divide the number of days by 365 to obtain years, then use I = P × r × t. This Tool does not use a 360-day banking convention.
Can simple interest rates be negative?
Yes. A negative rate produces negative modeled interest and reduces the total amount over positive time. This calculator accepts entered rates from −100% to 100% for mathematical completeness, not as a forecast or recommendation.
This calculator provides general mathematical planning estimates only and is not financial, investment, lending, tax or legal advice.