The Simple Interest Calculator computes interest earned or owed using the basic interest formula where interest is calculated only on the original principal amount. Unlike compound interest, simple interest does not earn interest on previously accumulated interest, making it easier to calculate and commonly used for short-term loans, auto loans, and some savings instruments.
Simple interest is most commonly used for short-term financial products. Auto loans, personal loans, and some bonds use simple interest. It's straightforward and transparent — you always know exactly how much interest you'll pay. This makes it easier to budget compared to compound interest, where the total interest can grow unexpectedly over long periods.
How the Formula Works
Simple Interest = P × r × t, where P = principal, r = annual interest rate (as decimal), t = time in years. Total Amount = P + Interest.
I = P × r × t (Interest)
A = P + I = P(1 + rt) (Total amount)
For months: t = number of months ÷ 12
For days: t = number of days ÷ 365
How to Use This Calculator
Enter the principal amount
Enter the annual interest rate
Enter the time period (years, months, or days)
View the interest earned and total amount
Tips & Best Practices
Simple interest is always less than compound interest for periods longer than 1 year
For very short periods (under 1 year), simple and compound interest are nearly identical
A 'flat rate' loan using simple interest has a higher effective APR than the stated rate
Use simple interest calculations to quickly estimate short-term loan costs
Important Limitations
Not used for most real-world savings accounts or long-term investments (those use compound interest)
Flat-rate simple interest can be misleading — the effective rate is higher since the principal decreases over time