DIY Cost Tools
Simple Interest Calculator
FINANCIAL CALCULATOR
Simple Interest Calculator
Calculate simple interest and ending amount from principal, annual rate, and time.
How it works
Enter the starting principal, annual simple-interest rate, and duration. Choose years, months, or days.
Formula
Simple interest I = P × r × t, where P is principal, r is the annual decimal rate, and t is time in years. Ending amount A = P + I.
Worked example
$5,000 at 6% simple interest for three years earns $900, producing an ending amount of $5,900.
Important limits
- Simple interest does not earn or charge interest on prior interest.
- Actual loans may accrue daily on a declining balance, compound, include fees, or use a different day-count convention.
- This is an educational estimate, not financial or lending advice.
Frequently asked questions
How is simple interest different from compound interest?
Simple interest applies the rate only to the original principal. Compound interest also applies to accumulated interest.
Does this calculator use APR?
It uses the annual rate entered as a simple rate. APR may include additional fees and is not automatically equivalent.
Sources and review notes
The Consumer Financial Protection Bureau notes that loan simple interest may be calculated on the outstanding balance daily or monthly; this tool instead demonstrates the textbook P × r × t model. Last reviewed September 29, 2026.