Simple Interest Calculator
Simple Interest Terms
Calculation Results
| Interest Earned | — |
|---|---|
| Total Ending Balance | — |
How to Use This Simple Interest Calculator
Simple interest is the math you learned in week one of personal finance—and the math many promissory notes still use today. No compounding curves, no reinvestment: just principal × rate × time. Handy when a contract explicitly says "simple interest" and you need a quick sanity check.
- Enter principal amount. This is the starting loan or investment balance.
- Enter annual interest rate. Use the stated % per year from your note or agreement.
- Set time in years, months, and/or days. All three fields combine into total elapsed time.
- Press Calculate. Review interest earned and total ending balance (principal + interest).
Most savings products compound. Compare results on the Compound Interest Calculator or use the Interest Calculator to toggle both methods.
Simple Interest Formulas and Practical Applications
Picture simple interest like a flat monthly rent on your principal—it never increases even as time passes. Compound interest, by contrast, raises the "rent" because the balance itself keeps growing.
Core formula
I = P × r × t
Ending Balance = P + I
Where r is the annual rate as a decimal and t is time in years. On $8,000 at 7% for 2 years, interest is $1,120 and ending balance is $9,120—the same $560 interest every year.
Mixed time inputs
Entering 1 year, 6 months, and 15 days converts to about 1.54 years internally, useful for short bridge loans or legal interest accrual windows.
Frequently Asked Questions
What is simple interest?
Simple interest is calculated only on the original principal. It does not earn interest on prior interest, unlike compound interest.
Can I enter years, months, and days together?
Yes. All three fields combine into total time, converted to years for the simple interest formula.
When is simple interest used in real finance?
Some short-term personal loans, student loan grace periods, and legal judgments use simple interest. Most bank deposits and credit cards use compound interest instead.
How is total ending balance calculated?
Ending balance equals principal plus simple interest earned over the entered time period at the stated annual rate.
How does simple interest compare to compound interest?
Compound interest grows faster because each period's interest is added to the balance. Simple interest produces straight-line growth and lower totals over long horizons.