How to use
- Choose what to solve for at the top: interest, principal, rate or time. Interest is the default.
- Fill in the other values. Time can be in days, months or years; for days, pick whether a year counts as 365 or 360 days.
- The answer and the principal-plus-interest total update instantly.
- Switching what you solve for carries the value you just calculated into its input, so you can explore the same scenario from another angle.
The simple interest formula
With simple interest, only the original principal earns interest — interest never earns interest of its own.
| Solve for | Formula |
|---|---|
| Interest (I) | P × r × t |
| Principal (P) | I ÷ (r × t) |
| Annual rate (r) | I ÷ (P × t) × 100 |
| Time in years (t) | I ÷ (P × r) |
| Converting time | months ÷ 12, days ÷ 365 (or 360) |
Examples
| Situation | Calculation | Result |
|---|---|---|
| 10,000 at 5% for 3 years | 10,000 × 0.05 × 3 | 1,500 interest, 11,500 total |
| 10,000 at 5% for 18 months | 10,000 × 0.05 × 1.5 | 750 interest |
| 10,000,000 at 5% for 90 days, 365-day year | 10,000,000 × 0.05 × 90 ÷ 365 | 123,287.67 |
| Same, 360-day year | 10,000,000 × 0.05 × 90 ÷ 360 | 125,000.00 |
| 150 earned on 1,000 in 3 years | 150 ÷ (1,000 × 3) | 5% a year |
Things to keep in mind
- Results are for reference only. Real accounts and loans differ in which days they count, how they round and whether interest is taxed.
- Some lenders use a 360-day year (the “banker’s rule”), which slightly increases the interest for the same number of days.
- Most savings accounts and many loans compound interest; for those, use the compound interest calculator or the loan calculator.
- To compare a nominal rate with its compounded yield, try the APY calculator.
FAQ
How big is the gap between simple and compound interest?
Small over short periods, large over long ones. 10,000 at 5% for 10 years earns 5,000 in simple interest but about 6,289 compounded yearly.
Where is simple interest actually used?
Typical cases are short-term loans and notes, certificates of deposit that pay interest out instead of reinvesting it, and bond coupons, which pay a fixed percentage of the face value each period.
Is anything I enter saved or sent anywhere?
No. All calculations run in your browser.