Calculate Simple Interest or Solve for P, R and T
Calculate interest that is charged only on the original principal, with time in years, months or days. Switch the unknown to work backwards and find the principal, rate or time needed for a given amount of interest.
How to calculate simple interest
- Choose what to solve for: interest, principal, rate or time.
- Enter the known values. Rates are a percentage per year.
- Pick the time unit. Days use a 365-day year.
- Read the answer, the total amount and the rearranged formula with your numbers.
Benefits
- Solves the simple interest equation for any one unknown.
- Accepts time in years, months or days and shows the conversion to years.
- Shows the formula with your values substituted, which suits homework and quick checks.
Limitations
- Day counts use a 365-day year (Actual/365). Some lenders use 360 days, which gives slightly higher interest.
- Simple interest never compounds. For balances where interest is added to the principal, use the compound interest calculator.
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Frequently asked questions
What is the simple interest formula?
SI = P × R × T ÷ 100, where P is the principal, R the annual rate in percent and T the time in years. 10,000 at 8% for 3 years earns 2,400, so the total amount is 12,400.
How do I calculate simple interest for days?
Convert the days to years by dividing by 365, then apply the formula. 73 days is 0.2 years, so 10,000 at 10% for 73 days earns 200.
How do I find the rate if I know the interest?
Rearrange the formula: R = SI × 100 ÷ (P × T). Choose "Rate" as the value to solve for and enter the interest, principal and time.