Simple Interest Calculator
I = P × R × T — solve for interest, principal, rate or time, in days or years.
About this tool
Choose what to find — interest, principal, rate or time — and fill in the other values. Time can be in years, months or days, with a 365-day year or the 360-day banker's year. The result is shown with the total and a comparison with compound interest at the same rate.
Simple interest, explained
Simple interest is calculated only on the original amount, never on interest already earned: I = P × R × T, with R as a decimal per year and T in years. Lend 10,000 at 5% for three years and you earn 500 every year, 1,500 in total. Rearranging gives the other three: P = I ÷ (R × T), R = I ÷ (P × T) and T = I ÷ (P × R). Simple interest is used for many car and personal loans, short-term deposits, bond coupons, and late-payment charges, and it is the first interest formula taught in school. For periods in days, lenders use either a 365-day year or the 360-day banker's year; the 360 basis makes the same rate slightly more expensive for borrowers, which is worth knowing when comparing quotes. The compound figure shows how much more reinvesting the interest would earn — the gap widens quickly with time. General information, not financial advice. The compound interest calculator reinvests the interest, the FD calculator handles deposits, the percentage calculator does quick rate sums, and Fractions, decimals and percentages: the same number three ways, and how to convert explains converting rates.
Frequently asked questions
What is the formula for simple interest?
I = P × R × T: principal × annual rate (as a decimal) × time in years. 10,000 × 0.05 × 3 = 1,500.
What is the difference between simple and compound interest?
Simple interest is paid only on the principal; compound interest is also paid on earlier interest, so it grows faster over time.
How do I calculate simple interest for days?
Divide the days by 365 (or 360 on the banker's basis) to get years. 90 days at 365 is 0.2466 years.
Is my data saved?
No. Calculations happen in your browser and nothing is stored or sent.