Simple Interest Calculator
Calculate simple interest on a principal over time — no compounding.
Results update as you type.
About this calculator
Simple interest is charged only on the original principal, never on interest that has already accrued. The formula is Interest = Principal × Rate × Time, with the rate as a decimal and the time in years. Because the base never grows, the interest is the same every year and the balance climbs in a straight line rather than a curve.
For example, 5,000 borrowed or invested at 5% a year for 3 years earns 5,000 × 0.05 × 3 = 750 in interest, for a total of 5,750. Double the time to 6 years and the interest simply doubles to 1,500 — there is no compounding to accelerate it.
Simple interest is common on short-term and some car, personal or bridging loans, and on certain bonds and savings products. Use this calculator to check the cost of such a loan, and compare it with the compound interest calculator to see how much difference compounding makes over longer periods.
Frequently asked questions
What is the simple interest formula?
Interest = Principal x Rate x Time, with the rate as a decimal. 5,000 at 5% for 3 years earns 5000 x 0.05 x 3 = 750.
How is simple interest different from compound interest?
Simple interest is calculated only on the principal, so it grows in a straight line. Compound interest is calculated on the principal plus prior interest, so it grows faster over time.
How do I calculate the total amount owed?
Add the interest to the principal. For 5,000 at 5% over 3 years the interest is 750, so the total to repay is 5,000 + 750 = 5,750. The calculator shows both the interest and this total.
Can I enter a partial year?
Yes. The time field accepts fractions, so 18 months is 1.5 years and 6 months is 0.5. The interest scales directly with time, so half the years means half the interest.
How do I find the interest rate from the interest paid?
Rearrange the formula: Rate = Interest ÷ (Principal × Time). If 5,000 over 3 years earned 750, the rate is 750 ÷ (5,000 × 3) = 0.05, or 5% a year.
When is simple interest used instead of compound?
It is typical for short-term loans, some car and personal loans, and certain bonds and notes where interest is paid out rather than reinvested. Over long horizons lenders and savers usually use compound interest instead.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/simple-interest/curl
curl "https://calculator.free/api/v1/simple-interest/?principal=5000&rate=5&years=3"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/simple-interest/?" + new URLSearchParams({
"principal": "5000",
"rate": "5",
"years": "3"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.