Use this free simple interest calculator to work out interest on a loan or deposit in seconds. Enter the principal, annual rate and time, and get the interest and the total amount — the classic school and banking formula, done for you.
What Is the Simple Interest Calculator?
Simple interest is interest charged or earned only on the original principal for the whole period, so the amount grows by the same amount each year. Unlike compound interest, it is not calculated on interest already earned. It is commonly used for short-term loans, some types of bonds, and in school mathematics, where the classic formula makes costs easy to work out by hand and to check quickly.
How to Use This Calculator
- Enter the principal amount.
- Enter the annual interest rate.
- Enter the time in years.
- Click Calculate Interest.
- See the interest and the total amount.
Formula
SI = P × R × T ÷ 100, where P = principal, R = annual rate in percent, T = time in years. Total amount = P + SI.
Worked Example
Principal 10,000 at 5% per year for 3 years. SI = 10,000 × 5 × 3 ÷ 100 = 1,500. Total amount = 11,500.
How It Works, In Plain Words
Three numbers go in, and the formula treats each one plainly. P is the principal, the 10,000 you lend or deposit. R is the annual rate in percent — 5 here — and the ÷ 100 in the formula simply turns that percent into a fraction of the principal: one year of interest is 10,000 × 5 ÷ 100 = 500. T is the time in years, and because simple interest never pays interest on interest, every year earns that same 500. Three years therefore earn 500 × 3 = 1,500, which is exactly what the formula does in one line: 10,000 × 5 × 3 ÷ 100 = 1,500. The second answer adds the principal back: total amount = P + SI = 10,000 + 1,500 = 11,500, the sum you repay on a loan or receive back on a deposit. The straight-line behaviour is the thing to remember: double the time and the interest doubles, double the rate and the interest doubles. Nothing accelerates, which is why this formula suits the short-term loans and exam questions the page mentions, and why long-term growth belongs to the compound calculator.
Real-World Uses
First scenario: the short loan check. The Benefits list below calls this a quick cost check for short loans, and the worked example is the pattern: borrowing 10,000 at 5% for 3 years costs 1,500 in interest and 11,500 in total. Knowing both figures before signing turns a vague “5%” into a real price. Second scenario: homework and exams. The page notes this is the standard formula for exams and school mathematics, so a student can work a question by hand — 10,000 × 5 × 3 ÷ 100 — and check the answer here in seconds, including the 18 months = 1.5 years conversion the FAQ explains for part-year questions. Third scenario: the comparison baseline. Run 10,000 at 5% for 3 years here and note the 1,500; then run the identical figures in the Compound Interest Calculator from Related Calculators. The gap between the two interest figures is exactly what compounding adds, which makes sales claims about growth easy to judge.
Benefits
- Quick cost check for short loans.
- The standard formula for exams and homework.
- Easy comparison baseline against compound offers.
- No hidden steps — one transparent formula.
Features of Our Tool
- Instant results as you click Calculate — no page reload
- Shows the full breakdown, not just one number
- Free forever, no sign-up or download
- Works on phones, tablets and computers
- Your numbers stay in your browser — nothing is stored
Common Mistakes to Avoid
- Entering months in the years field.
- Using compound interest formula for a simple interest question.
- Forgetting the rate is per year unless stated otherwise.
More Common Mistakes to Avoid
Three more traps deserve a mention. First, the rate box takes the percent figure itself — type 5 for 5% — because the formula’s ÷ 100 does the converting. Typing 0.05 instead shrinks the interest a hundredfold and produces a suspiciously tiny answer. Second, reading the total amount as the interest: the two print on separate lines — 1,500.00 is the interest, 11,500.00 is principal plus interest — and mixing them up makes a loan look far costlier, or a deposit far richer, than it is. Third, expecting this tool to price a repayment schedule on its own: it gives the interest and the total, not monthly payments. For a loan repaid in instalments, take the total from here as a sense check and use the Loan EMI Calculator in Related Calculators for the monthly figure.
Frequently Asked Questions
What is the simple interest formula?
SI = P x R x T / 100, where P is the principal, R is the annual rate in percent and T is the time in years.
What is the simple interest on 10,000 at 5% for 3 years?
The interest is 1,500 and the total amount is 11,500.
When is simple interest used?
Simple interest is used in many short-term loans, car loans in some regions, bonds, and school and exam mathematics.
What is the difference between simple and compound interest?
Simple interest is earned only on the original principal; compound interest is earned on principal plus past interest, so it grows faster.
Can time be in months?
Yes — convert months to years first (months / 12). For example 18 months = 1.5 years.
Why does each year earn the same interest?
Because the interest is always calculated on the original principal alone, never on interest already earned. In the worked example each of the 3 years earns the same 500 — a third of the 1,500 total — which is the straight-line growth the What Is section describes.
Can I calculate interest for part of a year?
Yes. The time box accepts fractions — its step is 0.25 of a year — and the FAQ above gives the conversion: months ÷ 12, so 18 months = 1.5 years. Enter 1.5 as the time and the same formula applies.