Simple Interest Calculator

SI = P × R × T / 100. You probably learned this in class 5. But when you actually need to calculate the interest on a car loan, a friend's lending arrangement, or a homework problem, it's easier to just plug the numbers in here than do the arithmetic. This simple interest calculator handles it instantly — and shows you the compound interest comparison so you can see what you're leaving on the table.

Four modes, because sometimes you're solving for different things

Calculate Interest: The classic — enter principal, rate, time, get the interest and total amount. Find Principal: "I want to earn ₹50,000 interest at 8% in 3 years — how much should I invest?" Find Rate: "I invested ₹2 lakh, earned ₹36,000 in 3 years — what rate did I get?" Find Time: "How long will ₹1 lakh take to earn ₹40,000 at 10%?"

Simple vs. compound — the real difference

₹1,00,000 at 8% for 5 years: Simple interest gives you ₹40,000. Compound interest gives you ₹46,933. That's ₹6,933 more — and the gap widens dramatically with time. Over 20 years, compound interest earns more than double what simple interest does. The calculator shows both side by side with visual bars so the difference is obvious.

Where simple interest actually applies

Most real-world investments use compound interest, but simple interest still shows up in car loans (some use flat rate which is basically SI), short-term lending between individuals, certain government bonds, and of course — math exams. The time unit toggle (years/months/days) handles all the conversion so you don't have to think about fractions of years.