Simple Interest Calculator

A simple interest calculator works out interest charged only on the original amount, never on accumulated interest. The formula is principal × rate × time ÷ 100, so the total grows in a straight line — the same amount is added every year, however long the arrangement runs. That makes it easy to work out without a spreadsheet, which is exactly why it survives in short-term and informal lending. It is also why a quoted simple, or 'flat', rate needs reading carefully: the same number expressed on a reducing balance is a very different deal, and the gap is larger than most borrowers expect. Everything runs in your browser.

How to use it

1. Enter the principal amount.
2. Add the yearly interest rate and how many years.
3. See the interest and the total amount to repay.
4. For a period in months, enter it as a fraction of a year — six months is 0.5, eighteen months is 1.5.

Examples

• ₹50,000 at 8% for 3 years → ₹12,000 interest, ₹62,000 total.
• ₹1,000 at 10% for 1 year → ₹100 interest.
• ₹1,00,000 at a flat 10% for 3 years → ₹30,000 interest, ₹1,30,000 to repay, ₹3,611.11 a month.
• That same monthly payment on a reducing-balance loan corresponds to about 17.9% a year — the flat figure is close to half the comparable rate.

Frequently asked questions

What is the simple interest formula?

Simple interest = principal × rate × time ÷ 100, where the rate is per year and time is in years. On ₹50,000 at 8% for 3 years that's 50,000 × 8 × 3 ÷ 100 = ₹12,000.

Where is simple interest used in real life?

Mostly where the period is short or the arithmetic has to be done without a spreadsheet: lending between people, short-term advances, some vehicle and consumer loans quoted as a flat rate, and fixed-income products that pay interest out rather than reinvesting it. It also turns up wherever a penalty or a late fee is expressed as a rate per month. Bank savings, fixed deposits and long-term loans use compound interest instead, so in formal finance simple interest is the exception rather than the rule.

Simple vs compound interest — which grows faster?

Compound interest always grows faster over time, because it also earns interest on the interest already added. Over one year at the same rate they're nearly identical; over ten years the gap becomes large.

How do I calculate interest for months, not years?

Convert months to a fraction of a year — 6 months is 0.5, 18 months is 1.5 — and enter that as the time. The formula works the same way with fractional years.

What is a flat rate, and why is it not the rate I am paying?

A flat rate charges simple interest on the whole amount you borrowed for the whole term, even though you are paying the loan down every month. Borrow ₹1,00,000 at a flat 10% over three years and the interest is ₹30,000, the repayment ₹1,30,000, and the instalment ₹3,611.11. By the last year you owe a fraction of ₹1,00,000, yet you are still charged as though you owed all of it. That same instalment on an ordinary reducing-balance loan corresponds to a rate near 17.9%. A flat rate is not dishonest, but it is roughly half the number you would need to compare the loan with anything else — ask for the reducing-balance rate, or the annual percentage rate, before signing.

A lender quoted me 2% — per month or per year?

Ask, because the difference is enormous. Two per cent a month is 24% a year in simple interest, and rates are quoted per month in short-term and informal lending precisely because the number sounds small. To use this calculator, convert first: multiply a monthly rate by 12, or a weekly rate by 52, and enter the yearly figure. If a lender will not state the rate per year and the total amount repayable, that refusal is itself the useful answer.

Is simple interest ever better for the borrower?

At the same stated rate, yes — simple interest is genuinely cheaper than compound over any period longer than one compounding interval, because nothing is charged on interest already added. The trap is that the two are almost never quoted at the same number: a flat 10% and a reducing-balance 10% are not competing offers. Compare the total amount repayable rather than the percentages, because that is the one figure both methods express in the same way.