What is the Simple Interest?
Simple interest (SI) is interest calculated only on the original principal — never on accumulated interest. Borrow ₹2 lakh at 12% simple interest and you owe exactly ₹24,000 per year, every year, no matter how long the loan runs.
You will meet simple interest in gold loans, informal/family lending, some vehicle and consumer loans quoted "flat", security deposits, court-ordered payments and short-term business credit. Knowing the SI math protects you: a "1% per month" informal loan is 12% a year — and a "flat rate" car loan at 8% flat is really ~14.5% reducing.
Formula & worked example
Worked example: ₹2,00,000 at 12% p.a. for 3 years → SI = 2,00,000 × 12 × 3 / 100 = ₹72,000; total repayable ₹2,72,000, or ₹2,000/month in interest. The same loan compounded yearly would cost ₹80,986 — ₹8,986 more — which is why lenders prefer compound and borrowers prefer simple.
How to use this simple interest calculator
- Enter the principal, annual rate and time in years (use 0.5 for 6 months via the input box).
- Read the interest, the total payable and the effective monthly cost.
- The last line shows what compounding would cost instead — useful when negotiating loan terms.
Smart tips
- Convert monthly rates before comparing: "2% per month" is 24% per year — higher than most personal loans.
- For flat-rate vehicle loans, the true reducing-balance rate is roughly 1.8× the flat rate. Ask for the APR.
- Put every informal loan in writing with rate, tenure and repayment schedule — this calculator gives you the numbers for the agreement.
- If you lend to friends or family, even a modest 8–10% simple interest keeps the arrangement fair versus your FD alternative.
- For deposits, always prefer compound products — simple interest is for short, fixed obligations, not for growing savings.
Frequently asked questions
Where is simple interest used in real life?
Gold loans, informal personal lending, "flat-rate" vehicle/consumer loans, security deposits, delayed-payment penalties and many court awards. Banks use compound (reducing-balance) interest for regular loans and deposits.
What is the difference between flat rate and reducing rate?
A flat rate charges simple interest on the full original amount for the whole tenure even as you repay monthly, so the effective cost is much higher — an 8% flat car loan equals roughly 14–15% reducing. Always compare loans on the reducing/APR basis.
How do I calculate simple interest for months or days?
Convert time to years: 8 months = 8/12 = 0.667 years; 90 days = 90/365. Type the decimal directly into the years box — the sliders snap to whole years, the input accepts decimals.
Is simple or compound interest better?
As a borrower, simple is cheaper; as a saver, compound is better. That single asymmetry is most of personal finance.
What monthly rate equals 12% yearly simple interest?
Exactly 1% per month (12 ÷ 12). Simple interest scales linearly, so per-month and per-year quotes convert by plain division — unlike compound rates.
Want the theory behind the numbers? Read our money basics guides on the Money Blog.