Simple Interest Calculator

Interest that never compounds

Borrow ₹50,000 at 8% for three years under simple interest and you pay ₹12,000 — exactly ₹4,000 a year, three times. The total repayable is ₹62,000. Under compound interest the same loan would cost ₹12,986, because each year's interest would join the balance and start earning interest itself.

The ₹986 gap looks trivial here, and over three years at a modest rate it is. Stretch the same figures to twenty years and simple interest charges ₹80,000 while compounding charges ₹1,83,048 — more than twice as much. Time is what separates the two, which is why simple interest survives mainly on short-dated arrangements.

Simple interest is calculated only on the original principal, for the entire period, regardless of how much interest has accrued. That single property makes it linear: the same amount is added every year, forever.

The formula

Simple interest = P × R × T ÷ 100

Total amount = P + Simple interest

• P — the principal, the original sum
• R — the annual rate as a percentage
• T — time in years

Working the example: 50,000 × 8 × 3 ÷ 100 = ₹12,000. Total: ₹62,000.

If your period is in months, convert to years first — eighteen months is 1.5, not 18. Using months directly is far and away the most common arithmetic error here, and it inflates the answer twelvefold.

Where it is actually used

Simple interest is rarer than compound interest in modern finance, but it is not obsolete:

Short-term and informal lending. Loans between individuals, and many moneylender arrangements, are quoted as simple interest per month.

Car and consumer loans quoted "flat". A flat rate applies interest to the original principal for the whole tenure even though you are repaying monthly. This is why a flat 9% is roughly equivalent to a reducing-balance rate of 16–17% — you keep paying interest on money you have already returned.

Some fixed deposits paying out interest. A non-cumulative FD that pays interest to your account quarterly effectively behaves as simple interest on the deposit, since nothing is retained to compound.

Certain government savings schemes and post-office instruments, and some bond coupon structures.

Penalty and delay interest, where statutes frequently specify simple interest on an overdue amount.

Reading the result

For a deposit, simple interest is the conservative case — you will do better with any instrument that compounds, so a simple-interest return should be compared against compound alternatives before accepting it.

For a loan, the critical question is whether the quoted rate is simple, flat or reducing-balance, because the same headline number means wildly different costs:

• Reducing balance — interest on what you still owe. This is what home loans and most bank loans use, and it is the cheapest.
• Flat or simple — interest on the original amount throughout. Roughly 1.8–1.9 times more expensive than the same reducing-balance rate.

If a lender quotes a rate that seems unusually low, ask explicitly which basis it uses. A "9% flat" car loan and a "9% reducing" car loan differ by a large margin, and the flat quote is legal, common, and rarely explained.

Common mistakes

Entering months as years. Eighteen months is 1.5 years.

Assuming a low simple rate is a good deal. Convert flat to reducing-balance before comparing — roughly, multiply the flat rate by about 1.8 for a typical tenure.

Using simple interest for an EMI loan. An amortising loan needs the EMI formula; interest is charged monthly on a falling balance.

Forgetting tax. Interest received is taxable at your slab rate, exactly as with any deposit interest.

Applying it to long horizons. Over a decade or more, simple interest so badly understates real-world growth or cost that the answer is misleading rather than merely approximate.