Simple Interest Calculator

Simple interest is interest calculated only on the amount you started with — the principal — for the whole duration. It never earns interest on itself.

That one sentence is the whole idea, and everything else follows from it. Because the base never changes, the amount added is the same every year: ₹1,00,000 at an assumed 10% adds ₹10,000 in the first year, ₹10,000 in the fifth and ₹10,000 in the twentieth. Plot it and you get a straight line, not a curve.

Compare that with compoundinterest, where each year’s interest is added to the balance and goes on to earn interest itself. Same principal, same rate — different rule, and over a long period a very different number.

Simple interest is a mathematical convention, not a product. Some loans and deposits are calculated this way; many are not. Knowing which applies is a question about a specific contract, and this page is about the arithmetic.

Four quantities, and each does one job.

• Principal (P). The original amount. In simple interest this number never changes — it is the base for every calculation, in every period.
• Rate (R). The annual percentage applied to the principal. Always per year unless stated otherwise, so a rate quoted for a different period has to be converted first.
• Time (T). The duration in years. A duration in months must be divided by 12 before it goes into the formula.
• Interest (SI). What the three above produce. This is the interest alone — not the total.
• Total amount (A). Principal plus interest. Also called the maturity value. Confusing this with the interest is the most frequent misreading of any interest figure.

Take ₹10,000 at an assumed 8% for 3 years:

1Year 1   interest = 10,000 × 8 ÷ 100 = ₹800   balance for the next calculation: still ₹10,0002Year 2   interest = 10,000 × 8 ÷ 100 = ₹800   still ₹10,0003Year 3   interest = 10,000 × 8 ÷ 100 = ₹800   still ₹10,0004 5Total interest  ₹2,4006Total amount    ₹12,400

The third column is the point. The base stays at ₹10,000 the whole way through, which is why the interest column repeats. Under compound interest that column would grow, because year two would be charged on ₹10,800.

1        P × R × T2SI  =  ───────────3           1004 5SI  =  simple interest — the interest alone6P   =  principal — the original amount7R   =  annual interest rate, as a percentage (10 means 10%)8T   =  time in years9 10A  =  P + SI            the total amount, or maturity value

The ÷ 100 is there because R is written as a percentage. If you would rather work with a decimal rate, use r = R ÷ 100 and the formula becomes SI = P × r × T — the same thing, with the division moved. This calculator uses the percentage form throughout, so there is one representation and no chance of applying the ÷ 100 twice.

For durations in months, convert first: T = months ÷ 12. So the whole thing becomes SI = P × R × months ÷ 1200, since 1200 is 100 × 12. That is exactly what this tool computes — deferring the only division to the end, so a duration like 7 months never passes through a repeating decimal on its way to the answer.

The formula has four quantities, so any three of them fix the fourth. Rearranged:

1P  =  SI × 100 ÷ (R × T)      the principal you would need2R  =  SI × 100 ÷ (P × T)      the rate you would need3T  =  SI × 100 ÷ (P × R)      the time you would need

All three are in the Solve tab above. Each refuses rather than guesses when its divisor is zero — at 0% there is no principal that reaches a target, and saying so is more useful than printing infinity.

1. 1. Enter the principal. The original amount. The field accepts 100000, ₹1,00,000 and shorthand like 1L.
2. 2. Enter the annual interest rate. As a percentage per year. Decimals are fine — 7.25% works.
3. 3. Enter the duration. In years or months; switch the unit and the conversion is handled for you.
4. 4. Read the interest. The middle card is the interest alone — what the principal earned or cost.
5. 5. Add it to the principal for the total.The third card. This is the maturity value, and it is the figure people usually mean when they ask “how much will I have?”

By hand it is three multiplications and a division. The value of doing it here is everything around the answer: the schedule that shows the interest repeating, the chart that shows the straight line, and the comparison against compounding.

The standard worked example, in full.

1P = 10,0002R = 103T = 54 5        10,000 × 10 × 5      5,00,0006SI  =  ─────────────────  =  ─────────  =  ₹5,0007              100               1008 9A  =  10,000 + 5,000  =  ₹15,000

So the interest is ₹5,000 and the total amount is ₹15,000. Total growth is 50% — which is 10% × 5 years, and is not an annual figure.

Year by year, it looks like this:

1Year   Interest   Cumulative   Total2  1      ₹1,000       ₹1,000   ₹11,0003  2      ₹1,000       ₹2,000   ₹12,0004  3      ₹1,000       ₹3,000   ₹13,0005  4      ₹1,000       ₹4,000   ₹14,0006  5      ₹1,000       ₹5,000   ₹15,000

The interest column repeats. That is not a rounding artefact — it is the definition of simple interest, written out.

The textbook figure, because the arithmetic is small enough to check in your head. At an assumed 5% for one year, ₹1,000 earns ₹50, for a total of ₹1,050.

Halve the duration and you halve the interest: six months gives ₹25. Double the rate to 10% and you double it: ₹100 for the year. Every input behaves the same way, because simple interest is linear in all three of them.

Enter ₹1,000 above with any rate and duration to see it worked through — the preset button does it in one tap.

At an assumed 10% for 5 years, ₹10,000 earns ₹5,000 — a total of ₹15,000. At 8% for 3 years it earns ₹2,400.

For a duration in months, remember the conversion: 18 months is 1.5 years, so ₹10,000 at 10% earns ₹1,500. Treating 18 as 18 years — which the formula will happily do if you feed it that — gives ₹18,000, and it is the commonest mistake with this calculation.

The most-searched figure of the lot. At an assumed 10% for 5 years, ₹1,00,000 earns ₹50,000, for a total of ₹1,50,000. At 7% for 3 years it earns ₹21,000.

Over ten years at the same 10%, the interest reaches ₹1,00,000 — the money doubles. Worth noting what compounding would have done over the same period at the same rate: about ₹2,59,374 in total, or ₹59,374 more. That gap is the whole argument for understanding which convention applies to a given product.

These are calculations, not returns. Nothing here says any deposit or investment pays 10%, and this tool has no way to know what any of them pay.

At an assumed 12% for 10 years, ₹5,00,000 earns ₹6,00,000 — a total of ₹11,00,000.

Notice that the interest exceeds the principal. That happens whenever rate × years passes 100, which at 12% takes a little over eight years. It is a useful sanity check in both directions: as a saver it is the point where the calculation is worth more than the original sum, and as a borrower it is the point where you have paid more in interest than you borrowed.

At an assumed 8% for 10 years, ₹10,00,000 earns ₹8,00,000, for a total of ₹18,00,000.

The growth percentage here — 80% — is identical to what ₹10,000 would show at the same rate and duration. The percentage never depends on the size of the principal, only on the rate and the time. That is worth internalising: when comparing two scenarios, the rupee figures scale with the amount, and only the rate and duration change the shape of the answer.

The difference is one rule: whether interest is added to the balance and goes on to earn interest itself. Everything else follows.

These answer different questions, and treating them as interchangeable is how a result gets overstated by a factor of five.

A simple interest rate says how much is added each year relative to the original principal. CAGR — compound annual growth rate — says what constant compounded rate would take a starting value to an ending value. And total growth is neither: it is the whole-period change, with no annual dimension at all.

Simple interest describes a mathematical relationship. Whether a particular loan uses it is a fact about that loan’s contract, and most loans in India do not.

Home loans, car loans and most term loans are calculated on a reducing balance: interest is charged each month on what is still outstanding, and the outstanding amount falls with every instalment. That is a different calculation, and it is what our EMI calculator and home loan EMI calculator handle.

Where simple interest does show up in lending is the flat rate: interest charged on the full original amount for the whole tenure, even as the loan is repaid. Arithmetically that is simple interest, and it is why a flat rate and a reducing-balance rate of the same number are not comparable — the flat one costs considerably more, because you go on paying interest on money you have already returned.

Some short-term and informal borrowing is genuinely simple interest. If a lender quotes you a rate, the question worth asking is not just “how much?” but “on what balance?”

As a mathematical model, simple interest is the easiest way to see what a rate is worth: the same amount added every year, no acceleration, no surprises.

As a description of a savings product, it is usually incomplete. Most savings accounts and deposits compound at a stated frequency, and some pay interest out rather than reinvesting it — which is closer to simple interest from the account holder’s point of view, because the interest leaves and stops earning.

This calculator will not tell you what any account pays or how it credits interest. It has no rate feed and no connection to any institution. Use it to understand the mechanics, and read the terms of a specific product for what that product actually does.

Fixed deposits vary, and the variation matters more than most people expect. Two deposits at the same headline rate can produce visibly different amounts depending on how interest is treated.

• Cumulative deposits reinvest the interest, so it compounds — commonly quarterly, though the frequency is set by the product.
• Non-cumulative deposits pay the interest out at intervals. The money leaves and stops earning, which behaves like simple interest as far as the deposit is concerned.
• Short tenures — under a year — are often quoted on a simple basis, because there is little or nothing to compound.

Do not assume every fixed deposit uses simple interest.Many do not. Check the specific product’s terms, including the compounding frequency, whether interest is paid out or reinvested, what happens on premature withdrawal, and how interest income is taxed — none of which this calculation models.

Interest and inflation measure different things, and a positive interest figure does not mean your purchasing power grew.

If a calculation shows 6% a year while prices also rise around 6%, the money buys roughly what it did before — despite the larger number on screen. The rate you entered is a nominal rate; what matters for what you can actually buy is the real return, which is roughly the nominal rate minus the inflation rate.

1nominal rate      6.0%2inflation        -6.0%3                 ─────4real return       0.0%   → the same purchasing power, a year later

This is worth doing before treating any interest figure as a gain, and it is the reason a low-rate calculation over a long period can look impressive in rupees while being flat in real terms. This calculator does not adjust for inflation and makes no claim about future price levels.

• Forgetting to convert months into years. The formula takes years. Entering 18 where 1.5 belongs turns ₹1,500 into ₹18,000 — a twelve-fold error, and by far the commonest one. Use the months unit above and the conversion is done for you.
• Confusing the interest with the total amount. ₹50,000 of interest on ₹1,00,000 means ₹1,50,000 at the end, not ₹50,000. The two are shown in separate cards above for exactly this reason.
• Dividing by 100 twice. If you convert the rate to a decimal (r = R ÷ 100) then also divide the result by 100, the answer comes out a hundred times too small. Pick one representation and stay in it.
• Applying compound logic to the simple formula.Adding each year’s interest to the principal and recalculating is compound interest. If the interest column grows year on year, you are not doing simple interest.
• Reading total growth as an annual rate. 50% over five years is 10% a year simple, and 8.45% a year compounded. Quoting the 50% as annual overstates it several times over.
• Assuming every loan or deposit uses simple interest. Most substantial loans in India are reducing-balance, and many deposits compound. The convention is a fact about the contract.
• Ignoring fees and taxes. The formula knows nothing about processing charges, penalties for early withdrawal, or tax on interest income. Any of them can change the outcome materially.
• Treating a calculated figure as a guaranteed return. This is arithmetic on numbers you typed. It is not a projection, not an offer, and not a prediction of what any product will pay.