What Is Simple Interest? The Formula and the Flat-Rate Trap

Search for simple interest and you land in a maths class. Of the pages ranking for it, nearly all are school and exam sites: BYJU'S, Cuemath, GeeksforGeeks, Testbook, Unacademy. They teach the formula properly, work through Rs 5,000 invested for 2 years, and set practice questions. Almost none of them tells you where simple interest turns up in a real loan agreement, which is a shame, because that is the one place it can quietly cost you a year's savings.

A Rs 1,00,000 personal loan at "10% flat" over three years charges Rs 30,000 in interest. The same Rs 1,00,000 at 10% on a reducing balance charges Rs 16,162. Same principal, same tenure, same advertised percentage, and Rs 13,838 of difference.

That gap is simple interest doing exactly what it is defined to do.

What is simple interest?

Simple interest is interest calculated only on the original amount of money, called the principal, and never on interest that has already been added. The amount you earn or owe each year stays flat, because the base it is worked out on never moves.

Put Rs 50,000 into something paying 8% simple interest for three years. Year one earns Rs 4,000. So does year two. So does year three. Total interest is Rs 12,000, and you finish with Rs 62,000. Nothing accelerates, because the Rs 4,000 you earned in year one sits to one side and never joins the pot that is earning.

That last point is the entire concept. Everything else is arithmetic.

How do you calculate simple interest?

Simple interest is worked out by multiplying the principal by the rate by the time, which Indian textbooks write as SI = P x R x T / 100 and American ones write as I = P x r x t. Both give the same answer. The Indian version writes the rate as a whole number and divides by 100 at the end; the American version writes the same rate as a decimal and skips the division.

Run the Rs 50,000 example through each:

Convention	Working	Result
India: SI = P x R x T / 100	50,000 x 8 x 3 / 100	Rs 12,000
US: I = P x r x t	50,000 x 0.08 x 3	Rs 12,000

Add the interest back to get the total, which is written A = P + SI. Here that is Rs 62,000.

Two things trip people up. Time has to be in years, so a nine-month deposit is 0.75 and not 9. And the rate has to match the period, so an annual rate needs an annual time figure. Neither is deep, but both account for most wrong answers.

How do you find the rate, time or principal?

The same formula rearranges to solve for whichever value is missing, because SI = P x R x T / 100 has four quantities and any three of them give you the fourth. Textbooks tend to teach only the forward version, which is why people get stuck the moment the question flips.

All three rearrangements come from moving one term across:

To find	Rearranged formula	Using the same numbers
Rate (R)	R = SI x 100 / (P x T)	12,000 x 100 / (50,000 x 3) = 8%
Time (T)	T = SI x 100 / (P x R)	12,000 x 100 / (50,000 x 8) = 3 years
Principal (P)	P = SI x 100 / (R x T)	12,000 x 100 / (8 x 3) = Rs 50,000

Every row above uses the one deposit from earlier, Rs 50,000 at 8% for three years earning Rs 12,000, so you can see each version landing back on a figure you already know. That's the quickest way to check you've rearranged it correctly: feed a solved example back in and see whether the answer you get is the one you started with.

One case needs a small extra step. If you know the total amount but not the interest, subtract first, because A = P + SI means SI = A minus P. Someone who repaid Rs 62,000 on a Rs 50,000 loan paid Rs 12,000 in interest, and only then can the rate formula do its work.

Where does simple interest actually show up?

In India the main real-world home of simple interest is the flat-rate loan, where interest is charged on the full original amount for the whole tenure even as you repay it every month. Vehicle dealers, consumer finance companies and smaller lenders quote this way. The number sounds low, which is the point.

The mechanics are worth being slow about. You borrow Rs 1,00,000 for three years. You start repaying immediately, so by month 18 you might owe around half of it. A reducing balance loan notices that and charges interest on what is left. A flat-rate loan does not. It charges interest on Rs 1,00,000 in month 35, when you owe almost nothing.

Simple interest also turns up in short informal loans, in bank and post-office interest that gets paid out to you each period and never reinvested, and in the way a bond coupon works.

The American usage is different enough to cause real confusion. A "simple interest loan" there usually means a car loan, personal loan or student loan where interest accrues daily on the outstanding balance. That is closer to what India calls reducing balance. Same two words, different machinery. Anyone comparing an Indian offer with an American article is comparing two things that share a name.

Why does a 10% flat rate cost more like 17.9%?

A flat rate roughly doubles because it charges you for money you have already given back. Converting one to the other means asking a single question: what reducing balance rate would produce the same monthly payment?

Take the Rs 1,00,000 loan at 10% flat over three years. Interest is 1,00,000 x 10 x 3 / 100, or Rs 30,000. Total repayable is Rs 1,30,000, spread over 36 months, so the EMI is Rs 3,611. Now work backwards: the reducing balance rate that produces an EMI of Rs 3,611 on Rs 1,00,000 over 36 months is 17.9%.

Rs 1,00,000 over 3 years	10% flat	10% reducing
Monthly EMI	Rs 3,611	Rs 3,227
Total interest	Rs 30,000	Rs 16,162
Total repaid	Rs 1,30,000	Rs 1,16,162
Real annual rate	17.9%	10%

The two loans in that table advertise the same number. One costs 86% more in interest than the other. Our simple interest calculator runs this conversion on your own figures, and it solves the formula in all four directions if what you're missing is the rate, the time or the principal.

Does the "multiply by 1.8" rule of thumb hold?

The usual shortcut of multiplying a flat rate by about 1.8 works on short loans and overstates the cost on long ones, because the multiplier falls as the tenure stretches. Most pages quoting the shortcut give a single figure and leave it there. It moves.

Running the same 10% flat rate across different tenures:

Tenure	Real annual rate	Multiplier
1 year	18.0%	1.80x
2 years	18.2%	1.82x
3 years	17.9%	1.79x
5 years	17.3%	1.73x
7 years	16.7%	1.67x

It peaks around the two-year mark and drifts down from there. On a seven-year loan, using 1.8x would have you believe the rate is 18% when it is nearer 16.7%. Still close enough to be useful as a mental check, and wrong enough that it is worth knowing which direction the error runs.

What does RBI actually require?

The claim that RBI mandates reducing balance for every lender is repeated widely and does not appear in the Master Direction on Interest Rate on Advances, 2016 in those terms. What that Direction says, at paragraph 4(a)(vii), is that interest shall be charged on all advances at monthly rests. That fixes how often interest is applied. It is not the same as naming a calculation method, and I could not find a clause in it that does.

The rule that genuinely drags a flat rate into the light is newer. Since 1 October 2024, under the RBI circular of 15 April 2024, regulated lenders must give retail and MSME borrowers a Key Facts Statement carrying an all-in annual percentage rate, together with the computation sheet and a full amortisation schedule. A flat-rate quote can hide inside a brochure. It cannot hide inside an amortisation schedule, because the schedule shows exactly how much you still owe each month while the interest carries on being charged on the original figure.

Worth being precise about who this binds. The KFS regime covers banks and NBFCs including housing finance companies. A vehicle dealer arranging finance in the showroom sits in a murkier place, and that is where flat quoting has lasted longest. Our APR vs interest rate explainer goes through what the KFS must contain and what India's disclosure rules leave out.

How is simple interest different from compound interest?

Simple interest is calculated on the principal alone, while compound interest is calculated on the principal plus all the interest added so far, so compound interest grows and simple interest does not. Over one year at the same rate they are identical. Over thirty, they are not remotely comparable.

On Rs 1,00,000 at 8% held for ten years, simple interest earns Rs 80,000 and compound interest earns Rs 1,15,892. Stretch it to thirty years and the gap reaches Rs 6,66,266.

The asymmetry is worth sitting with. Simple interest is better for you when you are borrowing and worse when you are saving, which is roughly the opposite of how the two are usually presented in a maths class, where compound interest is introduced as the more advanced and therefore better one.

The two are compared properly, with the year-by-year gap and the shortcut formula for computing the difference, in simple vs compound interest. For the other side on its own, see compound interest explained, and for how a rate gets set in the first place, what an interest rate is.

What this post deliberately does not cover

This explains what simple interest is, how it is calculated, and how a flat-rate quote converts into a real annual rate. It isn't advice on which loan to take, which lender to use, or whether a particular offer is reasonable for you. Those depend on your income, your other borrowing and what you are buying.

It also doesn't cover amortisation schedule construction, day-count conventions such as 30/360 and actual/365 that change short-period interest slightly, penal charges, prepayment mechanics, or the American precomputed-interest loan, which is a distinct product from the American simple interest loan and behaves differently when you pay early.

On sourcing: we calculated every flat-to-reducing conversion in this post ourselves, by solving for the reducing balance rate that reproduces the flat loan's EMI, and you can check any of them against the Key Facts Statement your lender must give you. Where a quote confuses you, the lender is obliged to hand over the computation sheet, and a chartered accountant or the lender's own grievance channel is the right place to press.

Frequently asked questions

What is simple interest?

Simple interest is interest calculated only on the original amount of money, called the principal, and never on interest that has already been added. If you deposit Rs 50,000 at 8% simple interest for 3 years, you earn Rs 4,000 in year one, Rs 4,000 in year two and Rs 4,000 in year three, for Rs 12,000 in total. The yearly figure never changes because the base it is calculated on never changes. That is the whole difference from compound interest, where each year's interest joins the principal and starts earning on its own.

What is the formula for simple interest?

The Indian textbook formula is SI = P x R x T / 100, where P is the principal, R is the rate per year written as a whole number, and T is the time in years. The American version is I = P x r x t, where r is the same rate written as a decimal. Both produce identical answers: Rs 50,000 at 8% for 3 years gives 50000 x 8 x 3 / 100 = Rs 12,000, and 50000 x 0.08 x 3 = Rs 12,000. To get the total repayable, add the interest to the principal.

How do you find the rate, time or principal in simple interest?

Rearrange the same formula, because SI = P x R x T / 100 has four quantities and any three give you the fourth. To find the rate, R = SI x 100 / (P x T). To find the time, T = SI x 100 / (P x R). To find the principal, P = SI x 100 / (R x T). Checking them against one known example keeps you honest: a Rs 50,000 deposit at 8% for 3 years earns Rs 12,000, and all three rearrangements land back on 8%, 3 years and Rs 50,000. If you know the total amount instead of the interest, subtract the principal first, since A = P + SI.

What is a flat interest rate, and how is it different from a reducing balance rate?

A flat interest rate applies simple interest to the full original loan amount for the entire tenure, even though you are paying the loan down every month. A reducing balance rate charges interest only on what you still owe, which falls with every EMI. On a Rs 1,00,000 loan over 3 years, 10% flat produces Rs 30,000 of interest while 10% reducing produces Rs 16,162. Same principal, same headline number, Rs 13,838 difference, because the flat calculation ignores every repayment you have already made.

What does a 10% flat rate work out to as a real annual rate?

About 17.9% a year on a 3-year loan. Working it out means finding the reducing balance rate that produces the same EMI: Rs 1,00,000 at 10% flat over 36 months gives a total of Rs 1,30,000 and an EMI of Rs 3,611, and the reducing balance rate that produces an EMI of Rs 3,611 is 17.9%. The common rule of thumb is to multiply a flat rate by roughly 1.8, which holds up well on short loans but overstates the gap on long ones.

Does RBI require lenders to use reducing balance instead of flat rates?

Not in the terms usually claimed. The Master Direction on Interest Rate on Advances, 2016 requires that interest on advances be charged at monthly rests under paragraph 4(a)(vii), which sets how often interest is applied rather than mandating a reducing balance method by name. The rule that actually exposes a flat rate is the Key Facts Statement circular of 15 April 2024, which since 1 October 2024 requires regulated lenders to disclose an all-in annual percentage rate along with the computation sheet and an amortisation schedule.

Which loans and investments actually use simple interest?

In India, flat-rate loans from vehicle dealers, consumer finance companies and some smaller lenders use simple interest, and so do many short-tenure informal loans. In the United States, most auto loans, personal loans and student loans are described as simple interest loans, which there means interest accrues daily on the outstanding balance rather than on the full original amount. The two markets use the same words for different mechanics, which is worth knowing before comparing offers across them.

Sources

• Reserve Bank of India, Master Direction on Interest Rate on Advances, 2016 (paragraph 4(a)(vii), interest on advances to be charged at monthly rests) rbi.org.in
• Reserve Bank of India, Key Facts Statement (KFS) for Loans and Advances, RBI/2024-25/18, 15 April 2024 (the all-in annual percentage rate, the computation sheet, the amortisation schedule, and the 1 October 2024 commencement) rbi.org.in
• Consumer Financial Protection Bureau, What's the difference between a simple interest rate and precomputed interest on an auto loan? (the American usage, where simple interest is charged on the outstanding balance daily or monthly, and the contrast with precomputed interest) consumerfinance.gov
• Our own calculations for every flat-to-reducing conversion in this post, produced by solving for the reducing balance rate that reproduces the flat loan's EMI on the same principal and tenure.