How simple interest works
Simple interest is charged only on the original principal. Nothing earns interest on interest, so the amount grows by exactly the same figure every year — which makes it easy to calculate and, for a borrower, easy to misjudge.
Formula
Simple interest = (P × R × T) ÷ 100
- P — principal amount
- R — annual rate of interest in percent
- T — time in years
- Total amount = P + simple interest
How it compares with compounding
₹2 lakh at 8% for three years earns ₹48,000 as simple interest. Compounded quarterly, the same deposit earns about ₹53,648 — a difference of ₹5,648 that grows quickly with time.
Over ten years the gap widens sharply: ₹1.60 lakh of simple interest against roughly ₹2.42 lakh compounded quarterly on the same ₹2 lakh. Time is what turns a small difference into a large one.
Where simple interest is actually used
You will meet it in short-term personal borrowing, some gold and agricultural loans, bank penalty and delay calculations, and non-cumulative deposits where interest is paid out rather than reinvested.
It is also the basis of the flat rate quoted by many vehicle and consumer-durable financiers — which is where the trouble starts.
The flat-rate trap for borrowers
A flat rate charges interest on the full original loan for the entire tenure, even though your EMIs are steadily reducing the balance. A 6% flat rate on a three-year loan works out to roughly 11-12% on a reducing balance.
Since banks quote reducing-balance rates and dealers often quote flat rates, the two are not comparable. Always ask for the reducing-balance rate or the total amount payable before signing.
When simple interest favours you
As a borrower on a short, genuinely simple-interest loan you pay less than you would under compounding, because unpaid interest never joins the principal.
As a saver it is the weaker arrangement. For any deposit you intend to hold for years, a cumulative compounding option is worth more at the same headline rate.
Example: ₹2 lakh at 8%
- Principal
- ₹2,00,000
- Rate
- 8% p.a.
- Simple interest for 3 years
- ₹48,000
- Total after 3 years
- ₹2,48,000
- Compounded quarterly instead
- ≈ ₹2,53,648
- Simple interest for 10 years
- ₹1,60,000
Simple interest adds ₹16,000 a year, every year, without acceleration. Compounding starts at the same point and pulls steadily ahead.