Simple vs Compound Interest: What's the Difference?
Same rate, same principal, same years — and a Rs 2.7 lakh gap. Here's where it comes from.
Simple interest is calculated only on the original principal (SI = P × R × T / 100). Compound interest is calculated on the principal plus all accumulated interest (A = P(1 + r/n)^(nt)), so you earn interest on your interest. On Rs 5,00,000 at 8% for 20 years, simple interest returns Rs 13 lakh while annual compounding returns Rs 23.3 lakh — a gap of over Rs 10 lakh from the same rate.
| Years | Simple interest total | Compound interest total | Gap |
|---|---|---|---|
| 1 | Rs 5.40 lakh | Rs 5.40 lakh | Rs 0 |
| 5 | Rs 7.00 lakh | Rs 7.35 lakh | Rs 0.35 lakh |
| 10 | Rs 9.00 lakh | Rs 10.79 lakh | Rs 1.79 lakh |
| 15 | Rs 11.00 lakh | Rs 15.86 lakh | Rs 4.86 lakh |
| 20 | Rs 13.00 lakh | Rs 23.30 lakh | Rs 10.30 lakh |
| 30 | Rs 17.00 lakh | Rs 50.31 lakh | Rs 33.31 lakh |
The gap is invisible early and enormous late
Read the last column top to bottom. In year 1 the two are identical — there's no accumulated interest to compound yet. By year 10 compounding is ahead by Rs 1.79 lakh. By year 30 it's ahead by Rs 33 lakh, roughly double the entire simple-interest return.
This is why compounding gets described as magic when it's just arithmetic being patient. The curve is nearly flat for years and then bends sharply upward — and most of the benefit lands in the final third. That's also why starting to invest at 25 rather than 35 matters far more than the rate you pick: you're buying the steep part of the curve.
Where each one actually shows up
Simple interest is rarer than people think:
- Most car and personal loans quoted on a “flat rate”
- Short-term informal or bridging loans
- Some fixed deposits that pay out interest rather than reinvesting it
Compound interest is nearly everything else — PPF, cumulative FDs, mutual funds, EPF, home loans, and credit card debt.
A warning on flat rates: a car loan advertised at a “7% flat rate” is not 7%. Flat rate charges interest on the full original principal for the entire tenure, even as you repay it. The effective reducing-balance rate is roughly 1.8–1.9× the flat rate — so 7% flat is about 13% real. Always ask for the reducing-balance rate and compare that.
Compounding frequency matters less than you'd expect
Rs 5,00,000 at 8% for 20 years:
- Annually: Rs 23.30 lakh
- Quarterly: Rs 24.35 lakh
- Monthly: Rs 24.60 lakh
- Daily: Rs 24.73 lakh
Moving from annual to daily compounding adds about 6% over 20 years. Meaningful, but small next to what time does — adding 10 more years more than doubles the return.
There's a mathematical ceiling here: continuous compounding gives Rs 24.73 lakh, so daily is already within a rupee or two of the theoretical maximum. Chasing compounding frequency is optimising the wrong variable. Chase time in the market instead. Model it in the compound interest calculator.
Tools used in this guide
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal; compound interest is charged on the principal plus all interest earned so far, so you earn interest on interest. On Rs 5,00,000 at 8% for 20 years, simple interest yields Rs 13 lakh while annual compounding yields Rs 23.3 lakh. In year 1 they're identical — the gap only opens as accumulated interest builds.
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. Compound interest alone is A − P. For Rs 5,00,000 at 8% compounded annually for 20 years: 500000 × (1.08)^20 = Rs 23.30 lakh, of which Rs 18.30 lakh is interest.
Is a 7% flat rate car loan actually 7%?
No — it's closer to 13%. A flat rate charges interest on the full original principal for the entire tenure, even though you're repaying it monthly and your outstanding balance is falling. The equivalent reducing-balance rate is roughly 1.8–1.9 times the flat rate. Always ask lenders for the reducing-balance rate, since that's the only figure comparable across loans.
Does daily compounding make a big difference?
Less than most people assume. Rs 5,00,000 at 8% for 20 years gives Rs 23.30 lakh compounded annually and Rs 24.73 lakh compounded daily — about 6% more. Daily is already within a hair of the continuous-compounding maximum, so there's little left to gain. Time matters far more: adding 10 years to the same investment more than doubles the return.
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