Simple vs. compound interest, explained
Why the same rate can produce very different results over time.
Quoted the same 8% rate, a simple-interest instrument and a compound-interest one produce identical results for exactly one year — and diverge more with every year after that. The difference isn’t a rounding quirk; it’s two fundamentally different formulas that happen to agree at a single point.
Two different formulas
- Simple interest is calculated only on the original principal, every year, for the whole term:
Interest = P × R × T / 100. Whether it’s year 1 or year 20, the interest earned that year is always the same flat rupee amount — growth is a straight line. - Compound interest is calculated on the principal plus whatever interest has already accumulated:
Amount = P × (1 + R/100)^T. Each year’s interest is itself added to the base the next year’s interest is calculated on — growth curves upward, not a straight line.
That one difference — whether interest earns further interest, or just sits alongside the principal doing nothing — is the entire distinction. Everything else about the two formulas follows from it.
The gap widens the longer you wait
₹1,00,000 at 8% for 10 years, using the real computeSimpleInterestformula: simple interest totals ₹1,80,000 (₹80,000 interest, a flat ₹8,000 every single year). Compound interest on the identical principal and rate totals ₹2,15,892 (₹1,15,892 interest) — ₹35,892 more, purely from interest earning interest on top of interest.
| Years | Simple interest total | Compound interest total | Gap |
|---|---|---|---|
| 5 | ₹1,40,000 | ₹1,46,933 | ₹6,933 |
| 10 | ₹1,80,000 | ₹2,15,892 | ₹35,892 |
| 20 | ₹2,60,000 | ₹4,66,096 | ₹2,06,096 |
| 30 | ₹3,40,000 | ₹10,06,266 | ₹6,66,266 |
Simple interest grows in a straight line — ₹8,000 more every year, no matter how long the money sits. Compound interest grows on a curve that gets steeper the longer it runs, because the base it’s calculated on keeps growing too. By year 30, compounding hasn’t just added a bit more — the same ₹1,00,000 principal has produced almost 3x as much total interest as simple interest did, at the identical 8% rate.
See how compounding grows a deposit at different rates and frequencies.
Work out interest on a non-compounding loan or deposit quickly.
Where each actually shows up
Most everyday savings and investment products — fixed deposits, PPF, EPF, mutual funds — compound. Simple interest shows up mainly in a handful of short-term loan products and a few older, specific instruments. When comparing two products’ quoted rates, check which convention each one actually uses before assuming the higher headline number wins — a compounding 7% can end up paying more than a simple-interest 8% over a long enough term.
The formula matters most exactly where it’s easiest to overlook: long tenures. Over a single year the two are nearly identical; over 20–30 years — the horizon most retirement and long-term goal planning actually runs on — the difference compounds (quite literally) into hundreds of thousands of rupees on the same starting amount.
All figures are indicative and for educational purposes only — not financial advice.
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