Finance · 6 min read
Compound interest and SIPs: how small monthly amounts become big
Einstein supposedly called compound interest 'the eighth wonder of the world'. The boring truth is: it's just math. But the math has surprising consequences — small amounts compounded for long periods beat large amounts compounded for short periods, by a lot. Here's the intuition, plus numbers you can plug into your own situation.
By UltraConvert Editorial · Published · Updated
Simple vs compound: what's the difference
Simple interest: you earn interest only on the principal. ₹100,000 at 10%/year for 5 years earns ₹50,000 — total ₹150,000. Compound interest: you earn interest on the principal AND on previously-earned interest. Same ₹100,000 at 10%/year compounded annually for 5 years earns ₹61,051 — total ₹161,051. Tiny difference at 5 years. Massive difference at 30 years (₹100,000 → ₹1,744,940 vs ₹400,000 simple).
The compounding frequency trap
Banks advertise 'X% per annum' but compound monthly, quarterly, or daily. The effective annual rate is higher than the headline. At 12% nominal compounded monthly, the effective rate is ~12.68%. Daily compounding at 12% gets you ~12.75%. The differences are small percentages but they add up over decades — daily-compounded ₹100,000 at 12% over 30 years beats annually-compounded by ~5% in final value.
Why SIPs work better than lump sums
A Systematic Investment Plan (SIP) — investing a fixed amount every month — does two things. First, it forces consistency: you don't try to time the market. Second, it averages your purchase price (rupee-cost averaging) across high and low months. ₹10,000/month over 20 years at 12% returns is worth ~₹1.0 crore. The same ₹2.4 crore total deposited as a single ₹2.4-crore lump sum 20 years ago would actually be worth more (~₹26 crore at 12%) — but most people don't have ₹2.4 crore lying around. SIPs make wealth-building accessible.
Step-up SIPs: the trick most people miss
Most people start a SIP and forget about it. But your salary grows ~10%/year. Increasing your SIP amount by even 5%/year (a 'step-up SIP') massively compounds the final corpus. ₹10,000/month flat for 20 years at 12% → ₹1.0 crore. ₹10,000/month with 5% annual step-up → ₹1.7 crore. Same starting amount, 70% more wealth.
Inflation-adjust everything
₹1 crore in 30 years won't buy what ₹1 crore buys today. At 6% inflation, ₹1 crore today is worth only ~₹17 lakhs in 30 years' purchasing power. Whenever you see 'in 20 years you'll have ₹X', mentally divide by ~3 to see the real-money value. Our compound-interest calculator does this automatically — it shows both nominal and inflation-adjusted future value side by side.
What rate to assume
Long-term equity returns in India have averaged ~12% nominal (~6% real after inflation). Bond/debt returns are ~7-8% nominal (~1-2% real). A diversified portfolio (60% equity, 40% debt) is ~10% nominal long-term. Don't plan on more than 12% — back-of-envelope estimates that assume 15%+ returns are basically gambling.