Simple vs Compound Interest: the Difference, with One Example

Simple interest pays you on your principal only. Compound interest pays you on your principal plus all the interest you have already earned — and that one difference is most of what people mean by 'making money work for you'.

Try the Simple Interest Calculator →

The formulas

Simple interest: SI = P × R × T ÷ 100 — the same rupee amount of interest every year.

Compound interest: A = P × (1 + R/100)^T for annual compounding — each year's interest joins the base for the next year.

Same numbers, ten years apart

Invest ₹1,00,000 at 8% for 10 years:

Simple interest: ₹8,000 every year → ₹80,000 total. Final amount ₹1,80,000.
Compound (annual): final amount ≈ ₹2,15,892 — interest of ₹1,15,892, about 45% more.
Stretch it to 20 years and compounding earns ₹3.66 lakh vs ₹1.6 lakh — the gap accelerates with time.

Compounding frequency and the rule of 72

More frequent compounding earns slightly more: 8% compounded quarterly is an effective 8.24% per year, which is why bank FDs (which compound quarterly) quote both a nominal rate and a higher annualised yield.

The rule of 72 is the quick mental check: divide 72 by the annual rate to estimate doubling time. At 8%, money doubles in about 9 years; at 12%, about 6.

Where each shows up in real life

Simple interest: some short-term deposits and bonds, certain personal/gold loans, most penalty interest.
Compound interest working for you: fixed deposits, reinvested mutual funds and SIPs, PPF/EPF.
Compound interest working against you: credit card balances and unpaid loan interest — the same acceleration, in reverse.

Frequently asked questions

What is the rule of 72?

A quick estimate of doubling time: 72 ÷ annual rate ≈ years to double. At 8% your money doubles in roughly 9 years under compound interest.

Does compounding frequency really matter?

Somewhat — 8% compounded quarterly is an effective 8.24% annually. Frequency matters less than the rate and far less than time invested.

Are SIP returns compound interest?

Mutual fund returns are market-linked, not a fixed rate, but growth is reinvested so the compounding effect applies — which is why SIP projections use the compound formula.