You deposit ₹1 lakh in an FD. After 5 years, the bank says your maturity amount is ₹1,41,478. Where did the extra ₹41,478 come from? The answer is compound interest — your interest earning interest, month after month, year after year. This guide explains everything.
Compound interest is interest calculated on both the original principal and the interest already accumulated from previous periods. Every time interest is added to your principal, the new — larger — total becomes the base for the next interest calculation. This creates a snowball effect: the more time passes, the faster your money grows.
The standard compound interest formula is:
Key insight: When n=1 (annual compounding), the formula becomes A = P × (1 + r)^t — the simplest form. Increasing n always increases the maturity amount, but the gains shrink as n gets very large. Monthly vs daily compounding is almost negligible in practice.
All Indian banks must disclose compounding frequency. For the same principal and annual rate, more frequent compounding always means higher returns. Here is how ₹1,00,000 at 7% per year grows over 5 years:
| Compounding | Formula (n) | Maturity | CI Earned | vs Annual |
|---|---|---|---|---|
| Annually | 1 | ₹1,40,255 | ₹40,255 | — |
| Semi-Annually | 2 | ₹1,41,060 | ₹41,060 | +₹805 |
| Quarterly | 4 | ₹1,41,478 | ₹41,478 | +₹1,223 |
| Monthly | 12 | ₹1,41,763 | ₹41,763 | +₹1,508 |
| Daily | 365 | ₹1,41,906 | ₹41,906 | +₹1,651 |
The difference between quarterly and daily is only ₹428 on ₹1 lakh over 5 years — essentially negligible. What matters far more is the interest rate and time period, not chasing daily vs monthly compounding.
Simple interest (SI) calculates interest only on the original principal: SI = P × r × t. Compound interest always outperforms simple interest — and the gap widens dramatically with time.
| Time Period | Simple Interest | Compound Interest (Quarterly) | CI Advantage |
|---|---|---|---|
| 1 year | ₹7,000 | ₹7,186 | +₹186 |
| 3 years | ₹21,000 | ₹23,144 | +₹2,144 |
| 5 years | ₹35,000 | ₹41,478 | +₹6,478 |
| 10 years | ₹70,000 | ₹99,151 | +₹29,151 |
| 20 years | ₹1,40,000 | ₹2,98,372 | +₹1,58,372 |
| 30 years | ₹2,10,000 | ₹7,68,609 | +₹5,58,609 |
P = ₹1,00,000 at 7% per year. The 30-year CI advantage is over ₹5.5 lakh on the same principal and rate.
The Rule of 72 lets you estimate how many years it takes to double your money at compound interest, without any calculator:
| Investment / Scheme | Approx. Rate | Years to Double (Rule of 72) |
|---|---|---|
| Savings Account | 3.5% | 72 ÷ 3.5 ≈ 20.6 years |
| Post Office RD | 6.7% | 72 ÷ 6.7 ≈ 10.7 years |
| SBI FD (5 yr) | 6.5% | 72 ÷ 6.5 ≈ 11.1 years |
| PPF | 7.1% | 72 ÷ 7.1 ≈ 10.1 years |
| HDFC FD (3 yr) | 7.25% | 72 ÷ 7.25 ≈ 9.9 years |
| Equity Mutual Fund (est.) | 12% | 72 ÷ 12 = 6 years |
| Credit Card Debt | 40% | 72 ÷ 40 ≈ 1.8 years (debt doubles!) |
Credit Card Warning: Credit card outstanding at 40% per year doubles in under 2 years. A ₹50,000 unpaid balance becomes ₹1 lakh in less than 24 months if you pay only the minimum due.
| Bank | 1 Year | 3 Years | 5 Years | Senior Citizen Bonus |
|---|---|---|---|---|
| State Bank of India | 6.80% | 6.75% | 6.50% | +0.50% |
| HDFC Bank | 6.60% | 7.25% | 7.00% | +0.25–0.75% |
| ICICI Bank | 6.70% | 7.20% | 7.00% | +0.25% |
| Axis Bank | 6.70% | 7.20% | 7.10% | +0.25% |
| Kotak Mahindra Bank | 7.10% | 7.25% | 6.20% | +0.50% |
| Bank of Baroda | 6.85% | 7.25% | 6.50% | +0.50% |
All major bank FDs compound quarterly. Senior citizens typically receive 0.25–0.75% extra. Rates as of May 2026 — verify directly with the bank.
| Scheme | Rate | Compounding | Maturity / Lock-in | Tax Benefit |
|---|---|---|---|---|
| PPF | 7.1% | Annual | 15 years | EEE (fully tax-free) |
| NSC | 7.7% | Annual | 5 years | 80C deduction |
| SCSS (Senior Citizens) | 8.2% | Quarterly | 5 years | 80C deduction |
| Post Office TD (5 yr) | 7.5% | Annual | 5 years | 80C deduction |
| Sukanya Samriddhi | 8.2% | Annual | 21 years | EEE (fully tax-free) |
| KVP | 7.5% | Annual | ~9.6 years | No 80C |
Best for long-term wealth: PPF and Sukanya Samriddhi are EEE (Exempt-Exempt-Exempt) — contributions, interest, and maturity are all tax-free. At 7.1–8.2% compounded annually for 15–21 years, these are among the best safe investments available in India.
A SIP of ₹5,000/month for 20 years at an assumed 12% CAGR grows to approximately ₹49.9 lakh, on a total investment of just ₹12 lakh. The ₹37.9 lakh difference is pure compounding — your returns earning returns, month after month.
Units bought in Month 1 compound for the full 20 years; units bought in Month 240 only compound for 0 months. This is why the first few years of a SIP — not the last — are the most valuable.
₹1 lakh invested at 25 at 8% grows to ₹10.06 lakh by age 65 (40 years). The same amount invested at 35 grows to only ₹4.66 lakh (30 years). Starting 10 years earlier more than doubles the outcome — with the same money and rate.
Withdrawing interest defeats compounding entirely. Choose cumulative FDs, select growth option over IDCW in mutual funds, and let PPF interest compound within the account rather than withdrawing annually.
A 10% annual step-up on your SIP can double or triple your corpus compared to a flat SIP. Starting at ₹5,000/month and stepping up 10% each year for 20 years creates a corpus almost 2x larger than a flat SIP over the same period.
If comparing two FDs with the same annual rate, prefer the one compounding more frequently. Quarterly compounding always beats annual at the same stated rate. Most major bank FDs already compound quarterly — check the fine print for small bank or NBFC FDs.
Compound interest works against you on credit card balances (36–48% per year). Paying off a 40% credit card balance is the equivalent of earning 40% guaranteed returns — better than any investment available. Eliminate compounding debt before aggressively investing.