How Compound Interest Works
With compound interest, interest earned in each period is added to the principal, and future interest is calculated on the new (larger) total. This creates exponential growth over time. Formula: A = P ร (1 + r/n)^(nt) Where P = principal, r = annual rate, n = compounding frequency per year, t = years โน1,00,000 at 7% annual interest: โข Simple interest (10 years): โน1,70,000 โข Compounded annually (10 years): โน1,96,715 โข Compounded quarterly (10 years): โน2,00,160 โข Compounded monthly (10 years): โน2,00,966 Quarterly compounding beats annual by โน3,445 on โน1 lakh over 10 years โ a meaningful difference at scale.
FD Interest Rates and Compounding in India
Most Indian bank FDs compound quarterly. The headline rate is the annual rate, but the effective yield (because of quarterly compounding) is slightly higher. For example, an FD at 7% compounded quarterly has an effective annual yield of 7.19%. Current FD rates (2025โ26) range from 6.5% to 8% depending on tenure and bank. Small finance banks like Unity, Suryoday, and Jana offer 8โ9% on certain tenures but carry higher risk. Senior citizens typically receive 0.25โ0.50% extra on FDs. For FDs, the interest is taxable as income in the year it accrues โ this reduces the effective post-tax return significantly for those in the 30% bracket.
PPF vs FD: Which Is Better?
PPF (Public Provident Fund) currently offers 7.1% annual interest, compounded annually, with a 15-year lock-in. The key advantage is full tax exemption: contributions qualify for Section 80C deduction, interest earned is tax-free, and the maturity amount is tax-free (EEE status). FD returns are fully taxable. If you are in the 30% tax bracket, a 7.5% FD effectively yields 5.25% after tax โ lower than PPF at 7.1% tax-free. For long-term savings, PPF almost always wins on post-tax returns unless you need liquidity. The main drawback of PPF is the 15-year lock-in and the annual contribution limit of โน1.5 lakh.
The Rule of 72: How to Quickly Estimate Doubling Time
The Rule of 72 gives a quick estimate of how long it takes money to double: Years to double โ 72 รท interest rate โข FD at 7%: 72 รท 7 = ~10.3 years to double โข PPF at 7.1%: 72 รท 7.1 = ~10.1 years to double โข Equity mutual fund at 12% CAGR (historical average): 72 รท 12 = 6 years to double โข Inflation at 5โ6%: 72 รท 5.5 = ~13 years to halve purchasing power This rule shows why matching inflation is not enough โ you need returns above inflation to actually grow wealth in real terms.