Compound Interest Calculation in India

Summary

Compound interest is the foundation of wealth building in India, whether through Fixed Deposits (FDs), Public Provident Fund (PPF), or equity mutual funds via SIPs. The compounding frequency — quarterly for FDs, annually for PPF, or daily for mutual fund NAVs — significantly affects the effective return.

How it works

The formula

A = P x (1 + r/n)^(n x t)

Where A = final amount, P = principal, r = annual rate, n = compounding frequency per year, t = number of years.

Compounding frequency by product

Tax impact on compounding

The real power of compounding is eroded by tax. FD interest is taxed at slab rate annually (even if not withdrawn in a cumulative FD, TDS is deducted). PPF is fully tax-free (EEE), and equity mutual fund LTCG is taxed at 12.5% above Rs 1.25 lakh annually. This makes post-tax compounding rates very different from headline rates.

Worked example

Rs 10 lakh invested for 10 years at 7.25% compounded quarterly

1. Quarterly rate: 7.25% / 4 = 1.8125%
2. Number of quarters: 40
3. A = Rs 10,00,000 x (1.018125)^40 = Rs 20,54,082
4. Interest earned: Rs 10,54,082
5. Effective annual rate: (1.018125)^4 - 1 = 7.44%

Compare with annual compounding at 7.25%: Rs 10,00,000 x 1.0725^10 = Rs 20,10,947. Quarterly compounding earns Rs 43,135 more.

For a 30% tax bracket investor, the post-tax FD return drops to approximately 5.08%, reducing the 10-year corpus to Rs 16,38,700.

Key differences from other markets

• TDS on FD interest: Indian banks deduct TDS at 10% if annual FD interest exceeds Rs 40,000 (Rs 50,000 for seniors), even on cumulative deposits. This disrupts the compounding effect compared to tax-sheltered accounts in the UK (ISA) or US (401k/IRA).
• PPF as a compounding vehicle: The combination of 7.1% guaranteed return with EEE tax status makes PPF one of the most efficient compounding instruments globally, despite its annual (not quarterly) compounding.