Compound Interest Calculator

1. Simple Interest vs Compound Interest: The Core Distinction

Simple Interest (SI) treats every year as day one. Interest is calculated solely against the initial principal investment regardless of tenure:

Compound Interest (CI), by contrast, reinvests your earned interest back into the principal at every compounding interval. From that moment onward, your interest begins earning interest of its own. Over short durations (1–2 years), the difference appears modest; but over 10, 20, or 30 years, compounding yields an exponential curve that dwarfs simple interest.

2. The Impact of Compounding Frequencies in Indian Banking

The compound interest formula relies heavily on the frequency of compounding ($n$ times per year):

• Quarterly Compounding (n = 4): Mandated by the Reserve Bank of India (RBI) for bank fixed deposits (FDs) and recurring deposits (RDs). A 7.00% nominal rate actually yields an Effective Annual Rate (AER) of 7.19%.
• Annual Compounding (n = 1): Standard for Public Provident Fund (PPF) and National Savings Certificates (NSC), where interest is credited on March 31 annually.
• Monthly Compounding (n = 12): Typical for corporate NBFC deposits or mutual fund SIPs that reinvest cash monthly.
• Daily Compounding (n = 365): Common in overnight debt mutual funds and liquid funds where Net Asset Values (NAVs) fluctuate daily.

3. The Rule of 72 & The Compounding Acceleration Horizon

The famous Rule of 72 provides a mental math estimate for the time required to double your wealth:

At an 8% return, ₹1 Lakh doubles to ₹2 Lakh in ~9 years, then doubles again to ₹4 Lakh in 18 years, and hits ₹8 Lakh in 27 years. Notice that in the third decade, your wealth grows by ₹4 Lakh — four times the initial principal — in the exact same time window!