Quick Answer: What Is Compound Interest?
Compound interest is interest calculated on the initial principal plus all previously accumulated interest. In simple interest, you only ever earn returns on your original deposit. In compound interest, your earnings generate their own earnings in every successive period. For example, on a ₹1,00,000 investment at 10% annual interest over 10 years: • Simple Interest: ₹1,00,000 + (₹10,000 × 10) = ₹2,00,000 total (₹1,00,000 gain). • Compound Interest (Annual): ₹1,00,000 × (1.10)^10 = ₹2,59,374 total (₹1,59,374 gain). Compound interest delivers nearly 60% more profit on the exact same capital because your money snowballs over time.
The Universal Compound Interest Formula
The universal mathematical equation for compound interest is: A = P × (1 + r / n)^(n × t) Where: • A = Final maturity amount (Principal + Accumulated Interest) • P = Initial principal amount invested (e.g. ₹1,00,000) • r = Annual nominal interest rate expressed as a decimal (10% = 0.10) • n = Compounding frequency per year (1 for annual, 4 for quarterly, 12 for monthly, 365 for daily) • t = Total investment duration in years Total Compound Interest (CI) = A - P.
Step-by-Step Worked Example: ₹5,00,000 at 8% for 10 Years
Let us calculate the growth of ₹5,00,000 invested at 8% annual interest across 10 years, comparing annual vs quarterly compounding: Case 1: Annual Compounding (n = 1) A = 5,00,000 × (1 + 0.08 / 1)^(1 × 10) A = 5,00,000 × (1.08)^10 ≈ 5,00,000 × 2.158925 = ₹10,79,462 Interest earned: ₹5,79,462. Case 2: Quarterly Compounding (n = 4, like Indian Bank FDs) A = 5,00,000 × (1 + 0.08 / 4)^(4 × 10) A = 5,00,000 × (1.02)^40 ≈ 5,00,000 × 2.208040 = ₹11,04,020 Interest earned: ₹6,04,020. Quarterly compounding generates an additional ₹24,558 over annual compounding because interest is reinvested every three months.
The Rule of 72: How Fast Does Your Money Double?
The Rule of 72 is an accurate mental shortcut that tells you how many years it takes for your investment to double in value: Years to Double ≈ 72 / Annual Return Rate (R) Where R is entered as a percentage number: • At 6% interest (savings/bonds): 72 / 6 = 12 years to double. • At 8% interest (fixed deposits/debt): 72 / 8 = 9 years to double. • At 12% interest (equity index funds): 72 / 12 = 6 years to double. Over 24 years, an investment at 12% doubles four times (2x, 4x, 8x, 16x). A ₹10,00,000 investment becomes ₹1.60 Crores.
The Companion Shortcuts: Rule of 114 and Rule of 144
Just as the Rule of 72 estimates doubling time, mathematical shortcuts exist for tripling and quadrupling your capital: • Rule of 114 (Years to Triple): 114 / Annual Rate. At a 12% return, your money triples in: 114 / 12 = 9.5 years. • Rule of 144 (Years to Quadruple / 4x): 144 / Annual Rate. At a 12% return, your money quadruples in: 144 / 12 = 12 years. These shortcuts enable you to evaluate retirement goals without complex calculations.
Inflation: Compounding Working in Reverse
While compounding builds investment portfolios, it also drives the erosion of purchasing power through inflation. If average inflation runs at 6% annually: 72 / 6 = 12 years. In 12 years, general consumer living expenses double, cutting the real purchasing power of cash in half. A monthly grocery bill that costs ₹15,000 today will cost ₹30,000 in 12 years. Keeping long-term wealth in low-interest accounts yielding 3.5% causes real purchasing power to decline by 2.5% every year.
Why Time Matters More Than Capital
The most critical ingredient in compounding is time, not initial capital. Consider two friends: • Amit invests ₹10,000 monthly from age 25 to 35 (10 years, ₹12 Lakhs invested total) and then stops, letting the money compound at 12% until age 60. • Rahul waits until age 35 and invests ₹10,000 monthly from age 35 to 60 (25 years, ₹30 Lakhs invested total). At age 60: • Amit (invested ₹12 Lakhs early): Corpus is roughly ₹2.35 Crores. • Rahul (invested ₹30 Lakhs late): Corpus is roughly ₹1.90 Crores. Starting ten years earlier allowed Amit to invest less than half as much money yet end up with ₹45 Lakhs more wealth.
Test Your Numbers With a Compound Interest Tool
You can model custom deposit sums, annual rates, and compounding frequencies (annual, semi-annual, quarterly, monthly, daily) with our QuickCalc Compound Interest Calculator.