Whether you are saving money, investing, or paying off debt, compound interest is one of the most powerful forces in personal finance. Understanding it could be worth thousands of rupees (or dollars) over your lifetime. This guide explains it clearly and shows you how to calculate it for free.
What is Compound Interest?
Simple interest is earned only on your original deposit. Compound interest is earned on your deposit AND on the interest you've already earned. In other words, your interest earns interest.
A Simple Example
You invest ₹10,000 at 10% annual interest:
- Simple interest: Year 1 = ₹1,000. Year 2 = ₹1,000. Year 5 = ₹5,000 total interest.
- Compound interest: Year 1 = ₹1,000. Year 2 = ₹1,100 (10% of ₹11,000). Year 5 = ₹6,105 total interest.
That extra ₹1,105 comes from compounding alone — and the difference grows exponentially over 20, 30, and 40 years.
The Compound Interest Formula
The mathematical formula for compound interest is:
A = P × (1 + r/n)^(n×t)
- A = Final amount
- P = Principal (initial deposit)
- r = Annual interest rate (decimal, e.g. 0.10 for 10%)
- n = Number of times interest is compounded per year
- t = Time in years
Instead of doing this math yourself, use our free calculator to get instant results.
Compounding Frequency Matters
The same annual interest rate produces different results based on how often interest is compounded:
- Annually (once per year) — lowest compounding
- Quarterly (4 times per year)
- Monthly (12 times per year)
- Daily (365 times per year) — highest compounding
The more frequently interest compounds, the more money you accumulate. Daily compounding is best for savings accounts and investments.
The Rule of 72 — Quick Mental Math
Want to estimate how long it takes for your money to double? Divide 72 by your annual interest rate:
- At 6% interest: 72 ÷ 6 = 12 years to double
- At 12% interest: 72 ÷ 12 = 6 years to double
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