💰 Finance & Math

Compound Interest Calculator

Q: What is compound interest?

Compound interest is interest calculated on both the initial principal and all previously accumulated interest. Unlike simple interest (which is only on the principal), compound interest causes investments to grow exponentially over time — often called 'interest on interest'.

Q: What is the compound interest formula?

A = P × (1 + r/n)^(n×t), where P = principal amount, r = annual interest rate (as a decimal), n = number of compounding periods per year, t = time in years. Total interest = A − P.

Q: What does compounding frequency mean?

Compounding frequency is how often interest is calculated and added to your balance. Options: Annually (1×/year), Semi-annually (2×), Quarterly (4×), Monthly (12×), Daily (365×). More frequent compounding = slightly higher effective return because each period's interest begins earning sooner.

Q: How much does ₹1 lakh grow at 8% for 10 years?

₹1,00,000 at 8% compounded annually for 10 years grows to ₹2,15,892. With monthly compounding at 8%, it grows to ₹2,21,964. The difference comes from more frequent compounding of accumulated interest.

Q: What is the difference between simple and compound interest?

Simple interest = P × r × t, calculated only on the original principal. Compound interest adds accumulated interest back to the principal each period, so you earn interest on interest. Over 20+ years, the difference is enormous.

Q: What is the Rule of 72?

The Rule of 72 estimates doubling time: divide 72 by the annual interest rate. At 6%, money doubles in ~12 years. At 8%, ~9 years. At 12%, ~6 years. It is a quick mental math shortcut for compound growth.

See exactly how your money grows with compound interest. Adjust principal, rate, time, and compounding frequency for an instant result.

Principal Amount (₹)

Annual Interest Rate (%)

Time Period (years)

Compounding Frequency

Results

Principal —

Total Interest —

Final Amount —

Effective Annual Rate —

Rule of 72 (doubles in) —

Principal Interest

How the Compound Interest Calculator works

Enter your Principal (initial investment), Annual Interest Rate, Time Period in years, and Compounding Frequency. The calculator uses the formula A = P × (1 + r/n)^(n×t) to compute the final amount. Results update instantly as you change any input. The bar chart shows the proportion of your final balance that is original principal versus accumulated interest — visually illustrating the power of compounding.

Why the Compound Interest Calculator is Useful

Compound interest is what separates a mediocre investment from a life-changing one. The difference between starting to invest at 25 versus 35 is not just 10 years — it can mean double or triple the final amount. This calculator makes that abstract concept concrete: enter your numbers and watch exactly how your money grows, with a breakdown showing how much is your original investment versus earned interest.

Key Features

Five compounding frequencies: Annual, semi-annual, quarterly, monthly (most common for FDs), and daily
Effective Annual Rate (EAR): Shows the true annual yield after compounding, so you can compare products accurately
Rule of 72: Instantly estimates when your money will double based on the interest rate
Visual principal vs. interest split: Bar chart shows how much of the final amount is growth versus original investment
Instant updates: All outputs recalculate live as you adjust any input

Real-Life Use Cases

Comparing Fixed Deposit returns across different banks — use monthly vs. annual compounding to see the effective difference
Projecting how much ₹50,000 invested today grows in 20 years at a historical 10–12% mutual fund return rate
Teaching children or students the practical difference between saving early versus late
Understanding why daily-compounding savings accounts (though by a small margin) outperform monthly compounding ones

Who Can Use This Tool

Personal finance enthusiasts, students, investors at any level, parents planning for education or retirement funds, bank customers comparing FD rates, and anyone who wants to understand how investments grow over time.

Tips & Best Practices

The difference between monthly and annual compounding is small at low rates and short periods — but significant at 10%+ over 15+ years
Use the Rule of 72 for quick mental math: at 9% return, money doubles in roughly 8 years; at 12%, roughly 6 years
Compare your bank FD rate with long-term equity mutual fund returns (8–12% historically) — the gap in final amounts over 20 years is eye-opening
This calculator assumes a fixed rate — real investments have variable returns; use it for planning, not guarantees

Frequently Asked Questions

What is compound interest?

Compound interest is interest earned on both the principal and the accumulated interest. It causes exponential growth — the longer you invest, the faster it accelerates.

What is the compound interest formula?

A = P × (1 + r/n)^(n×t). P = principal, r = annual rate (decimal), n = compounding periods/year, t = years. Interest earned = A − P.

What does compounding frequency mean?

How often interest is added to your balance. Monthly (12×/year) earns slightly more than annual (1×/year) because each month's interest starts earning immediately.

How much does ₹1 lakh grow at 8% for 10 years?

₹1,00,000 at 8% compounded annually → ₹2,15,892. Monthly compounding at 8% → ₹2,21,964. More frequent compounding = higher effective yield.

What is the difference between simple and compound interest?

Simple interest only accrues on principal. Compound interest accrues on principal + all past interest. Over long periods the difference is dramatic.

What is the Rule of 72?

Divide 72 by the annual interest rate to estimate doubling time. At 8%: ~9 years. At 6%: ~12 years. At 12%: ~6 years. A handy mental math shortcut.

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