Compound Interest Calculator — How Your Money Grows Over Time

Albert Einstein allegedly called compound interest "the eighth wonder of the world." Whether or not he actually said it, the sentiment is entirely true — compound interest is the single most powerful force in personal finance. It can turn modest savings into substantial wealth over time, and it can turn a manageable loan into a debt trap if you are not careful.

This guide breaks down exactly what compound interest is, how the formula works, why compounding frequency matters, and how everyday financial products like fixed deposits, SIPs, and loans use it. Use our free compound interest calculator to model your own scenarios.

What Is Compound Interest?

Compound interest is interest calculated on both the initial principal and the interest that has already been added to the account. In other words, your interest earns interest. This is in contrast to simple interest, which is calculated only on the original principal.

The compounding effect creates exponential growth. In the early years, the difference between compound and simple interest seems small. Over decades, it becomes enormous. This is why starting early — even with small amounts — is the most powerful financial decision most people can make.

Compound interest works for you when you are an investor or saver. It works against you when you are a borrower. Understanding which side of the equation you are on is essential to making smart financial decisions.

Compound Interest vs Simple Interest

Let us compare both methods on the same ₹1,00,000 investment at 10% per year for 5 years.

Year	Simple Interest Balance	Compound Interest Balance
1	₹1,10,000	₹1,10,000
2	₹1,20,000	₹1,21,000
3	₹1,30,000	₹1,33,100
4	₹1,40,000	₹1,46,410
5	₹1,50,000	₹1,61,051

After 5 years, simple interest gives ₹1,50,000 and compound interest gives ₹1,61,051 — an extra ₹11,051 for doing nothing different. Over 20 years at the same rate, the gap grows to over ₹3.7 lakh in favour of compound interest. Use our simple interest calculator alongside the compound interest calculator to see the difference for your own figures.

The Compound Interest Formula

The standard compound interest formula is:

A = P × (1 + r/n)^(n×t)

Where:

• A = Final amount (principal + interest)
• P = Principal (initial investment)
• r = Annual interest rate (as a decimal — 8% = 0.08)
• n = Number of times interest is compounded per year
• t = Time in years

The compound interest earned is: CI = A − P

Example: ₹50,000 invested at 8% per year, compounded quarterly (n=4) for 3 years.
A = 50000 × (1 + 0.08/4)^(4×3) = 50000 × (1.02)^12 = 50000 × 1.26824 = ₹63,412
CI = 63412 − 50000 = ₹13,412

Compounding Frequencies Explained

The more frequently interest is compounded within a year, the more interest you earn (or owe). Here is how the same 10% annual rate performs across different compounding periods on a ₹1,00,000 principal for 1 year:

Compounding Frequency	n value	Amount after 1 year	Interest Earned
Annually	1	₹1,10,000	₹10,000
Semi-annually	2	₹1,10,250	₹10,250
Quarterly	4	₹1,10,381	₹10,381
Monthly	12	₹1,10,471	₹10,471
Daily	365	₹1,10,516	₹10,516

The difference within a single year is modest, but over 10–30 years, even small differences in compounding frequency add up to significant amounts. Most Indian bank FDs compound quarterly, while savings accounts typically compound daily.

The Rule of 72

The Rule of 72 is a simple mental shortcut to estimate how long it takes for an investment to double in value at a given compound interest rate. Divide 72 by the annual interest rate percentage, and you get the approximate number of years to double.

Years to double ≈ 72 ÷ Interest Rate (%)

• At 6%: 72 ÷ 6 = 12 years to double
• At 8%: 72 ÷ 8 = 9 years to double
• At 12%: 72 ÷ 12 = 6 years to double
• At 18%: 72 ÷ 18 = 4 years to double

The Rule of 72 also works in reverse: if you want to know what interest rate you need to double your money in a given number of years, divide 72 by the number of years. To double your money in 8 years, you need a rate of approximately 72 ÷ 8 = 9% per year.

This rule is particularly useful for comparing investment options quickly. An 8% FD doubles your money in about 9 years; an equity mutual fund targeting 12% doubles it in about 6 years — a 3-year difference that has enormous implications for long-term wealth creation.

Fixed Deposit Example

Fixed Deposits (FDs) are the most popular savings instrument in India. Most banks offer quarterly compounding on FDs. Let us see how ₹2,00,000 grows in an FD at 7% per annum, compounded quarterly, over 5 years.

A = 2,00,000 × (1 + 0.07/4)^(4×5) = 2,00,000 × (1.0175)^20 = 2,00,000 × 1.4148 = ₹2,82,960

The FD earns ₹82,960 in interest over 5 years. Compare this to a simple interest FD at the same rate: SI = (2,00,000 × 7 × 5) / 100 = ₹70,000. The quarterly compounding adds an extra ₹12,960 over simple interest — about 18% more income just from the compounding effect.

SIP & Mutual Funds

A Systematic Investment Plan (SIP) works differently from a lump-sum investment — you invest a fixed amount each month, and each instalment compounds from the date it is invested. The formula for SIP maturity value is:

M = P × [((1 + r)^n − 1) / r] × (1 + r)
Where r = monthly rate, n = number of instalments

Example: ₹5,000 per month SIP for 10 years at 12% annual return (1% per month).

M = 5000 × [((1.01)^120 − 1) / 0.01] × 1.01 = 5000 × 230.04 × 1.01 ≈ ₹11,61,695

Total invested = 5000 × 120 = ₹6,00,000. Total returns ≈ ₹5,61,695 — nearly doubling the invested amount through compound growth. This is the fundamental math behind why financial advisors emphasise starting SIPs early.

Compound Interest on Loans

Compound interest on loans is why debt can spiral. Credit card companies typically charge 2–3% per month, compounded monthly — that translates to an effective annual rate of 26–42%. A ₹50,000 credit card balance unpaid for 2 years at 3% monthly (36% annually, compounded monthly) becomes:

A = 50000 × (1 + 0.03)^24 = 50000 × 2.0328 = ₹1,01,640

The debt has doubled in just two years. This is why financial advisors urge paying credit card balances in full each month. For home loans and car loans in India, EMI calculations use monthly compound interest via the formula: EMI = P × r × (1+r)^n / ((1+r)^n − 1), where r is the monthly rate.

Tips to Maximise Compound Growth

• Start early: The most powerful variable in compound interest is time. Starting at 25 instead of 35 can more than double your retirement corpus.
• Reinvest returns: Never withdraw interest if your goal is growth. Let it compound.
• Choose higher frequency: When comparing FDs at the same rate, prefer quarterly over annual compounding.
• Increase contributions over time: Even small annual increases in your SIP amount have a disproportionate impact on the final corpus.
• Avoid debt with high compound rates: Credit card debt at 36% annual compound interest can undo years of patient investing in a matter of months.
• Use tax-advantaged accounts: In India, instruments like PPF, EPF, and ELSS allow compound growth within a tax-efficient framework.

Frequently Asked Questions