Compound Interest: Definition, Formula, and Calculation Method

Compound interest is calculated on both the principal and previously earned interest, resulting in exponential growth over time. Explore this article to understand its formula, calculation process, and benefits.
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Compound Interest
OPEN FD
4 min
12-March-2025

Compound interest is the process of earning interest on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is calculated only on the principal amount, compound interest allows money to grow at an accelerated rate over time. This concept is widely used in banking, investments, and financial planning.

In India, compound interest plays a crucial role in savings accounts, fixed deposits, mutual funds, and loan repayments. Financial institutions apply compound interest on various financial products, making it essential for individuals to understand how it impacts their savings and borrowings. The frequency of compounding—daily, monthly, quarterly, or annually—affects the final amount significantly.

Investors benefit from compound interest by reinvesting earnings, leading to exponential wealth growth. Borrowers, on the other hand, must carefully consider the interest accumulation on loans. Learning how to calculate and utilise compound interest effectively can help individuals make informed financial decisions and optimise their returns.

Compound interest formula

The formula to calculate compound interest is:

A = P (1 + r/n)^(nt)

Where:

A = Final amount after interest

P = Principal amount

r = Annual interest rate (decimal)

n = Number of times interest is compounded per year

t = Number of years

This formula helps determine the total amount accumulated over a specific period. The compounding frequency plays a vital role in maximising returns. Higher compounding frequencies result in greater interest accumulation.

For example, if Rs. 10,000 is invested at an annual interest rate of 8% compounded annually for five years, the total amount can be calculated using the formula. Understanding this formula enables individuals to compare investment options and make better financial decisions.

Pro tip

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Derivation of compound interest formula

The compound interest formula is derived from the basic principle of repeatedly applying interest on an initial principal amount. The derivation follows these steps:

The principal amount (P) accumulates interest at a rate of (r/n) per period.

After one compounding period, the amount becomes P(1 + r/n).

After two periods, it further grows to P(1 + r/n)^2.

Extending this logic to ‘t’ years, the final amount is expressed as A = P (1 + r/n)^(nt).

This derivation explains why compound interest leads to faster growth compared to simple interest. Understanding the derivation helps in financial planning and evaluating different interest-bearing products such as bank deposits, bonds, and loans.

Quarterly compound interest formula

When interest is compounded quarterly, the frequency of compounding increases to four times a year. The formula modifies as follows:

A = P (1 + r/4)^(4t)

Here:

r/4 represents the interest applied per quarter

4t represents the total number of quarters over the investment period

For instance, if an investor deposits Rs. 20,000 at an annual rate of 10%, compounded quarterly for three years, the formula helps determine the final amount.

Quarterly compounding is widely used in Indian fixed deposits and corporate bonds, providing higher returns compared to annual compounding. It is crucial to check the compounding frequency before investing to maximise benefits.

Calculate compound interest

To calculate compound interest manually, follow these steps:

Identify the principal amount (P), interest rate (r), compounding frequency (n), and duration (t).

Convert the interest rate to a decimal (e.g., 8% becomes 0.08).

Apply the values in the formula A = P (1 + r/n)^(nt).

Compute the final amount (A) and subtract the principal to get the compound interest earned.

For example, if Rs. 50,000 is invested at an annual rate of 12% compounded semi-annually for five years, the compound interest can be calculated using the given formula.

Many online calculators and financial tools can simplify these calculations, helping individuals and businesses make better financial decisions.

Conclusion

Compound interest is a powerful financial tool that significantly impacts savings, investments, and borrowings. It allows money to grow exponentially by reinvesting earned interest. Understanding its formula and calculation methods helps individuals optimise their financial planning.

In India, compound interest is commonly used in bank savings, fixed deposits, mutual funds, and loan repayments. The compounding frequency plays a crucial role in determining returns, making it essential to choose financial products wisely.

By leveraging the power of compounding, individuals can maximise their wealth over time. Whether investing or borrowing, understanding compound interest can lead to smarter financial decisions and improved long-term financial stability.

If you are looking for safe investment option, then you can consider investing Bajaj Finance Fixed Deposit. With a top-tier AAA rating from financial agencies like CRISIL and ICRA, they offer one of the high returns of up to 7.30% p.a.

Calculate your expected investment returns with the help of our investment calculators

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Frequently asked questions

What is compound interest and example?
Compound interest is the interest earned on both the initial principal and the accumulated interest from previous periods. It helps investments grow exponentially over time. For example, if Rs. 10,000 is invested at 10% annual interest, it becomes Rs. 11,000 in one year and Rs. 12,100 in the second year.

Why invest in compound interest?
Investing in compound interest helps money grow faster than simple interest. It maximises long-term wealth creation by reinvesting earnings. The longer the investment period, the greater the returns. It is ideal for retirement funds, fixed deposits, and mutual funds. For instance, compounding can turn Rs. 1 lakh into Rs. 6.72 lakh in 20 years at 10% interest.

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As regards deposit taking activity of Bajaj Finance Ltd (BFL), the viewers may refer to the advertisement in the Indian Express (Mumbai Edition) and Loksatta (Pune Edition) furnished in the application form for soliciting public deposits or referhttps://www.bajajfinserv.in/fixed-deposit-archivesThe company is having a valid Certificate of Registration dated March 5, 1998 issued by the Reserve Bank of India under section 45 IA of the Reserve Bank of India Act, 1934. However, the RBI does not accept any responsibility or guarantee about the present position as to the financial soundness of the company or for the correctness of any of the statements or representations made or opinions expressed by the company and for repayment of deposits/discharge of the liabilities by the company.

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