Which of the following best defines compound interest?

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Compound interest is a fundamental concept in finance that plays a crucial role in various aspects of our lives, from saving money to investing for the future. It refers to the interest earned on both the initial principal amount and the accumulated interest from previous periods. In simpler terms, compound interest can be defined as “interest on interest.”

Understanding Compound Interest

To better understand compound interest, let’s consider a hypothetical scenario. Imagine you deposit $1,000 in a savings account with an annual interest rate of 5%. At the end of the first year, you would earn $50 in interest, bringing your total balance to $1,050. However, in the second year, the interest is calculated not only on the initial $1,000 but also on the additional $50 of interest earned in the first year. This compounding effect continues year after year, resulting in exponential growth of your savings.

The formula to calculate compound interest is:

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

A = the future value of the investment/loan, including interest
P = the principal amount (initial deposit or loan amount)
r = the annual interest rate (expressed as a decimal)
n = the number of times that interest is compounded per year
t = the number of years the money is invested or the loan duration

For example, if you invest $1,000 at an annual interest rate of 5%, compounded annually for 5 years, the formula would be:

A = 1000(1 + 0.05/1)^(1*5)
A = 1000(1.05)^5
A ≈ $1,276.28

As you can see, compound interest allows your savings to grow significantly over time.

Compound Interest vs. Simple Interest

To understand the power of compound interest, it is important to differentiate it from simple interest. Simple interest is calculated only on the initial principal amount and remains constant throughout the investment period. In contrast, compound interest takes into account the accumulated interest, resulting in a higher return on investment.

Let’s compare the two using the same example as before. If the interest were simple instead of compound, the calculation for the final balance after 5 years would be:

A = P(1 + rt)
A = 1000(1 + 0.05*5)
A = 1000(1.25)
A = $1,250

As you can see, the difference between compound interest and simple interest is $26.28 in this case. Over longer periods or with higher interest rates, the difference becomes even more significant.

Applications of Compound Interest

Compound interest has various applications in personal finance, investing, and banking. Some common examples include:

1. Savings Accounts: Many savings accounts offer compound interest, allowing individuals to grow their savings over time.

2. Investments: Compound interest plays a crucial role in long-term investments such as stocks, bonds, and mutual funds. The compounding effect allows investments to grow exponentially over time.

3. Loans and Debt: Compound interest is also relevant when it comes to loans and debt. Borrowers need to understand the impact of compound interest on their repayment obligations.

4. Retirement Planning: Compound interest is a key factor in retirement planning. By starting early and taking advantage of compound interest, individuals can build a substantial retirement nest egg.


Compound interest is a powerful concept that allows money to grow exponentially over time. It takes into account both the initial principal amount and the accumulated interest from previous periods. By understanding and harnessing the power of compound interest, individuals can make informed financial decisions, save for the future, and maximize their investment returns.


– Investopedia: www.investopedia.com/terms/c/compoundinterest.asp
– The Balance: www.thebalance.com/what-is-compound-interest-315426
– Khan Academy: www.khanacademy.org/math/ap-calculus-ab/ab-differential-equations-new/ab-7-5/v/compound-interest-as-a-exponential-function