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Compound Interest

Categoria — Rates
By Konstantin Vasilev Member of the Board of Directors of Cbonds, Ph.D. in Economics
Updated October 06, 2023

What Is Compound Interest?

Compound interest refers to the process of earning interest on not only the initial amount of money you deposit or invest, known as the principal, but also on the interest that accumulates over time. In simpler terms, it’s interest that "interests" itself. Compound interest rate is a powerful financial concept that allows your money to grow more quickly than simple interest, where you only earn interest on the original principal amount.

Compound interest is a key concept in finance and is used in various financial instruments, including savings accounts, certificates of deposit (CDs), investment accounts, and more. It’s a crucial factor to consider when evaluating the growth potential of your investments or when borrowing money, as it can significantly impact the final amount you receive or owe.

How Compound Interest Works

Compound interest works by allowing the interest you earn on your savings to generate additional interest over time. This concept creates a compounding effect, where your money grows not only on the initial principal amount but also on the already accumulated interest. As a result, your savings begin to multiply at an exponential rate.

Imagine it as a snowball effect. Initially, your interest might seem small, but as it accumulates, it starts to grow more rapidly. The more time passes, the larger your savings become. This compounding effect can significantly impact your financial growth, potentially leading to substantial wealth accumulation.

In essence, compound interest is a powerful financial tool that rewards individuals for keeping their money invested or saved for an extended period. It’s a key factor to consider when making long-term financial decisions, as it can dramatically influence the growth of your wealth over time.

Compound Interest Formula

The compound interest formula is a fundamental equation used to calculate the future value of an investment or savings account when compound interest is applied. It is expressed as:

  • The initial amount of money you invest or deposit is known as the principal (represented as "p" in formulas).

  • The interest rate (often denoted as "r") determines how much interest you earn on your principal.

  • The compounding period (represented as "n") refers to how frequently the interest is calculated and added to the principal.

The formula to calculate compounded interest is often expressed as A = P(1 + r/n)^(nt), where:

  • A represents the future value of the investment.

  • P is the principal amount.

  • r is the annual interest rate.

  • n is the number of times interest is compounded per year.

  • t is the number of years the money is invested or borrowed for.

How to Calculate Compound Interest?

Calculating interest can be done using various methods, and Microsoft Excel provides a convenient tool to do so. Here are three approaches using Microsoft Excel, along with a summary of each method:

Approach One: Multiplication

  1. Enter "Year" into cell A1 and "Balance" into cell B1.

  2. Input the years (0 to 5) into cells A2 through A7.

  3. In cell B2, enter the initial principal amount, which is $1,000.

  4. In cell B3, calculate the balance for the next year with the formula "=B2*1.05" (assuming a 5% annual interest rate).

  5. Continue this calculation in cells B4 through B7.

  6. In cell B7, you will find the balance in your savings account after five years, which is $1,276.28.

  7. To determine the compound interest, subtract the initial principal amount ($1,000) from the final balance ($1,276.28), resulting in $276.28.

Approach Two: Fixed Formula

  1. Use the compound interest formula: ((P*(1+i)^n) - P).

  2. Enter "Principal value" into cell A1 and input the principal amount ($1,000) into cell B1.

  3. In cell A2, enter "Interest rate," and in cell B2, input the annual interest rate (0.05 for 5%).

  4. In cell A3, enter "Compound periods," and in cell B3, input the number of periods (5 years).

  5. Calculate the compound interest in cell B4 using the formula "=(B1*(1+B2)^B3)-B1," which yields $276.28.

Approach Three: Macro Function

  1. Open the Visual Basic Editor in Excel.

  2. Insert a module (click on "Insert" and then "Module").

  3. In the module, type the function for compound interest: Function Compound_Interest(P As Double, I As Double, N As Double) in the first line.

  4. In the second line type: Compound_Interest = (P*(1+I)^N) - P

  5. In the third line type: End Function

  6. Now, you can use this custom function in your worksheet. For example, in cell A6, enter "Compound interest," and in cell B6, input =Compound_Interest(B1, B2, B3) to calculate $276.28.

You can also use free online calculators:

  • Compound Interest Calculator. This SEC site offers a simple compound interest calculator with options for monthly deposits.

  • Compound Interest Calculator. Allows calculations in multiple currencies, factoring in monthly deposits or withdrawals.

  • Council for Economic Education Compound Interest Calculator. Geared toward students, it shows the power of compounding with inputs like age, annual interest rates, initial investment, and monthly savings.

Unveiling the Power of Compound Interest

The power of compound interest is a financial concept that highlights the remarkable effect of allowing interest to accumulate on both the initial principal and the interest earned in previous compounding periods. This compounding process leads to exponential growth in your savings or investments over time.

One of the key characteristics of compound interest is its ever-accelerating nature. As interest accumulates over time, it starts to grow at an increasingly rapid rate. This is in stark contrast to simple interest, where the interest is calculated solely on the initial principal amount.

For example, consider a loan with a total interest payable over three years of $1,576.25. In the case of compound interest, the interest amount at the end of each year is not the same as it would be with simple interest. This is because, with compound interest, the interest earned in previous years contributes to the calculation of interest for subsequent years.

To illustrate the power of compound interest in a savings or investment context, let’s take a look at a scenario. Suppose you have a $100,000 deposit receiving 5% simple annual interest. Over 10 years, this deposit would earn you a total interest of $50,000. However, if the same $100,000 deposit were subject to monthly compounding at a 5% interest rate, the interest would accumulate to approximately $64,700 over the same period.

This example demonstrates how compound interest can significantly boost investment returns over the long term. By allowing your money to grow through the power of compounding, you can harness the potential for your wealth to increase at an accelerating pace, ultimately leading to greater financial gains over time. This is why compound interest is often considered a powerful tool for wealth accumulation and financial planning.

Advantages and Disadvantages of Compounding

Advantages of Compound Interest

  1. Long-Term Wealth Building. Compound interest is a powerful tool for building wealth over the long term. It allows your returns to earn additional returns, leading to exponential growth in your investments and savings. This can be particularly beneficial for retirement planning and achieving long-term financial goals.

  2. Mitigates Wealth Erosion Risks. Compound interest helps mitigate wealth-eroding factors such as inflation. Inflation reduces the purchasing power of money over time, but with compound interest, your investments have the potential to outpace inflation, preserving and growing your wealth.

  3. Loan Repayments. Compound interest can work in your favor when repaying loans. By making payments above the minimum required amount, you can reduce your loan balance faster and ultimately save on the total interest paid. This approach can help you pay off debts more efficiently.

Disadvantages of Compound Interest

  1. Minimum Payments on High-Interest Debt. Compound interest works against consumers who make only the minimum payments on high-interest loans or credit card debts. When you pay only the minimum, your debt balance can grow exponentially due to the compounding of interest. This situation can lead to a cycle of increasing debt and financial stress.

  2. Taxation. Earnings from compound interest are typically taxable at your applicable tax bracket. Unless your money is invested in a tax-sheltered account such as a retirement account or a tax-free savings account, you may owe taxes on your investment gains.

  3. Complexity. Calculating compound interest can be more complex compared to simple interest. While simple interest calculations involve straightforward multiplication, compound interest calculations require more intricate mathematics. As a result, many individuals find it challenging to calculate compound interest manually and may prefer to use online calculators or financial tools.


Initial Values:

  • Principal Amount (P): $1,000 (the initial amount of money deposited)

  • Annual Interest Rate (rate of return, annual rate): 5% (0.05 as a decimal)

  • Compounding Period (compounding frequency): Quarterly (interest compounded four times a year)

  • Number of Years (n number, years): 5 years

Formula: The compound interest formula is A = P(1 + r/n)^(nt), where:

  • A represents the future value of the investment.

  • P is the principal amount.

  • r is the annual interest rate.

  • n is the number of times the interest is compounded per year.

  • t is the number of years the money is invested or borrowed for.

Calculation: Using the formula, we can calculate the future value of the investment:

A = $1,000 (1 + 0.05/4)^(45)

A = $1,000 * (1 + 0.0125)^20

A = $1,000 * (1.0125)^20

A ≈ $1,282.04 (rounded to two decimal places)

Result: After 5 years of quarterly compounding at a 5% annual interest rate, the $1,000 initial deposit will grow to approximately $1,282.04. The interest earned on this investment, which is $1,282.04 - $1,000 = $282.04, is the compound interest. This demonstrates how your money can grow significantly over time through the power of compound interest.

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