Compound interest calculator: everything you need to know to maximise your investments

Definition of compound interest

Compound interest is interest calculated on the initial capital, as well as on the accumulated interest from previous periods. In other words, it is "interest on interest", which enables exponential growth of your investment over time. The yield generated increases exponentially each year, and this mechanism is a powerful shortcut to reaching your financial goals.

How does compound interest work?

Compound interest works by reinvesting the interest earned on the initial amount. Each period (annual, quarterly, monthly) adds new interest to the accumulated capital, thereby increasing the calculation base. The interest rate applied to the total amount generates ever-higher gains at the end of each period.

Difference between simple and compound interest

Simple interest is calculated only on the initial capital, whereas compound interest also includes the accumulated interest. This means that, for the same initial investment, compound interest grows faster over the long term. This difference is crucial for the investor seeking to maximise their wealth.

Advantages of an online calculator

Using a compound interest calculator lets you easily simulate the outcome of your financial investments. It helps to enter initial amounts, forecast interest rates, and visualise the growth of invested capital over the long term. Such a calculator also simplifies complex calculations and reduces the risk of human error.

Pitfalls to avoid when using a calculator

It is essential to properly estimate the expected interest rate and to take into account fees, taxes, and the parameters of the financial investment. Forgetting these elements can distort the simulated future value. Errors in regular contributions or initial amounts entered can also reduce the calculator's accuracy.

What to enter in the calculator

To use the compound interest calculator, you need to enter the following parameters, which make it possible to calculate the future value of the investment accurately:

• The initial capital (initial amount invested)
• The interest rate (expected annual rate)
• The investment duration (number of periods)
• The compounding frequency (annual, quarterly, monthly)
• The regular contribution amount (if applicable)

Manual calculation vs automated calculator

Manual calculation of compound interest involves multiple steps and can be error-prone, especially when taking the compounding frequency into account. An automated calculator offers a faster and more reliable way to see the growth of your wealth. It is particularly useful for simulating various yield scenarios.

The compound interest formula

The compound interest formula is given by: A = P × (1 + r / n)^(n × t)

where:

• A is the future value
• P is the initial capital
• r is the annual interest rate
• n is the compounding frequency (annual, quarterly, monthly)
• t is the investment duration in years

A concrete example using the formula

Example of an investment via our Bricks platform:

• Initial amount (P): €10,000
• Annual interest rate (r): 10% (i.e. 0.10 in decimal)
• Number of compounding periods per year (n): 12 (monthly)
• Investment duration (t): 10 years

Let's calculate step by step

0.10 / 12 = 0.00833: this corresponds to the monthly interest. 1 + 0.00833 = 1.00833: adding the monthly interest to 1. 12 × 10 = 120: the total number of compounding periods (10 years × 12 months). (1.00833)^120 ≈ 2.707: value of the growth factor after 120 months. 10,000 × 2.707 = 27,070: future value of the investment after 10 years.

Result: after 10 years, the initial capital of €10,000 becomes approximately €27,070, assuming constant monthly reinvestment of the income.

Example with a monthly contribution

Let's add a €200 contribution every month for 10 years, with an initial capital of €10,000 and an annual interest rate of 10% compounded monthly.

200 × (2.707 - 1) / 0.00833 ≈ 50,969: accumulated value of the monthly contributions. 10,000 × 2.707 = 27,070: future value of the initial capital.

Result: after 10 years, with monthly contributions of €200 and an initial capital of €10,000, the future value of the investment reaches approximately €68,039.

The importance of time and contribution frequency

Starting to invest early is essential to fully benefit from the power of compound interest. Regular contributions, even modest ones, can significantly increase the total amount accumulated at the end of the investment period. The longer the duration, the higher the yield.

Factors influencing compound interest

The interest rate, the investment duration, the compounding frequency, and the initial amount are the four main factors that influence the future value of a compound investment. Adjusting these parameters helps optimise the balance between risk and return.

The snowball effect on micro-investments

Micro-saving apps make it possible to automatically invest small amounts. Thanks to compound interest, even these micro-investments can grow over the long term, offering a powerful shortcut to building a financial portfolio.

The impact of compound interest in daily life

Compound interest can also be observed in everyday aspects such as long-term financial planning and saving for future goals. For example, by using savings accounts with compound interest rates, you can gradually grow your savings and more easily reach important goals such as buying a house or preparing for retirement. Even modest and regular contributions can grow significantly thanks to compound interest.