Compound interest calculator

What is compound interest?

Put simply, compound interest means that not only your original capital can earn interest or returns. Interest you have already received can also contribute to further growth over time.

As a result, capital can grow more strongly over a longer period than it would if only the original amount earned interest. This effect becomes especially visible when money remains invested for many years and regular additional contributions are made.

The compound interest calculator helps you explore different scenarios. You can enter a starting amount, a monthly savings contribution, a time period, and an assumed interest rate. It then shows how contributions and assumed interest combine in the model.

Which values are calculated?

The calculator takes four key details into account.

Starting amount

The starting amount is the sum you begin with. It can be money you already have and want to set aside for a long-term goal.

If you are starting from zero, simply enter €0. Regular contributions can still add up to a meaningful amount over time.

Monthly savings contribution

The monthly savings contribution is the amount you add regularly.

It shows particularly clearly that long-term planning does not have to begin with a large starting amount. Even smaller, regular contributions can add up over many years.

For example, a monthly savings contribution of €50 equals €600 per year. Over ten years, that would already be €6,000 in contributions – before any possible interest or returns are taken into account.

Time period

The time period describes how long your money remains invested or earns interest in the model.

Time is an important factor in compound interest. The longer interest or returns can remain in the system, the more noticeable the effect can become. This does not mean that a long time period automatically guarantees gains. It does, however, explain why long-term planning and regular saving often matter more than individual short periods.

Annual interest rate

The interest rate describes the assumed annual interest rate or return as a percentage.

In this calculator, it is a model value. You can enter different interest rates to compare scenarios. The result shows how the capital could develop under those assumptions.

A constant interest rate cannot be assumed in real life. Especially for investments whose value fluctuates, results can differ significantly from year to year.

How does compound interest work mathematically?

If an amount of K₀ grows over n years at an annual interest rate of r, the capital can develop according to the following simplified formula:

Where:

• K₀ is the starting amount.
• r is the interest rate as a decimal, for example 0.05 for 5 percent.
• n is the number of years.
• Kₙ is the capital at the end of the time period.

With regular monthly contributions, the calculation is extended. Each contribution has a different amount of time to grow: the first contribution has almost the entire time period, while the last contribution has only a short period.

This is why the calculator takes your regular savings contribution and the chosen compounding frequency into account in addition to the starting amount.

An example

Suppose you start with €5,000, contribute €100 each month, and calculate with an assumed interest rate of 5 percent over ten years.

Over this period, you would contribute a total of €17,000 yourself:

• €5,000 starting amount
• €12,000 from contributing €100 per month for ten years

The calculator also shows which share of the modelled final balance comes from interest under the chosen assumption.

This lets you see not only the final result, but also the difference between:

• Contributions: money you have added yourself.
• Interest: the calculated return based on your entries.
• Final balance: the sum of contributions and calculated interest.

Monthly or yearly compounding

The calculator offers different compounding frequencies because, in theory, interest can be credited or calculated at different intervals.

With monthly compounding, the assumed annual interest rate is distributed across individual months. This means the compound interest effect is calculated slightly more often than with yearly compounding.

The difference is often small for short time periods or low amounts. Over longer periods, it can become more noticeable.

However, it is important to remember that the chosen frequency is part of the model calculation. It does not predict how a particular bank, investment, or product will actually develop.

How to use the calculator effectively

The compound interest calculator is particularly useful when you want to compare different scenarios.

For example, you can explore:

• What happens if you set aside €150 per month instead of €100?
• How much difference do time periods of 10, 15, or 20 years make?
• How does the result change with different assumed interest rates?
• What share do your own contributions make up compared with the modelled interest?
• What might a personal savings goal require at different savings contribution levels?

Use the "Save scenario" feature for this. It lets you compare different calculations directly without entering your figures again each time.

What the calculator cannot predict

The calculator is a calculation tool, not a forecast.

Actual developments may differ from the calculation. Reasons can include:

• Fluctuating or changing interest rates;
• Positive or negative changes in value;
• Costs and fees;
• Taxes;
• Interruptions or changes to the savings contribution;
• Deposits and withdrawals at different points in time;
• Inflation and the resulting changes in purchasing power.

The final balance shown is therefore not a promise or an expectation for a particular investment. It is the result of the assumptions you have chosen.

Plan for the long term without needing to be perfect

The calculator's greatest value is not predicting an exact future. It helps you develop a feel for the relationships between different factors.

You can see how regular contributions, an early start, and a longer time period can affect a possible result. At the same time, you can realistically assess which savings contribution fits your everyday life and your goals.

Even small steps can be worthwhile. What matters is not finding the perfect scenario immediately, but building a system you can maintain over the long term.

Note: This calculation is intended solely for personal guidance. It is based on your entries and an assumed constant interest rate. It does not constitute financial, tax, or investment advice and does not make any statement about future performance.