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Investment Growth Simulator
Legend
- Beginning of year balance: The amount carried over from the previous year, representing the starting balance at the beginning of the current period.BOY
- The total amount of interest accrued during the current year.Annual interest
- Annual Contribution: The total amount added to the balance during the current year, excluding any interest or carryover.Annual contr.
- Annual Withdrawal: The total amount withdrawn from the balance during the current year, reducing the overall value.Annual withdr.
Why Simulate Different Interest Rates, Contributions and Withdrawals?
Investment conditions change over time — different asset types yield different returns, and your contribution capacity might vary. By simulating multiple scenarios, you gain a more realistic understanding of long-term outcomes, especially when shifting your investment strategy near retirement or continuing to earn interest during withdrawal phases.
A single projection is a guess with a decimal point. Running the same horizon at two percentage points lower, or with the contribution stopping five years early, tells you something a single number cannot: how much of the result depends on assumptions you cannot verify. That is the question worth answering before committing money for thirty years.
A Realistic Investment Journey: Contributions, Shifts and Withdrawals
Imagine starting with 10,000 € and contributing 200 € per month for the first 5 years, then 400 € for the next 5, and finally 600 € for the following 10 years. During the first 15 years, you invest in higher-risk stocks earning an average return of 7%. After that, you shift to more secure bonds yielding 4% for 5 years. Once you begin withdrawing, your money remains in a savings account earning 3% annually. This example shows how changing contribution levels and risk profiles can shape long-term growth — and how compounding continues even during withdrawal.
Every element of that example is a field in the calculator, and the resulting address is shareable. Change one input, copy the link, and you have a second scenario you can put next to the first — which is usually more informative than refining a single one.
How Compound Interest Grows Your Wealth
Compound interest is the foundation of long-term investing. Unlike simple interest, it allows your earnings to generate their own earnings — resulting in exponential growth over time. The earlier and more consistently you invest, the more powerful this effect becomes. This app uses the compound interest formula to simulate how your capital evolves over time by factoring in your contributions, interest rate changes, and withdrawal phases. It recalculates the total balance at each time step by applying the current interest rate to the full amount — including all past interest gains.
Two details decide how the curve looks. The compounding frequency you set for the interest component defines the sub-periods within each year, and contributions and withdrawals are spread across those sub-periods. Where two periods overlap, the one defined last applies — so a contribution and a withdrawal covering the same years do not net against each other.
A = P × (1 + r/n)nt
A = final amount
P = initial principal (starting amount)
r = annual interest rate (as a decimal)
n = number of compounding periods per year
t = number of years
What Is My Investment Horizon?
Your investment horizon doesn’t end when you start withdrawing funds. It's the entire period your money stays invested — before and after retirement. Simulating beyond the withdrawal start point helps you plan sustainably for the long term and estimate how long your capital might last.
Shortening the horizon pulls every contribution and withdrawal period in with it, so nothing is left pointing past the end of the simulation. That matters when you compare a plan for twenty years against the same plan for thirty: only the horizon changes, everything else stays where you put it.
Understanding Investment Contributions
Regular contributions — whether monthly, quarterly, or annually — are key to building wealth steadily. Even small recurring amounts can grow substantially when combined with compound interest. Automating your contributions helps maintain discipline and reduce emotional decision-making.
The relationship is strictly linear: doubling the monthly amount doubles the contributions, the interest and the final balance. The horizon is not linear, which is why moving the start date earlier beats raising the amount. You can model a rising contribution as up to three separate periods.
Annual vs. Monthly Contributions: Does Timing Matter?
Contributing monthly instead of annually can slightly improve growth due to more frequent compounding. This effect, known as "contribution frequency benefit," isn't huge but adds up over time. More frequent investing also smooths out market volatility through cost averaging.
Put a number on it before you optimise: at 6 % over twenty years, switching a lump sum from annual to monthly compounding adds about 3 % to the final balance. One extra year at the same rate adds roughly twice as much. Frequency is worth understanding so nobody can use it to make a weaker product look stronger.
Withdrawals Don’t Stop Compounding
Even while taking money out, the remaining capital can continue to grow through compounding. Understanding this balance is key to making sustainable withdrawals without prematurely depleting your assets. Strategic planning allows you to withdraw while still benefiting from growth.
The sensitivity is sharp in this direction. On a portfolio of 500,000 € at 5 %, taking 2,500 € a month leaves money after thirty years; taking 3,000 € empties it in year 23. Twenty percent more income costs seven years of the plan, which is why a withdrawal rate deserves a stress test at a lower return.
Balancing Risk and Reward
Higher returns often come with higher risk. While it's tempting to aim for aggressive growth, balancing your risk tolerance with your financial goals is crucial. Diversification, time horizon, and consistency are more reliable than chasing quick gains.
Inflation belongs in the same trade-off. Six percent nominal with two percent inflation is 3.92 percent real, not four, and after half a percent of annual costs roughly 3.4 percent is left. Entering the real rate keeps every figure in the result in today's purchasing power, which is the only form comparable to a price you know.
Guides on compound interest
What each part of A = P × (1 + r/n)^(n·t) does to the result, how to solve it for rate or time, and where a single formula stops being enough.
How much more monthly compounding pays than annual, why the gap has a ceiling, and how to compare nominal against effective rates.
What different monthly amounts add up to over twenty-five years, what waiting five years costs, and when interest overtakes your own contributions.
Four withdrawal rates on the same portfolio, where the 4 % rule comes from, and why the order of the years matters once you are taking money out.
The exact way to turn a nominal return into a real one, what 2 % inflation costs over thirty years, and how to keep a whole plan in today's money.
Divide 72 by the interest rate and you have the doubling time. How accurate that is, where the 72 comes from, and the version worth carrying around.
What's changing for investors
- Trump Accounts open for contributionsUnited States