Simulate your net worth growth
Move the sliders to see how your estimated capital changes based on monthly savings, expected returns, and time horizon.
How to use this calculator
Enter your monthly contribution, an estimated annual return rate, and a time horizon. The calculation is illustrative and does not constitute financial advice, but it helps visualize orders of magnitude and compare scenarios quickly.
You can adjust each parameter independently to understand which variable has the greatest leverage on your final net worth. Spoiler: it is almost always time.
The mathematical compounding formula
The classic compound interest formula is: A = P × (1 + r/n)^(n×t), where A is the future value, P is initial principal, r is the annual rate, n is compounding frequency, and t is time in years.
With monthly recurring contributions, the formula expands to: FV = C × [(1 + r/n)^(n×t) - 1] / (r/n), where C is monthly contribution and FV is cumulative future value.
What makes compounding special is that gains generated in one period are added to principal, producing brand new gains in subsequent periods. Over time, growth transitions from linear to exponential.
- A = future total capital.
- P = initial deposit (assumed 0 in this calculator to isolate monthly contributions).
- r = annual interest rate in decimal form (6% = 0.06).
- n = compounding periods per year (12 for monthly).
- t = time horizon in years.
Practical examples: the real power of time
If you invest €200/month at a 7% return over 30 years, your direct contribution is €72,000, but your total accumulated wealth exceeds €243,000. Over €170,000 comes solely from reinvested compound growth.
With the exact same €200/month contribution over a 15-year horizon, your contribution is €36,000 and total wealth reaches ~€63,000. The €180,000 difference is explained almost entirely by 15 missing years of compounding.
Investing €500/month over 25 years at 6% yields ~€346,000 compared to €150,000 in personal savings. Doubling your monthly contribution triples your long-term wealth outcome.
How to interpret the results
The final figure is an estimate, not a guaranteed promise. Its primary value is testing scenarios: starting earlier versus later, contributing more or less, and seeing the compounding impact of time.
You can also use this tool to calculate the opportunity cost of delaying. Every year spent uninvested loses not just capital, but the cascading compound interest that capital would have created.
Video walkthrough: compound interest in 5 minutes
In this video we explain visually how compounding works, why time is your greatest asset in building wealth, and how to integrate these projections into your broader eXcenda balance sheet.
What to do next
Combine these projections with your net worth and cash flow. This ensures your investment targets are realistic within your current balance sheet.
To take your planning further, check our retirement simulator to see how recurring contributions fit into long-term retirement and tax strategies.
Apply This Knowledge to Your Real Finances
eXcenda combines net worth, expenses, simulations, and macro context into a single app so you can move from theory to action.
Frequently Asked Questions
No. The calculation is a simplified model to demonstrate compounding. You should account for expected taxes, broker fees, and fund expense ratios (TER) when making real-world decisions.
For globally diversified equity index funds, 5% to 8% real annual return is a reasonable long-term historical benchmark, though past performance never guarantees future results.
Both matter, but time exerts disproportionate leverage due to compounding. Given the choice between starting earlier with less or starting later with more, starting earlier almost always wins.
It provides a great baseline reference, but comprehensive retirement planning requires integrating expenses, state pensions, taxes, and asset allocation using our retirement simulator.
It describes how annual interest earned is added to principal, generating even more interest on a larger base. As decades pass, gains dwarf original contributions.