Questions and answers

Filisim helps you visualize how an amount invested in the markets could evolve over time, based on your own situation: a starting amount, regular contributions, and one or more life phases — for example "$10,000 today plus $100/month for 5 years", then "$0 initial and $200/month" once your saving capacity changes. The site does not tell you exactly how much money you will have on a given date — it shows you a range of possible outcomes, built from the historical behavior of the markets.

Filisim addresses three recurring methodological limitations found in typical financial simulators.

The first is the assumption of a constant investment value throughout the simulation: over a 20 or 30-year horizon, personal and professional situations change, and that assumption quickly drifts away from reality.

The second concerns how returns are represented: the smoothed growth curves used by many simple calculators show a single, deterministic trajectory that hides the real volatility of the markets.

The third concerns how results are processed statistically: an average alone doesn't capture the spread of possible outcomes. Filisim reintroduces the 10th/50th/90th percentiles, so you can see a range of outcomes rather than a single number.

Filisim tries to address these three points while staying simple and accessible, without requiring prior financial or statistical expertise. That said, this approach doesn't claim to remove every bias: the modelling assumptions, the historical data used, and the simplifications inherent to any simulator necessarily introduce limits of their own that are worth keeping in mind.

A quick thanks to Xavier Delmas, whose video Bourse = 10% par an ? Pas si sûr was the spark that led to building this site.

For someone managing their own personal finances who wants to explore different savings or investment scenarios on their own — not for financial professionals, and not as a regulated advisory service. This is not investment advice: it is a simulation and exploration tool for personal, educational use.

Worked example. Alice and Bob have €10,000 in savings. Alice is the only one working right now, while Bob finishes his last year of medical school, so together they can only set aside €100 a month. In a year, once Bob starts earning an income, they'll be able to save €500 a month. In three years, they're expecting a baby, and expect to save less during the baby's first three years — €200 a month. In five years, they plan to sell a side business for €100,000 and invest that lump sum with a 10-year horizon. In Filisim, this becomes four phases, entered exactly as described (including the fact that the "New baby" and "Selling the side business" phases overlap in time — phases don't need to follow one another, see the question on phases below):

• Before Bob's residency ends — starts now, lasts 1 year, €10,000 initial, €100/month.
• Bob starts earning — starts in 1 year, lasts 2 years, €0 initial, €500/month.
• New baby — starts in 3 years, lasts 3 years, €0 initial, €200/month.
• Selling the side business — starts in 5 years, lasts 10 years, €100,000 initial, €0/month.

With a 15-year total horizon (so the last phase fits exactly) and everything invested in a single historical stock market index, actually running this scenario in Filisim gives a total contributed of €129,600 (the sum of every euro Alice and Bob actually put in — fixed, not random), against a simulated combined outcome of around €178,000 at the 10th percentile, €343,000 at the median, and €653,000 at the 90th percentile.

These specific figures come from one real run of the simulator with these exact inputs — they are not invented, but running the same scenario again will give slightly different P10/P50/P90 values each time (the total contributed, on the other hand, never changes: it isn't simulated). Your own figures will of course depend entirely on your own situation.

The chart at the top of the page ("Combined outlook") shows the combined result of every phase you've created, added together. Below it, each phase you set up has its own detailed chart, specific to its own contributions and investment choices. Both views use the same colour language: a dashed line for the amount you actually contributed, and three lines for the simulated scenarios (10th, 50th and 90th percentile).

For every investment based on historical data, Filisim doesn't calculate a single outcome — it generates thousands of possible scenarios (10,000 by default), each one drawing random monthly returns from a statistical distribution calibrated on that asset's real history. Among all of those simulated scenarios, three complete, real trajectories are kept: the one that ends in the lower part of the range (10%), the one that ends in the middle (50%, the median), and the one that ends in the upper part (90%).

The parameters used for each asset — its average monthly return and its volatility — are calculated in advance from real historical prices, including reinvested dividends. These parameters are bundled into a single data file that the site loads when it starts. The site itself never connects to any external market data source while you are using it — it only reads this pre-built file.

They are estimated from the whole price history available on Yahoo Finance for each asset, sampled month by month. The catalogue is a fixed list of eleven: some are market indices, others are exchange-traded funds standing in for an asset class that has no index you can invest in directly.

• ^GSPC S&P 500
• ^NDX Nasdaq 100
• ^STOXX50E Euro Stoxx 50
• ^FTSE FTSE 100
• ^N225 Nikkei 225
• EEM Emerging Markets (MSCI EM)
• URTH World equities (MSCI World)
• GLD Gold
• TLT US Treasury 20+Y Bonds
• VNQ US Real Estate (REIT)
• BTC-USD Bitcoin

Each row links to that asset's page on Yahoo Finance, opened in a new tab, so you can look at the underlying history yourself.

One caveat worth keeping in mind: these histories are not the same length. The equity indices reach back decades, the exchange-traded funds start in the 2000s, and Bitcoin's history is barely a decade long — so the estimates are not equally solid from one asset to the next. Bitcoin in particular shows a measured volatility several times higher than anything else in this list, which is why any scenario using it produces an extremely wide gap between the P10 and P90 curves.

"Historical data" simulates uncertain behaviour, based on the real volatility observed in the past for that asset: results vary from one simulation to the next, with distinct P10/P50/P90 curves. "Fixed rate" applies a constant annual return that you choose yourself, with no simulated uncertainty at all — in that case, all three curves are strictly identical, because there is nothing to simulate.

Once a phase ends, there are no further contributions for that phase — but the capital already built up keeps growing (or fluctuating) according to the simulated returns until the end of the total horizon you defined. Phases are meant to represent changes in your saving capacity over time (for example, a raise at work), not withdrawals of capital.

"Volatile" shows the simulated path as it actually plays out, with its realistic ups and downs — a deliberate choice to remind you that markets never move in a straight line. "Smoothed" shows the exact same trajectory redrawn as a steady growth curve that starts at the same value and ends at exactly the same final value — purely to make it easier to read. This toggle changes nothing about the underlying calculation: it only changes how the same result is drawn.

No, not in the sense of a prediction. The P50 curve is the trajectory, among thousands simulated, whose final result sits exactly in the middle of the distribution: just as many simulated scenarios end higher as end lower than this value. It's a statistical estimate from a simplified market model — not an announcement of what your money will actually do. The real future of the markets could easily fall outside the P10–P90 range shown here.

No — this is a common and important misreading. The P10 curve is a complete, real simulated trajectory, chosen because its final value is low, not because every point along it represents "the 10th percentile at that exact date". At an intermediate date, the true 10th percentile across all simulated scenarios could actually be higher or lower than what the displayed curve shows at that same moment. The correct reading only applies to each curve's final value.

A wide gap means the chosen investment is simulated as highly volatile: possible outcomes are spread across a wide range, from the least to the most favourable. A narrow gap (or none, for a fixed rate) means the opposite — a more stable, or fully deterministic, simulated behaviour. This isn't a quality judgment on the investment itself — a more volatile asset can also have a higher median outcome.

Not necessarily. A higher median outcome often comes with a wider gap between P10 and P90 — meaning a larger risk of an unfavourable result too. Filisim does not compute any "best choice" score: it lets you compare possible trajectories, and it's up to you to weigh the level of risk you're willing to accept against the potential gain.

No. The amounts shown are gross figures, in current (non inflation-adjusted) currency: no tax, no brokerage or management fee, and no inflation is subtracted from the simulated results. The real purchasing power of your future capital will likely be lower than what is displayed.

Because the model calibrates its parameters on the return and volatility actually observed in the available history for that asset. For an asset like Bitcoin, that history shows a very high average monthly return and monthly volatility compared to traditional stock markets — which, once projected over a long horizon, produces extremely dispersed simulated trajectories. This is not a bug: it's an honest reflection of a real, but historically short and exceptionally volatile, track record — one that may become less and less realistic the further it is projected into the future.

No. Each phase is calculated independently, like a separate savings pocket, based only on the starting amount and contributions you entered for it. The results of all phases are then simply added together for the combined view. There is no automatic transfer of a phase's final capital into the next phase's starting capital — if you want a phase to start from the result of a previous one, you need to enter that amount yourself as its initial investment.

Filisim is an exploration and educational tool. It is not financial advice, and it is not a prediction tool. Before acting on anything you see here:

• The curves shown are simulations built from a simplified statistical model — not forecasts of what will actually happen to your money.
• Past market performance, which the model is calibrated on, is no guarantee of future performance. Future markets could behave very differently from the historical data used here.
• Important real-world factors are not modeled at all: taxes, management fees, inflation, and the way different assets can move together during a crisis.
• Filisim provides no financial, tax, or legal advice, makes no guarantee of accuracy or of any outcome, and accepts no responsibility or liability for any financial gain or loss resulting from decisions made using this tool.
• For any decision that matters to you — a large amount, a retirement horizon, an irreversible choice — treat this tool as one input among others, and seek independent professional advice.