A deterministic simulation is the simplest form of financial projection. Three fixed assumptions (a constant return, stable inflation, regular contributions) are enough to draw a single trajectory of your wealth. It is your Plan A: the straight line to financial independence.
Starting capital: $10,000. Contribution: $200/month ($2,400/year). Return: 7% per year. Result after 25 years: ~$215,000. Of which ~$70,000 was contributed by you and ~$145,000 from compound interest. The curve is smooth, regular, reassuring... but reality is never that smooth.
Each year, the simulator applies the same recipe: current capital + monthly contributions + annual return − inflation. The result? A curve that rises (or falls) predictably. No surprises, no randomness. Just math, and nothing else.
Reality never follows a straight line. Markets fluctuate: +25% one year, -15% the next. The deterministic simulation does not capture this volatility. You can be perfectly on track on paper and watch a crash at the wrong moment erase three years of progress. That is precisely the risk Monte Carlo lets you quantify (probability, magnitude, recovery time).
Three solid use cases: answering 'how much should I save per month?', comparing two scenarios side by side (staying home vs Portugal), or setting a 10-year wealth target. To validate a real FIRE decision, follow up with Monte Carlo.