Historical simulation vs. average-return assumptions
Many calculators apply one flat expected return — say, 7-10% a year — evenly across every year of a projection. That erases the actual lumpy, uneven path real markets take. This simulator instead replays actual monthly total returns starting from any real historical date the underlying data supports, so the bumps stay in.
The difference shows up most clearly once withdrawals or contributions enter the picture: a flat 8%-a-year assumption treats every year identically, while the real historical path might have delivered -35% one year and +40% the next on the way to a similar long-run average — and when that volatility lands relative to a withdrawal plan changes the outcome substantially.
What “every crash, every bull run” actually covers
Depending on how far back a chosen ticker's history extends, the available data can span the dot-com crash of 2000-2002, the 2008 financial crisis, the 2020 COVID crash, and the 2022 bear market — alongside the recoveries and bull runs between them. Testing a plan against several genuinely different market regimes, not just the friendliest one, is the point.
Why starting month matters as much as strategy
Two investors running an identical strategy, starting even a year or two apart, can end up with meaningfully different outcomes purely from timing. That's what the historical outcomes chart is built to show: every possible historical starting month tested side by side, not just one backtest run picked in advance.
Simulating a single fund vs. a full portfolio
Testing one broad fund alone — the VOO-only preset loaded above, for example — is a useful starting point for understanding how a single asset class behaved across history, without the added variable of an allocation mix. Building out a full multi-holding portfolio afterward shows how combining assets changes that picture, which is the more realistic test for an actual investment plan.
How this differs from a projection
A pure projection applies one assumed constant return going forward — useful for a rough estimate, but it can't show what an actual downturn does to a withdrawal plan mid-stream. This simulator falls back to a flat-rate projection only in the one case where there's no market data to test against: a 100%-cash plan, modeled as a constant-rate savings account rather than a historical backtest.
How to use this simulator
Pick a time period (5 years, 10 years, full history, or a custom start month), build a portfolio by searching any of over 14,000 tickers or starting from a preset, and set a spending or contribution plan. Then check the year-by-year outcomes chart to see how sensitive the results are to exactly when the plan started.