Why retirement calculators disagree

Because they use different methodologies. A fixed-rate calculator assumes one average return every year and can only say pass or fail. A Monte Carlo calculator runs thousands of randomized market scenarios and reports a probability. A historical-backtest calculator tests your plan against real market history and reports how many actual past periods would have worked. Same inputs, three different lenses.

None of them is simply "correct." Fixed-rate projection is the simplest and the least realistic — real markets never return the same amount every year. Monte Carlo captures a wide range of possible futures but samples from randomized sequences, not sequences that actually happened. Historical backtesting is grounded in real market history, including real crashes, but is limited to the roughly one century of data available. Reading the three together is more honest than trusting any single number.

The Monte Carlo method here uses the same core approach — thousands of randomized trials drawn from historical return data — but institutional versions often use proprietary capital-market assumptions or require account linking. This version runs entirely in your browser: no balance or withdrawal figures are ever sent anywhere.

Annual S&P 500 total returns (with dividends) and 10-year Treasury bond returns from 1928 through the most recent complete year, the same category of academic dataset tools like cFIREsim use for historical backtesting.