Coast FIRE is the point where your investments can grow to your retirement number with no more saving. Most calculators stop at a single date, which is the average case. If you stop saving exactly on that date, a plan like that only works about half the time.

I built a free calculator that adds two things: your coast date to the month, and the share of 1,000 simulated markets where the plan reaches your goal (log-normal yearly returns, 15% volatility, fixed seed so results are reproducible). It also shows what-if rows for saving more, retiring later, lower returns or higher fees.

It runs entirely in your browser, with no sign-up. You can try it at Coast FIRE Planner, and the formulas and sources are on the methodology page.

Feedback on the model is welcome.