Lean FIRE is the minimalist's route to early retirement — you keep your spending low (think $25-40k a year), which means the pot you need is a lot smaller than the standard advice assumes. The trade-off is obvious: less buffer for surprises, so the maths has to be tighter. This calculator shows whether your number actually holds up on a lean budget over a long retirement, not just on paper.
Put in your age, what you've saved and when you want out, and watch what a lean drawdown looks like across 30-40 years. The bit most Lean FIRE guides gloss over is the first few years — on a thin margin, a rough market run early does real damage (sequence-of-returns risk, if you want the textbook name). Just try it, then nudge your target spending up a touch and see how much breathing room a small increase buys you.