Estimate the portfolio size needed to cover your annual spending indefinitely, then see how a monthly savings habit gets you there — after inflation, and across a range of outcomes.
How this is calculated: the projection grows your savings at the real return, (1 + return) ÷ (1 + inflation) − 1, compounded monthly, so every figure is in today's dollars and is directly comparable to your FIRE number. It assumes your monthly savings rise with inflation, returns are constant every year, and there are no taxes or fees. Real markets are uneven, which is why the sensitivity range matters more than any single estimate.