How the FIRE math works
FIRE (Financial Independence, Retire Early) asks a single question: how large a portfolio would cover this year's spending forever, using a conservative withdrawal rate? The conventional answer is the 4% rule from William Bengen (1994) and the Trinity Study (1998): withdraw 4% in year one, adjust for inflation, and a diversified portfolio historically lasted 30 years.
That makes the FIRE number annual expenses ÷ 0.04, or 25× spending. A 3.5% rate (more common for 40–50 year early-retirement horizons) is 28.6×. This calculator lets you set the rate yourself.
Years to FI assume a constant real (after-inflation) return and a constant savings contribution. If r is the real return,P today's portfolio, s annual savings, and F the FIRE number:
n = ln((F·r + s) / (P·r + s)) / ln(1 + r)
Savings rate is the sharpest lever because it works twice: you add more each year, and you shrink the pile you must accumulate. Coast FIRE is the balance that would grow to your FIRE number by age 65 with no further contributions. Lean and Fat FIRE are 70% and 150% of current spending.
This is a model, not a forecast. Sequence-of-returns risk, taxes, healthcare, and spending that refuses to stay flat all matter in real life. Use it to feel the sensitivity — then plan with a margin.