Quick answer
This calculator sizes two versions of financial independence: lean FIRE on a frugal budget and fat FIRE on a generous one. At $3,000 a month of spending, lean FIRE needs a pot of $900,000. At $10,000 a month, fat FIRE needs $3,000,000. With $80,000 invested and $1,500 a month saved at 7%, lean arrives in about 17.8 years and fat in about 32.5 — a 14.8-year gap.
How the Fat FIRE vs Lean FIRE works, step by step
Enter your lean and fat monthly spending figures, what you have invested and your monthly saving. The calculator multiplies each spending level by 12 and then by 25 to get the two target pots (the 4% withdrawal guideline), then simulates month by month — balance compounding at your return plus the monthly contribution — until each target is crossed.
The useful output is the gap in years, not just the gap in dollars. On the default figures the fat target is $2,100,000 larger but takes 14.8 extra years, because the pot is bigger and your contributions are a smaller share of its growth. The last stretch to fat FIRE is disproportionately long: by then the portfolio's own growth rivals your contributions, so each additional year of work buys more than it did at the start.
Most people land between the poles. Planning on $6,000 a month sets a $1,800,000 target and lands in about 25.9 years on these inputs — roughly 8 years after lean and 6.6 years before fat. The calculator's amber panel states the gap explicitly so you can see what the middle ground costs in years, not just in dollars.
Test the spending inputs before anything else. The difference between $6,000 and $7,000 a month of planned retirement spending is $300,000 of required pot — several years of saving for most households. Knowing which expenses actually drive your quality of life is worth more than any return assumption, and it is the input only you can set.
Remember what the 25× rule assumes. It is the inverse of the 4% withdrawal guideline, derived for roughly 30-year retirements from US market data. For a retirement starting at 40 that may last 50 years, a 3.25% to 3.5% withdrawal rate — a 29× to 31× pot — is a more common planning choice, and it pushes both targets and both dates further out.
The formula
Target pot = Monthly spending × 12 × 25. Time to target is simulated month by month: balance grows at return ÷ 12 and the monthly saving is added until the balance reaches the target.
Worked example — try it on the calculator above
Worked example: $80,000 invested, $1,500 a month saved, 7% return
Lean FIRE at $3,000 a month: target $900,000, reached in about 17.8 years. Fat FIRE at $10,000 a month: target $3,000,000, reached in about 32.5 years. The fat lifestyle costs roughly 14.8 extra years of saving on this plan. The middle ground — $6,000 a month, a $1,800,000 target — lands in about 25.9 years. Raising the monthly saving from $1,500 to $3,000 would cut the fat timeline from 32.5 years to about 25.5, which is why income growth directed entirely to saving is the lever that moves these dates most.
Frequently Asked Questions
What spending levels count as lean and fat FIRE?
The community usage is loose: lean FIRE usually means under about $40,000 to $45,000 a year of spending, fat FIRE above $100,000 a year, with regular FIRE between. The labels matter less than your own two numbers — price the life you would accept and the life you would choose, and look at the years between them.
Is the 4% rule safe for a 40- or 50-year retirement?
It was derived for roughly 30-year retirements. For very early retirees, a 3.25% to 3.5% withdrawal rate (a 29× to 31× pot) is a more common planning choice, because the portfolio must survive more market cycles and there is less room to return to work. Building the target with 25× and retiring at 35 leaves thin margins.
Does fat FIRE require a higher income, or just more time?
Either. In the worked example, the fat target takes about 32.5 years on $1,500 a month of saving. Raising saving to $3,000 a month cuts it to roughly 25.5 years. The mathematics is indifferent: more saved per month, or more months. Income growth directed entirely to saving is the lever that moves it most.
Should my FIRE spending include housing and healthcare?
Yes — the target must cover all spending the portfolio will fund, including housing (or rent), health insurance premiums before Medicare at 65 (a major early-retirement cost in the US), and taxes on withdrawals. Paid-off housing lowers the spending figure substantially, which is why many early retirees treat the mortgage payoff as part of the plan.
Estimates, not advice. This calculator provides educational estimates based on the inputs you enter and the assumptions shown on this page. It is not financial, tax or investment advice, and it cannot see your full situation. Verify important figures against official sources, and consider a qualified professional for decisions about your own money. See our Disclaimer and Methodology.