What This Calculator Does
The Microapp FIRE Number Calculator answers the headline question of the Financial Independence, Retire Early movement: how much money do I need invested before I can stop working? It applies the 4% safe-withdrawal rule from the 1998 Trinity Study — your FIRE number is approximately 25× your annual expenses — and projects how long it'll take to get there at your current savings rate. Three variants are shown side-by-side: Lean FIRE (12.5× expenses, the frugal anchor), Regular FIRE (25×, the standard), and Fat FIRE (50× expenses, the 2% withdrawal hedge). The years-to-FIRE projection uses inflation-adjusted compound math, so the answer is in today's dollars.
FIRE numbers: Lean $625,000 · Regular $1,250,000 · Fat $2,500,000.
Progress: 8% of the way to Regular FIRE.
Years to Regular FIRE: ~22.4 years.
Levers to test: bump the monthly contribution to $3,000 → years drops to ~18. Cut annual expenses to $40,000 → FIRE number drops to $1M and years drops further. The calculator makes the trade-offs visible.
Where the 4% Rule Comes From
In 1998, three finance professors at Trinity University in San Antonio — Philip Cooley, Carl Hubbard, and Daniel Walz — published Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable. They tested historical US market data from 1926 onward, simulating 30-year retirements with various withdrawal rates and stock/bond allocations. The headline finding: a portfolio split roughly 50/50 between stocks and bonds, with 4% of the initial balance withdrawn in year one and that figure increased for inflation each subsequent year, survived all 30 years in 95%+ of historical periods.
The 25× rule is just the inverse: if you can safely withdraw 4% per year, you need 1 ÷ 0.04 = 25× annual expenses invested. The math is simple; the genius is the empirical backstop. Earlier retirement-planning rules of thumb (3% withdrawals, 5% withdrawals) were largely vibes; Trinity actually ran the numbers against history. Subsequent studies — Wade Pfau's work, Big ERN's Safe Withdrawal Rate Series — have refined the picture (4% is closer to safe than absolutely safe; 3.5% is safer for very long horizons), but the 25× shorthand has stuck.
Lean vs Regular vs Fat — Three Flavors of the Same Math
The three FIRE variants don't change the formula; they change the expense baseline you apply it to. The convention in the FIRE community:
| Flavor | Multiplier | Withdrawal rate | Typical lifestyle |
|---|---|---|---|
| Lean FIRE | ~12.5× expenses | ~8% withdrawal | Minimalist. Lower-cost geography. Often involves geographic arbitrage (e.g. retire from a US salary to Lisbon or Mexico City). |
| Regular FIRE | 25× expenses | 4% withdrawal | Current lifestyle, sustained. Trinity Study target. |
| Fat FIRE | 50× expenses | 2% withdrawal | Upgraded lifestyle. Buffer against sequence-of-returns risk. Common in tech-FIRE communities. |
Most calculators show only the 25× number. We show all three because the trade-off is real: a 25× retirement at 4% has historical failure cases (specifically, retiring into the 1966–1981 stagflation period would have depleted a 4% portfolio in about 28 years). A 50× retirement at 2% has no historical failure cases in the US dataset. The cost is doubling the FIRE number, which extends working years by roughly the time it takes your invested savings to double — about 10 years at 7% real returns. That's the trade.
The Time-to-FIRE Math
The headline FIRE number tells you the destination. The years-to-FIRE figure tells you when you'll arrive. The formula is the future-value-with-contribution equation, run in real terms:
FV = P × (1+r)^t + C × ((1+r)^t − 1) / rwhere P is your current invested savings, C is your annual contribution (monthly × 12), r is the real return rate, and t is years. Solve for t when FV equals your FIRE number:
t = ln((FV·r + C) / (P·r + C)) / ln(1+r)The "real return" is the key input. Historical US equity real return (1926–2023, dividends reinvested) is roughly 7% per year. A 60/40 stock/bond portfolio is closer to 4–5% real. A 100% bond portfolio is currently near 2% real. If you use a nominal return number (say 10%) you'll under-estimate your time-to-FIRE because inflation will eat a chunk of the growth before you actually need the money. Always use real returns when comparing future spending to today's expense baseline.
The Savings-Rate Lever
One of the more counterintuitive results in the FIRE literature comes from a 2012 Mr Money Mustache post titled The Shockingly Simple Math Behind Early Retirement: at a given investment return, the years-to-FIRE depend almost entirely on your savings rate, not your salary. The math is straightforward — if you save 50% of take-home pay, you're investing 1 unit for every 1 unit you spend, so each year of work funds roughly one year of retirement (modified by compound growth). The full table at 5% real return looks roughly like:
| Savings rate | Years to FIRE |
|---|---|
| 10% | ~51 years |
| 25% | ~32 years |
| 50% | ~17 years |
| 65% | ~10.5 years |
| 75% | ~7 years |
This is why FIRE writers focus relentlessly on the spending side. A $200,000 salary spent down to a $180,000 lifestyle (10% savings rate) takes longer to retire from than a $60,000 salary spent down to $30,000 (50% savings rate), despite the income gap. Income matters, but only insofar as it makes a high savings rate possible. The lever is the gap between earning and spending.
What This Calculator Doesn't Model
Three real-world factors the basic 25× calculation glosses over:
Healthcare costs in the US. Pre-Medicare (before age 65), private health insurance for a couple via the ACA marketplace can run $15,000–$25,000/year depending on income, state, and plan. The 4% rule was developed when most retirees were Medicare-eligible immediately; early retirees face a 10–25-year gap. Most FIRE planners add a $10,000–$20,000/year healthcare line to their expense projection. Use the higher figure if you're under 50.
Taxes on withdrawals. The Trinity Study tested pre-tax returns; it assumed your expense figure already included any taxes you'd owe. If you're holding most of your savings in tax-deferred accounts (401(k), traditional IRA), withdrawals are taxed as ordinary income, and you'll need a higher portfolio to net the same after-tax spending. Roth accounts solve this if you have enough basis to draw from. The simplest fix is to enter your gross expenses including a tax cushion.
Sequence-of-returns risk. The 4% rule's "95% historical success" includes some periods where the portfolio came perilously close to running out. The worst case in US history was retiring in 1966 — high inflation, weak stock returns, and a 28-year-survival outcome for the 4% rule. Mitigations include (1) holding 1–2 years of cash as a buffer to avoid selling stocks in down years, (2) flexible withdrawals — drop spending by 10–20% in bad market years (a 'guardrails' approach popularized by Jonathan Guyton), and (3) maintaining some flexible income early in retirement. Fat FIRE (50×) is essentially the brute-force solution to all three.
How to Use This Tool With Other Calculators
The FIRE Number Calculator handles the destination question. Three related Microapp tools handle the path:
- Use the Compound Interest Calculator to test specific year-by-year growth scenarios. The FIRE calculator outputs a single years-to-target; compound-interest shows the trajectory.
- Use the Millionaire Calculator to set a fixed dollar target ($1M, $2M, etc.) regardless of expense level — useful if you want to hit a number rather than fund a lifestyle.
- Use the 401(k) Calculator to project the employer-match-augmented portion of your contribution. Most FIRE plans assume you're capturing the full match before doing anything else.
- Use the Emergency Fund Calculator to size the cash buffer that protects against sequence-of-returns risk in the early years.
Educational Tool — Not Financial Advice
The 4% rule and the 25× target are historical heuristics, not guarantees. Real-world outcomes depend on your specific portfolio mix, tax situation, healthcare costs, family circumstances, geographic choices, and the markets' actual behavior over the decades that will determine your retirement. This calculator is one input to a planning conversation — useful for setting an order-of-magnitude target and seeing the impact of savings-rate changes — not a substitute for a personalized plan. For real retirement planning, consult a fee-only fiduciary advisor (one paid by you, not by the products they recommend). For tax-optimization questions, a CPA or qualified tax planner.