The FIRE number: 25x spending and the 4% rule
The core idea of FIRE is simple: spend much less than you earn, invest the gap in a diversified portfolio, and stop working for money once the portfolio can fund your lifestyle on its own. The threshold is called your FIRE number, and the classic shortcut is 25 times your annual expenses.
The 25x figure is just the inverse of the 4% safe withdrawal rate: if you can withdraw 4% of a portfolio in year one (then adjust that dollar amount for inflation) without running out, then a year of spending divided by 4% equals 25 years of spending. The 4% rate comes from the 1998 Trinity Study, which tested every historical 30-year retirement in US market data; a 4% initial withdrawal from a stock-heavy portfolio survived nearly all of them, including retirements that started right before the Great Depression. It is a rule of thumb built on one country’s past, not a guarantee about anyone’s future.
| Assumption | Multiple | FIRE number |
|---|---|---|
| 4% withdrawal rate (classic) | 25x | $1,250,000 |
| ~3.3% withdrawal rate (conservative) | 30x | $1,500,000 |
Two things to notice. First, the number keys off spending, not income: a household that spends $50,000 needs the same $1,250,000 whether it earns $80,000 or $300,000. Second, early retirees planning for 40 or 50 years often use 30x instead of 25x, because the Trinity Study only tested 30-year windows and longer retirements deserve more margin.
Savings rate is the lever that matters
The single most useful insight in FIRE math: how fast you reach independence depends almost entirely on your savings rate, the percentage of take-home income you keep. A higher savings rate works on both ends at once. It grows the portfolio faster, and it proves you can live on less, which shrinks the target itself.
The table below runs the math honestly: start from $0, earn 5% real (after-inflation) returns, and stop when the portfolio hits 25 times what you spend. The formula is plain compound interest: if you save a fraction s of income, your target is 25 × (1 − s) years of income, and you solve for how many years of saving s it takes to compound there.
| Savings rate | Years to FI | Retire by (if you start at 25) |
|---|---|---|
| 10% | 51.4 years | Age 76 |
| 15% | 42.8 years | Age 68 |
| 25% | 31.9 years | Age 57 |
| 35% | 24.6 years | Age 50 |
| 50% | 16.6 years | Age 42 |
| 60% | 12.4 years | Age 37 |
| 70% | 8.8 years | Age 34 |
Read down the column and the story is clear. A traditional 10% saver retires in about 51 years, which is why the standard advice produces retirement at 65. Push to 25% and it drops to roughly 32 years. At 50% it is about 17 years, and at an aggressive 70% you are done in under 9. This is the Mr. Money Mustache result that launched a thousand FIRE blogs, and the arithmetic behind it is not exotic: cutting spending simply counts twice.
Real life is messier than the table. Most people do not start from zero, savings rates move around, and returns arrive in lumps rather than a smooth 5%. But the direction of the result survives all of that: the gap between what you earn and what you spend is the whole game.
Lean FIRE: independence on a small budget
Lean FIRE means retiring on deliberately low expenses, commonly under $40,000 a year for a household (some draw the line at $25,000 per person). Because the target is spending × 25, a lean budget shrinks the required portfolio dramatically: $30,000 of annual spending needs only $750,000.
It suits people who already live simply and enjoy it: low-cost-of-living areas, paid-off housing, no expensive hobbies. The reachable target is the appeal; the thin margin is the risk. A lean budget has little discretionary spending left to cut when markets fall or when life adds costs (a health issue, a dependent parent, a roof). Run your own numbers in the Lean FIRE calculator to see how much a tighter budget shrinks the target, and stress-test it with spending a few thousand dollars higher.
Fat FIRE: independence without the frugality
Fat FIRE targets a comfortable or affluent retirement, usually $100,000 or more of annual spending, which puts the portfolio target at $2,500,000 and up. The trade is obvious: a much bigger number, reached later or on a higher income, in exchange for a retirement that requires no budget discipline at all.
It suits high earners (tech, medicine, law, business owners) who want to keep their current lifestyle, and anyone for whom the lean version of retirement sounds like a second job. The generous budget doubles as a safety margin: a Fat FIRE household that trims 20% of spending in a bad market year still lives well, which makes the 4% rule meaningfully safer at this level. Model your own target and date with the Fat FIRE calculator.
Coast FIRE: front-load the saving, let compounding finish
Coast FIRE is the point where your existing portfolio will grow to your full FIRE number on its own, with no further contributions, by a traditional retirement age. The mechanic is different from every other variant: instead of asking "how big must the portfolio be today?", it asks "how big must it be today so that compounding alone closes the gap?"
The math discounts your FIRE number backwards. If your full target is $1,250,000 and you have 25 years until retirement at a 5% real return, you only need about $369,128 invested today; growth does the rest. Hit that milestone in your early 30s and you can drop to a job that merely covers your bills, switch careers, or work part-time, without giving up the retirement you already funded.
The catch is that you are still working until 60 or 65; Coast FIRE buys freedom in the kind of work, not freedom from work. It also leans entirely on the return assumption, since there are no future contributions to correct a weak decade. The Coast FIRE calculator computes your coast number at any retirement age and shows how sensitive it is to the return you assume.
Barista FIRE: part-time work plus a partial portfolio
Barista FIRE splits the bill between a part-time job and your investments. Instead of waiting for the portfolio to cover 100% of spending, you leave full-time work once it can cover part of it, and a lower-stress job pays for the rest, ideally one with health insurance (the nickname comes from Starbucks famously offering benefits to part-time baristas).
The math is the same 25x rule applied to a smaller number. If you spend $50,000 a year and part-time work brings in $25,000, the portfolio only needs to fund the other $25,000, so the target falls to $625,000 instead of $1,250,000. That can pull the exit from a high-pressure career forward by a decade.
It suits people who want out of their main career years early and do not mind (or actively want) some ongoing work. The risks are practical: part-time jobs with good benefits are not guaranteed to exist when you need one, and a plan that depends on earned income is only semi-retired. The Barista FIRE calculator lets you model the part-time income, the reduced target, and the crossover date.
All five variants, side by side
Every variant runs on the same engine (spending × a multiple, funded by compounding). They differ on two axes: how much lifestyle the target funds, and whether work disappears entirely or just changes shape.
| Variant | Typical target | Work after the milestone | Who it suits |
|---|---|---|---|
| Lean FIRE | ~$625,000 to $1,000,000 (spending under $40k/yr) | None | Naturally frugal people in low-cost areas who value time over consumption |
| Regular FIRE | ~$1,000,000 to $2,500,000 (spending $40k to $100k/yr) | None | Middle-to-high earners who can hold a 30% to 50% savings rate for 15+ years |
| Coast FIRE | A fraction of the full number today (e.g. ~$369,128 for a $1,250,000 goal 25 years out) | Full- or part-time until a normal retirement age, but only to cover current bills | Aggressive early savers who want career flexibility more than an early exit |
| Barista FIRE | Roughly half the full number (e.g. ~$625,000 when part-time work covers half of $50,000/yr) | Part-time, ideally with health benefits | People escaping a high-pressure career who still want some work and income |
| Fat FIRE | $2,500,000+ (spending $100k+/yr) | None | High earners who want early retirement without changing their lifestyle |
The labels matter less than the levers behind them. Most real plans mix elements: a household might coast for five years, then barista for three, then fully retire on a budget between lean and fat.
The honest criticisms of FIRE
FIRE math is sound, but the movement has real weaknesses worth naming before you reorganize your life around it.
- Sequence-of-returns risk. The 4% rule survives most history, but a deep bear market in your first five retirement years forces you to sell depressed assets to eat, which permanently shrinks the portfolio. Early retirees face this risk over 40+ year horizons the Trinity Study never tested. Cash buffers, a lower withdrawal rate, and spending flexibility all help; none eliminate it.
- Healthcare before Medicare. A US retiree at 45 has 20 years to bridge before Medicare at 65. ACA marketplace plans make this workable, but premiums plus out-of-pocket costs can add $10,000 to $25,000 a year for a family, and subsidy rules change with politics. Any FIRE budget that ignores this line item is fiction.
- Life happens. Divorce, disability, a child with special needs, supporting aging parents: a plan calibrated to precise spending 30 years out will meet events the spreadsheet did not include. The honest answer is margin (a lower withdrawal rate, some earning power kept warm), not more decimal places.
- The deprivation critique. Critics argue that extreme savings rates trade your actual 30s for a hypothetical retirement, and for some people that trade goes badly: frugality becomes an identity, or the long-awaited retirement arrives and feels empty. The counterargument is that most FIRE followers land at moderate savings rates and report more agency, not less. Both things can be true; know which person you are.
How to get started
You do not need to commit to an early retirement date to benefit from the mechanics. The first steps are the same for every variant:
- Track your spending for 90 days. Your real annual spending is the input every other number depends on. Most people are off by 20% or more when they guess.
- Compute your FIRE number and your current savings rate. Spending × 25 (or × 30 if you are conservative), then check what fraction of take-home pay you actually keep. These two numbers locate you on the map.
- Raise the savings rate where it is cheap. Housing, cars, and food dominate most budgets; a structural fix there beats a hundred small sacrifices. Direct the freed-up cash to tax-advantaged accounts first (401(k), IRA, HSA).
- Invest in broad, low-cost index funds and leave them alone. The 4% rule’s evidence base assumes diversified market returns, not stock picking. Fees and tinkering are the quiet killers of compounding.
Then put your own numbers into the FIRE calculator: it takes your age, savings, spending, and return assumption and returns your FIRE number, your projected FI date, and how much a higher savings rate moves it.