🔥 FIRE Number Calculator

Key concepts

The FIRE number is the strategy, not the path. Most retirement planning fixates on the path (savings rate, contribution amounts, account choice), but FIRE collapses everything into one number: how much do you need invested today — in today’s dollars — for 4% of the balance to cover your annual spending forever. Annual Expenses ÷ Safe Withdrawal Rate. At a 4% SWR this is the classic “25× rule”: $40,000 of expenses needs $1,000,000 invested; $80,000 needs $2,000,000. The number is denominated in today’s purchasing power because the 4% rule’s underlying math already inflation-adjusts withdrawals annually with CPI — you don’t need to forecast future-dollar targets to plan the strategy. Knowing the number is what unlocks every downstream decision: which accounts to fund, how aggressively to save, when to downshift to Coast FIRE.

4% rule origin: Trinity Study (1998) and Bengen (1994). William Bengen’s 1994 paper “Determining Withdrawal Rates Using Historical Data” ran every 30-year period from 1926-1976 against a 50/50 stock/bond portfolio and found 4% was the highest rate that never failed within the dataset. The follow-up Trinity Study (Cooley, Hubbard, Walz 1998) extended the analysis to 50/50, 75/25, and 100% stock allocations and confirmed Bengen’s 4% conclusion for 30-year horizons with 100% historical success. The two critical fine-print items: (1) the studies assumed 30-year retirements (FIRE retirements are often 40-60), and (2) the data window was US-only and includes the strongest equity bull markets in human history. Most modern researchers (Wade Pfau, Michael Kitces, Karsten Jeske / Big ERN) suggest 3.0-3.5% is the more accurate “perpetual” safe rate for 50+ year horizons.

Lean, Regular, Fat, Coast — the FIRE tier system. Lean FIRE = annual spending of $30-50k (a deliberately frugal lifestyle, often urban and one-or-no-car). Regular FIRE = $50-100k (middle-class American median in 2026 dollars). Fat FIRE = $120k+ (comfortable upper-middle-class, with travel and discretionary spending built in). The tiers exist because the FIRE number scales linearly with spending: Lean is $750k-$1.25M, Regular $1.25M-$2.5M, Fat $3M+. Coast FIRE is the lower bar: enough invested today that compound growth alone gets you to your regular FIRE number by traditional retirement age (say 65) with zero further contributions. The Coast number is FIRE Number ÷ (1 + return)^(65 − current age) — at 7% real returns, the Coast number for a 30-year-old targeting $1M at 65 is about $115k. Hit Coast FIRE and you can downshift — switch to a lower-paying-but-fulfilling job, drop to part-time, or take a sabbatical — knowing the math catches up.

Sequence-of-returns risk is the silent FIRE killer. Two retirees with identical $1M portfolios and identical 4% withdrawal plans can end up very differently. The one who retires into a bear market in year 1 sells assets at the bottom to fund spending, depleting the share base permanently; the one who retires into a bull market builds enough cushion in years 1-5 to survive a later drawdown. This is sequence-of-returns risk and it hits the first 5-10 years of withdrawal disproportionately. Three classic defences: (1) hold 1-3 years of expenses in cash or short bonds at the moment of FIRE so you never sell equities during a drawdown; (2) use Guyton-Klinger guardrails (cut spending 10% in years where the portfolio drops >20%); (3) Barista FIRE — cover part of your spending with low-stress part-time work for the first 5-10 years so withdrawals are smaller while the sequence-of-returns danger is acute.

Three big mistakes that wreck FIRE plans. First, targeting Lean FIRE because it looks closer — lifestyle inflation, healthcare, or wanting kids later turns many Lean-FIRE plans into Regular-FIRE plans halfway through, with the lost compounding being brutal to recover. Most FIRE veterans now recommend planning for Regular FIRE with the optional ability to scale down rather than aiming for Lean from day one. Second, ignoring healthcare in early retirement: USA pre-Medicare (under 65) healthcare can be $15-30k/yr for a family on the ACA marketplace, and that’s a 20-25% addition to a Lean-FIRE budget. Third, anchoring on the 4% rule mechanically without re-running the math at 3.25-3.5% as a stress test — the dollar difference between 4% and 3.5% is roughly $200-400k for most FIRE plans, which is the difference between hitting FIRE at 45 versus 48. Always model both and treat the lower number as the upper bound of confidence.

Update annually, not monthly. Re-running your FIRE number every paycheque is anxiety-inducing and creates lifestyle-inflation drift — you’ll catch yourself revising the spending input upward to match incrementally higher actual spending. Once a year (ideally at tax-prep time, when your real annual spending is already in front of you) refresh four inputs: (1) actual annual expenses from last year’s spending data, not last month’s budget; (2) updated return-and-inflation assumptions checked against current 10-year inflation breakevens; (3) retirement-spending plan with a 10-20% healthcare buffer added to the post-65 figure; and (4) SWR trending down toward 3.5% as your specific retirement horizon becomes clearer. Major life events — marriage, kids, home purchase, divorce — warrant an off-cycle update.