FIRE is not a vibes-based dream — it is a solvency equation with three variables: your expenses, your portfolio, and time. This playbook covers the 4% rule’s real origin, the Coast FIRE shortcut, and the account-stacking order that decides how much of your number taxes quietly confiscate.
1. The 4% Rule: What the Trinity Study Actually Said
The 1998 Trinity Study back-tested portfolios of stocks and bonds against every 30-year retirement window since 1926 and asked: what withdrawal rate survived nearly all of them? The answer was roughly 4% of the starting portfolio, adjusted for inflation each year. Flip it and you get the FIRE number: annual expenses × 25.
Two honest caveats. First, 4% was the historical floor, not a guarantee — sequence-of-returns risk (a crash in your first retirement years) is what kills plans, not average returns. Second, early retirees face 40–60-year horizons, not 30; many planners now use 3.3–3.5% for retirements before 45, which raises the number to 28–30× expenses.
FIRE CalculatorYour FIRE number from expenses and withdrawal rate — plus the year your current trajectory crosses it.→2. Coast FIRE: The Milestone That Changes Everything Earlier
Full FIRE means your portfolio funds you forever starting tomorrow. Coast FIRE is weaker and more useful: your invested assets, with zero further contributions, grow to your full FIRE number by traditional retirement age. Once you hit Coast, you only need to cover current expenses — you can downshift careers, take the interesting lower-paying job, and let compounding finish the job.
// The FIRE number (4% rule, Trinity Study)
FIRE number = annual expenses / 0.04 = 25 x expenses
Coast FIRE = FIRE number / (1 + r)^years_left
// r = real annual return (e.g. 0.05 after inflation)
//
// $40k/yr expenses, age 30, retire at 60, r=5%:
// FIRE = $1,000,000
// Coast FIRE = 1,000,000 / 1.05^30 ≈ $231,000At a 5% real return with 30 years of runway, every $1 invested today is $4.32 at retirement — which is why Coast FIRE typically lands at 20–30% of the full number. It is the highest-leverage checkpoint in the entire FIRE journey.
3. The Savings Rate Is the Only Dial That Matters
FIRE math has a brutal asymmetry: raising your savings rate attacks the problem from both sides — you invest more and your target shrinks, because expenses define the FIRE number. At a 10% savings rate, the working horizon is ~50 years; at 25%, about 32; at 50%, roughly 17 years; at 65%, under 11.
This is why the movement obsesses over the big three — housing, transport, food — rather than lattes. Cutting $1,000/month of recurring expense both adds $12,000/year of investing and removes $300,000 from your required portfolio.
4. Tax-Advantaged Stacking: The Order of Operations
Two people with identical savings end up years apart depending on which accounts captured the money. The evidence-based priority queue:
- 401(k) up to the employer match — an instant 100% return; nothing else competes.
- HSA — triple tax-advantaged (deductible in, tax-free growth, tax-free out for medical); after 65 it behaves like a traditional IRA.
- Roth IRA — taxed now, never again; decades of compounding arrive tax-free, and contributions (not earnings) can be withdrawn anytime — a stealth early-retirement bridge.
- 401(k) to the max — then taxable brokerage last.
The Roth-versus-traditional question reduces to one comparison: your marginal tax rate today versus your expected rate in retirement. High earner in a peak year? Traditional. Early career or planning Roth conversions in low-income FIRE years? Roth.
Roth IRA CalculatorProject tax-free growth to retirement — and compare the Roth balance against a taxed account with the same contributions.→5. Sequence Risk and Guardrails: Retiring Into a Crash
Two retirees with identical average returns can end in opposite places: the one who crashed in year 1–3 runs out; the one who crashed in year 25 barely notices. Mitigations with real track records: hold 1–2 years of expenses in cash so you never sell into a bottom; use a flexible withdrawal rule (skip the inflation raise after down years); and consider a partial annuity or bond tent for the fragile first decade.
Retirement CalculatorNest-egg projection with the 4% rule income line — stress-test your plan before it stress-tests you.→ROI CalculatorAnnualized return on any investment — the honest input every projection above depends on.→The First Session, Thirty Minutes
- Compute your real annual expenses — tracking beats guessing by 20–30%.
- Get your FIRE number (×25, or ×30 if retiring before 45).
- Get your Coast FIRE number — the checkpoint you will hit shockingly early.
- Fix the account order before adding a dollar; the queue above is the free lunch.
- Re-run the numbers once a year. The plan is a living model, not a tattoo.