Methodology

1. FIRE number

Your FIRE number is the portfolio size that, at a chosen safe withdrawal rate (SWR), is expected to support a given annual spend:

FIRE number = Annual spending ÷ Withdrawal rate
Multiplier  = 1 ÷ Withdrawal rate

Example (4% rule): $60,000 ÷ 0.04 = $1,500,000 (25×)

Lean / Regular / Fat are only spending presets for convenience. They are not standards of living defined by academic research — edit them to your actual budget.

2. Safe withdrawal rates & the “4% rule”

The popular 4% starting withdrawal rate is associated with work on historical retirement portfolio survival in the United States — commonly traced to William Bengen (1994) and the “Trinity Study” (Cooley, Hubbard & Walz, 1998 and updates). These studies ask, in substance: for a given stock/bond mix and initial withdrawal rate adjusted for inflation, how often would a portfolio have lasted 30 years in past U.S. market history?

• 4% is a widely cited starting point for ~30-year horizons — not a guarantee.
• Early retirement (40–50+ year horizons) often motivates more conservative rates (e.g. 3–3.5%) or flexible spending rules.
• Outcomes depend on asset allocation, fees, taxes, sequence of returns, and spending flexibility — none of which our simple calculator fully models.

3. Years to FIRE

We project a constant real return with end-of-year contributions:

T = P(1+r)^n + C × ((1+r)^n − 1) / r

Solved for n (years):
n = ln((T·r + C) / (P·r + C)) / ln(1+r)

If r ≈ 0: n = (T − P) / C

Where P is current portfolio, C annual contribution, r real return, and T the FIRE target. This is a smooth compound-growth illustration — not a Monte Carlo simulation.

4. Coast FIRE

Coast FIRE asks: how much do you need today so that, with no further contributions, compound growth reaches full FIRE by a chosen traditional retirement age?

years = retirement age − current age
coast number = FIRE number ÷ (1+r)^years

Projected at retirement (stop saving now):
FV = current × (1+r)^years

Hitting Coast FIRE means you could stop saving for retirement — not that you must stop working or that early retirement is funded yet. Lifestyle, healthcare, and Social Security are out of scope of this simple model.

5. Barista FIRE (semi-retirement)

When part-time or flexible work covers some spending, the portfolio only needs to fund the remainder:

gap = max(0, annual expenses − work income)
barista number = gap ÷ withdrawal rate

Years to Barista uses the same compound-growth solver as Years to FIRE, with the barista number as the target. Job stability, benefits, and taxes are not modeled.

6. Savings-rate table

Holding lifestyle spending fixed, each savings rate s implies income = spending ÷ (1−s) and annual savings = s × income. We then solve years to the full FIRE target from your current portfolio. Rows near your current implied savings rate are highlighted in the UI.

7. Real vs nominal returns

By default, expected return is real (after inflation), matching targets expressed in today’s dollars. In nominal mode we convert:

real ≈ (1 + nominal) / (1 + inflation) − 1

Projections always compound at the effective real rate so FIRE targets stay in today’s purchasing power.

8. Default assumptions

9. What we deliberately omit (for now)

• Taxes (ordinary income, capital gains, RMDs, NIIT)
• Investment fees and advisory costs
• Social Security, pensions, and annuities
• Healthcare and ACA subsidy cliffs
• Sequence-of-returns risk and Monte Carlo paths
• Inflation shocks, currency risk, and home equity strategies

Omitting these keeps the tools understandable. Adding them later will always ship with the same transparency standard.

10. Further reading

• Bengen (1994) and Trinity Study papers (library / journal access)
• Peer-reviewed and practitioner work on variable withdrawal strategies (e.g. guardrails frameworks)
• Long-run asset return summaries from reputable research shops (always check methodology and period bias)