RetireFire methodology explained (human version)

Methodology pages are often written for people who already trust the product. This post is the human version: what each RetireFire tool computes, which assumptions are shared, what the free stress test does, and what we deliberately leave out. Pair it with the full Methodology page when you want formulas on one screen.

The one-sentence product idea

Show transparent FIRE math under named assumptions, keep core tools free, and prefer ranges over false confidence. If a result cannot be explained in plain language, it should not ship as a green checkmark.

Four questions, four tools

• FIRE Number — “How large a portfolio for this spending and withdrawal rate?” → spending ÷ SWR.
• Years to FIRE — “How long might accumulation take under constant real return and steady savings?”
• Coast FIRE — “Can growth alone finish full FIRE by a traditional retirement age if I stop contributions?” → full FIRE ÷ (1+r)^n.
• Barista FIRE — “What portfolio covers the gap after part-time income?” → max(0, spend − work income) ÷ SWR.

Shared assumptions (why that matters)

Spending, portfolio, savings, withdrawal rate, real return, ages, and part-time income live in one planner state. Change SWR once and FIRE, Coast, Barista, and Years stay consistent. Share URLs and CSV export freeze a snapshot so “my plan” is not a vibe.

Defaults are starting points, not forecasts

• Example default SWR: 4% (historical conversation starter; early retirees often stress 3–3.5%).
• Example default real return: 5% (illustrative; lower it when you want conservatism).
• Lean / Regular / Fat: spending presets ($40k / $60k / $100k examples) — not academic standards.
• Traditional retirement age default: 65 for Coast horizon math — edit it.

Constant return first, then ranges

Main calculators assume the same real return every year. That is intentional clarity, not a claim markets are smooth. Sensitivity chips and Scenario A/B let you change one lever at a time. Coast and Years also offer a free basic Monte Carlo: 1,000 paths, volatility presets, success rate and p10/p50/p90 terminals under a simple shock model (seed 42). It is educational dispersion — not historical cycle backtesting and not a life success probability.

What we leave out on purpose (for now)

• Taxes, account types, RMDs, withdrawal ordering
• Fees as a first-class drag model
• Social Security, pensions, annuities
• Healthcare premiums and subsidy cliffs (see Barista healthcare guide)
• Full historical market engines and multi-regime Pro-depth sims (teased, not gated on free core)

Research lineage we cite carefully

The 4% conversation tracks Bengen and Trinity Study–style historical U.S. portfolio survival research. We describe it as history-based guidance, not a warranty. Safe withdrawal rate posts and Methodology expand the nuance for early retirement horizons.

How to use the suite without lying to yourself

• Name spending honestly (including healthcare).
• Run at least two SWR and two return assumptions.
• Use Coast checklist before cutting contributions.
• Read stress-test tails, not only success rate.
• Compare Coast vs Barista when the question is work vs savings.
• Escalate high-stakes plans to full planning tools and professionals.

Free core promise

If optional Pro features ship later (more paths, regimes, saved scenarios, detailed reports), core calculators and a useful free stress-test tier are intended to remain free. Approach documents that contract in plain language.

FAQ

• Is Methodology the same as this post? Methodology is the formula reference; this is the narrative tour.
• Why not taxes first? Complexity that obscures the core identity question ships later, if ever, as optional depth.
• Why seed 42? Reproducibility for the free stress test under the same inputs.

Read the full Methodology page, Approach & roadmap, sequence-risk guide, and disclaimer. Educational only — not financial, tax, or investment advice.