FIRE Number Skill

Every FIRE calculation is three assumptions wearing a number's clothing: a withdrawal rate, a real return, and the pretense that returns arrive in a convenient order. This skill does the math properly and refuses the false precision — the deliverable is a surface of outcomes with the assumptions labeled, not a single date to organize a life around.

What This Skill Produces

• The FIRE number — annual spend ÷ withdrawal rate, with the withdrawal rate named as the choice it is
• Years to target — at stated savings, contributions, and real return
• The sensitivity grid — years-to-target across return (3/5/7%) × withdrawal rate (3/3.5/4%)
• The ignored-risks list — sequence-of-returns, taxes, spending drift — stated, not buried

Required Inputs

Ask for these if not provided:

• Current invested savings (invested — not home equity, not emergency cash)
• Monthly contribution (realistic, not aspirational — ask which)
• Target annual spend in retirement (today's dollars; if unknown, current spend is the honest starting guess, labeled)
• Return and withdrawal assumptions (defaults: 5% real, 4% withdrawal — both labeled as defaults)

Programmatic Helper

python3 scripts/fire_number.py --savings 120000 --monthly 3000 --spend 60000
python3 scripts/fire_number.py --savings 120000 --monthly 3000 --spend 60000 --return 5 --wr 4 --json

Deterministic monthly compounding at a constant real return (inflation already removed — never stack an inflation adjustment on top). The script prints the sensitivity grid and its own not-modeled list.

Framework: The Honesty Rules

• The 4% rule is a study, not a law — one country, one era, 30-year horizons; early retirees have longer horizons, which is why the grid includes 3% and 3.5%
• Sequence risk is unmodeled and largest near the finish — a crash in year 1 of retirement ≠ a crash in year 20; say this every time