FIRE number

A FIRE number is the total invested portfolio value at which you can comfortably stop relying on a paycheck and live off the portfolio’s returns indefinitely. It’s the central number in the Financial Independence, Retire Early (FIRE) movement — every other FIRE calculation feeds into it.

How it’s calculated

Under the standard “4% rule” derived from the Trinity Study (1998), a FIRE number is annual expenses divided by a safe withdrawal rate:

FIRE number = annual expenses ÷ (safe withdrawal rate / 100)

At a 4% safe withdrawal rate, that’s 25× annual expenses. Annual spending of $40,000 implies a FIRE number of $1,000,000. Annual spending of $80,000 implies $2,000,000. The relationship is linear, which is why expense reduction is the highest-leverage variable in FIRE planning.

Why 4% is the default

The Trinity Study examined how a 60/40 stock/bond portfolio held up under historical 30-year withdrawal periods. A 4% inflation-adjusted withdrawal rate survived nearly every historical 30-year window in U.S. market data. It’s a heuristic, not a guarantee, and it’s been challenged for very long horizons (50+ years), foreign markets, and current low-yield environments — but 4% remains the default planning number.

Conservative planners use 3–3.5% (30× expenses) for longer horizons. Aggressive planners use 4.5–5% (20–22× expenses) and accept more risk.

What it doesn’t capture

A FIRE number is a modelling tool, not a financial instruction. The headline figure does not account for:

• Tax treatment. The 4% rule is a pre-tax heuristic.
• Sequence-of-returns risk. A bad market in the first 5 years of retirement disproportionately damages outcomes.
• Healthcare costs. In some jurisdictions, healthcare in early retirement is materially more expensive than during employed years.
• Behavioural risk. Most retirements that fail under the 4% rule fail because the retiree sold during a downturn, not because the math broke.

The FIRE number tells you how big the goal is. Hitting it is a different problem — covered by your savings rate, expected return, and time horizon.