Key takeaways
- The FIRE number is annual spending ÷ withdrawal rate — at 4%, exactly 25× spending.
- The withdrawal rate is a judgment call, not a law; lower rates mean larger targets.
- Spending is a double lever: each trimmed dollar shrinks the target by roughly 25× and frees money at the same time.
The definition in one line
You are financially independent when work becomes optional — when the portfolio can pay for your life indefinitely without being exhausted. The FIRE number is the size that portfolio needs to be. Everything about it flows from two personal inputs: what your life costs, and how much you believe a portfolio can sustainably pay out each year.
The arithmetic
The formula is a single division: FIRE number = annual spending ÷ withdrawal rate. At a 4% withdrawal rate that works out to spending multiplied by 25 — the famous 25× rule — because dividing by 0.04 and multiplying by 25 are the same operation. There is no deeper mystery; the rule is algebra wearing a nickname.
Choosing a withdrawal rate
The rate is where judgment enters. The 4% figure comes from historical studies of diversified portfolios over roughly 30-year retirements; it is a well-worn reference point, not a law of nature. Longer horizons, heavier caution, or lower return expectations argue for 3–3.5%, which raises the target to roughly 29–33× spending. The honest move is to compute the number at more than one rate and know how sensitive your plan is.
A worked example
Example · one budget, two rates
Annual spending of $40,000 at a 4% withdrawal rate needs $1,000,000 — exactly 25×. The same budget at a more cautious 3.5% needs about $1,142,900.
The gap between those targets is the price of caution, stated in dollars. Whether it is worth paying is a personal call — the arithmetic just makes the trade visible.
Spending is the double lever
Notice that spending appears in the formula and nowhere else does income. Trim annual spending and two things happen at once: the target shrinks by 25 times the cut, and the money freed up accelerates progress toward it. That double effect is why FIRE planning obsesses over the cost of a life rather than the size of a salary — and why the number is worth recomputing whenever your life meaningfully changes.
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