Where the number comes from
The rule traces to the 1990s “Trinity Study” and William Bengen's research, which tested historical U.S. stock-and-bond portfolios against decades of market data. The finding: a 4% initial withdrawal rate, rising with inflation, survived almost every historical 30-year window. Flip it around and you get the famous corollary — you need roughly 25× your annual spending invested to retire (because 1 ÷ 0.04 = 25).
What it gets right
- It turns a vague goal (“enough to retire”) into a concrete target you can actually save toward.
- It bakes in inflation adjustments, so your spending power is held roughly constant.
- It's conservative enough that in many historical periods retirees ended richer than they started.
Where it breaks down
The 4% rule is a backward-looking average, and averages hide the cases that actually sink a plan:
- Sequence-of-returns risk. A bad market in your first few retirement years does far more damage than the same crash later — the rule's single percentage can't see that.
- Long horizons. 4% was calibrated to ~30 years. Retire early for a 45- or 50-year horizon and the safe rate drops.
- Taxes and account location. A withdrawal from a traditional IRA isn't worth the same as one from a Roth or brokerage account. The rule ignores tax entirely.
- Flat spending. Real spending isn't a smooth inflation- adjusted line — it lumps and dips across a retirement.
A better approach: model your own rate
Treat 4% as a sanity check, not a guarantee. The way to know whether your plan holds is to project it year by year — actual balances, actual expenses, inflation, and the taxes due on each withdrawal — and watch whether the portfolio survives your specific horizon.
FIRE Planner does exactly that in your browser: set your spending, returns, and time horizon and see your effective withdrawal rate evolve, instead of trusting one number. Open the planner — it's free and private, with no signup.
Related: tax-efficient withdrawal strategy covers which accounts to draw from, and the Coast FIRE calculator covers getting to the nest egg in the first place.