What's "the number"?
Your retirement number is the portfolio size that can sustainably fund your spending forever. At a 4% withdrawal rate, that's 25× your annual spending. At 3.5%, it's about 29×. At 3%, it's 33×.
The most common mistake is calculating it once, in today's dollars, and treating it as fixed. Inflation moves the target — what costs $50k/yr today will cost about $75k/yr in 20 years at 2% inflation. The calculator above adjusts the target line as it walks forward.
Why savings rate matters more than income.
Mr. Money Mustache popularized this insight: if you save 25% of your take-home, you can retire in ~32 years. Save 50%, and it's ~17. Save 75%, and it's ~7.
Income matters too, but only insofar as it enables savings. A high earner who spends every dollar is in worse shape than a moderate earner saving aggressively. The math is uncomfortably democratic.
Sequence-of-returns risk.
The deterministic age above assumes a smooth return path. Reality is bumpy. A bear market in the first 5 years of retirement does much more damage than the same loss in year 25 — the early withdrawals lock in losses on a shrinking base.
Hedges: 2–3 years of cash, a bond ladder, or a "guardrails" withdrawal strategy that flexes spending in down years. The calculator does not model these — it gives you the central estimate.
The four levers, ranked.
1) Spending — the single biggest lever. Spending less reduces the target and increases the savings rate simultaneously. 2) Savings rate — the next biggest. 3) Investment returns — meaningful, but partly outside your control. 4) Working longer — least powerful per year, but useful at the margin.
Common retirement-age mistakes
- Using nominal returns without adjusting the spending target for inflation.
- Forgetting that retirement-age healthcare is dramatically more expensive than employer-subsidized coverage.
- Treating Social Security as guaranteed when modeling early retirement — it doesn't start until 62+.
- Assuming a smooth 7% return every year (real returns come in lumps).
- Ignoring taxes — a $1M Traditional 401(k) is not the same as $1M in a Roth or brokerage.