The retirement math that keeps people up at night
The saving phase asks "how big a nest egg can I build?" The spending phase asks the scarier question: "will it last?" This calculator simulates your balance month by month — growing with returns, shrinking with withdrawals — until it either runs dry or clearly outlasts any reasonable retirement.
The 4% rule
The famous guideline from the Trinity study suggests that withdrawing about 4% of your starting balance per year (then adjusting for inflation) has historically lasted through a 30-year retirement. On $1 million that's $40,000/year, about $3,333/month. Withdraw much more and you risk running out; withdraw less and you'll likely die with a large balance. The calculator shows how your chosen withdrawal compares to this benchmark.
The variables that make or break it
- Withdrawal rate is king. The gap between a 4% and a 6% withdrawal is the difference between "lasts forever" and "gone in ~20 years."
- Sequence-of-returns risk: a market crash early in retirement is far more dangerous than the same crash later, because you're selling assets while they're down. Average returns hide this danger — real retirees should keep a cash buffer to avoid selling into downturns.
- Inflation: this tool uses a flat withdrawal; in reality your spending needs rise with inflation, which shortens how long the money lasts. Using a lower "real" return (say 4–5%) partly accounts for this.
Flexibility is the safety net
The 4% rule assumes rigid spending. Retirees who can trim withdrawals in bad market years — skipping the inflation raise, or cutting discretionary spending — dramatically improve the odds their money lasts. Treat the result here as a planning guide, not a guarantee, and build in the flexibility to spend less when markets demand it.