Two people retire with the same savings and earn the exact same average return over the next 30 years. One coasts comfortably; the other runs out of money. The only difference is the order their good and bad years arrived in. That's sequence-of-returns risk, and it's the part of retirement math that the averages quietly hide.
While you're still saving, the order of your returns barely matters. A crash early in your career is almost a gift, since you're buying shares on sale every month. The moment you stop adding money and start pulling it out, that flips. If the market drops early and you're selling investments to pay for groceries, you lock in those losses for good and shrink the base that's supposed to bounce back. Later gains then have fewer dollars to work on, and the math may never catch up, even if your long-run average ends up identical to someone who retired into a calm decade.
Sequence risk is front-loaded. Your portfolio is biggest right when you retire, and that's also when you start withdrawing, so early losses do outsized damage. People who study this closely find the first decade of returns drives most of the final result. And it isn't only crashes. A long, flat, inflation-eaten stretch can be just as corrosive as a sharp drop, which is exactly the trap a lot of 1960s and 70s retirees walked into.
Retire at 45 instead of 65 and the risk hits twice. Your retirement is longer, maybe 45 to 55 years, so there are simply more bad sequences you could stumble into. This is the same risk Bengen wrestled with when he built the 4% rule: his worst historical case wasn't the 1929 crash, it was the 1973–74 bear market and the stagflation that followed, because years of high inflation drained portfolios in slow motion. Stretch a retirement out to half a century and you've got more exposure to a run like that.
The most reliable defense is almost dumb in its simplicity: keep enough cash that you never have to sell investments at a bad time. A cash buffer, parked in a high-yield savings or money-market account, lets you cover expenses out of cash while the market is down and leave the portfolio alone to recover. You're cutting the chain that turns a temporary paper loss into a permanent one.
How much is enough? The research on cash reserves and bear-market spending points to about 2.5 years of expenses as the sweet spot, enough to ride out most historical recessions without touching stocks, without sitting on so much cash that it drags down your returns. Here's the scale the calculator grades your buffer against:
| Cash buffer | What it gets you |
|---|---|
| Under 1 yr | Too thin. You'll likely be forced to sell into a downturn. |
| 1–2 yrs | The minimum that's workable. Covers short corrections. |
| 2–2.5 yrs | Solid. Gets you through most recessions. |
| 2.5 yrs ★ | The research-backed target. |
One detail in how I model it: in a down year you spend from the buffer first and leave your investments alone, then refill the cash in strong years when the portfolio is healthy. A sequence only truly fails when both the portfolio and the buffer are exhausted. That's what lets a 3.5% withdrawal rate hold up where a buffer-free plan would need to drop to 2.5% or lower.
Most calculators assume one steady return every year and hand you a tidy answer like "retire at 43." But you won't live an average. You'll live one specific sequence, and that sequence can pull your real retirement age forward a few years or shove it back by five to ten. A point estimate gives you false confidence. The honest answer to "when can I retire" is a range, with the ugly sequences shown, which is the whole reason the calculator re-runs your plan through every market since 1928 instead of trusting an average.
See your range across every historical sequenceNot financial advice. Consult a fee-only fiduciary CFP for personalized guidance.