TL;DR. Success rate is a binary count: in how many historical cycles did your portfolio still have a positive balance at the end of the horizon? It does not tell you how big the surplus was, how close a near-miss was to depletion, or how comfortable the middle of the run felt. Pair it with the median, min, and max ending-balance numbers before drawing conclusions.

Binary survival vs. depletion magnitude

A cycle is "successful" if it finished the horizon with any positive portfolio balance. Ending with $1 and ending with $5,000,000 both count as one success. Likewise, a cycle that ran out two months before the horizon ends counts as a failure even though it almost made it. That is why success rate alone hides important information. It cannot distinguish "barely made it" from "left a fortune behind".

The numbers that fill in the gaps

• Median ending balance. The typical outcome. If this is many multiples of your starting portfolio, you are likely overfunded.
• Minimum ending balance (across successful cycles). How thin the worst survivor finished. A near-zero minimum says success rate is fragile to small input changes.
• Maximum ending balance. The upside under a lucky cycle. Useful for bequest planning conversations.
• Failure depth. For failed cycles, how many years short the portfolio ran. Failing in year 29 of 30 is recoverable; failing in year 12 is not.

Why 100% is not the goal

On a 30-year horizon, chasing 100% success often means underspending against your real lifestyle. Historical cycles include the Great Depression, stagflation, and the dot-com crash; clearing every one of them at full lifestyle requires a portfolio sized for the worst case in history, every year, even though you only live one timeline. A target of 85-95% with a healthy median ending balance is usually a better tradeoff than 100% with a permanently underfunded standard of living.

"Failure" rate as a concept

One of my personal pet peeves in DIY retirement planning is people fixating on the Failure Rate. Sure, in planning your own retirement, there is risk that you run out of money. But, I implore you to use tools like FIREproof in a way that teaches you about the power of flexibility. 10% "Failure" does not mean all is lost in 10% of all cycles. To me, it means that you would have had to change your plan 10% of the time. Explore variable withdrawal plans, look into how a part-time job effects your outcomes, simulate the downsizing of a house while you pocket the proceeds. These are all ways to mitigate "failure" when planning.