What the number is
Probability of success is the share of modelled market paths in which the portfolio never hits zero before the horizon, under a stated spending rule and a stated engine. It is not a grade, a forecast or a chance of dying broke. It is a statement about a model.
What is a good number?
Most planners treat 85% to 95% as a reasonable band. Below ~80% the plan is asking the market to be kind. Above ~97% you are usually buying certainty the model cannot actually sell — extra years of work, or spending you deferred and never took — because the assumptions (fees, taxes, spending flexibility, the regime model itself) are not known to that precision.
The ruin rate is one minus success. The methodology page is how Killion computes it. Does Monte Carlo overstate success is why a 92% from one engine is not a 92% from another.
How to use it
Compare two plans on the same engine, the same seed and the same household. A jump from 78% to 91% after a 10% spending cut is information. A lone 91% on a slide is a slogan.
Getting the number for your own plan
If you want the figure rather than the explanation, the Monte Carlo retirement calculator runs 1,000 seeded paths on a portfolio, a withdrawal and a horizon you type, and reports the success rate with the count of paths behind it. It is free and needs no account. The live demo runs the same question through the full five-regime household engine.