Probability of Expiring

Probability of expiring is an estimate, expressed as a percentage, of how likely it is that an option will be in-the-money (worth exercising) at the moment it expires. It is a statistic used in options trading, not a guarantee — it is derived from a pricing model, most commonly by looking at an option's delta as a rough stand-in for this probability.

The number is calculated using the option's strike price (the price at which the option lets you buy or sell the underlying stock), the current stock price, the time left until expiration, and the stock's implied volatility (the market's estimate of how much the stock might swing around). Plug those into an options pricing model and it will spit out a probability that the stock finishes beyond that strike price by expiration day.

The key nuance: this probability only looks at the finish line, not the race. A stock could swing wildly in the days before expiration — triggering a stop-loss, a margin call, or a panicked exit — and the probability of expiring would say nothing about any of that, because it only measures where the stock is likely to be on the single day the option expires. A trader who confuses this with "probability of touching that price at some point" will misjudge their risk.

It's also worth remembering this is a model output, not a fact about the future. It assumes the stock's future price moves resemble a certain statistical pattern (often a lognormal random walk), which real markets don't always follow, especially around earnings, news events, or thin trading.