Assignment probability calculator
The probability that a short option finishes in the money is N(d2) under Black-Scholes, and it runs a few points below the option delta that most traders use as a shortcut. This calculator shows both, plus the probability of touching the strike at any point before expiry, which is roughly double.
Delta is close, and it is not the same thing
Sell a 0.20 delta call and the rule of thumb says an 80% chance of keeping the premium. The rule is good. It is also consistently off by a couple of points in the seller's favor, because delta carries a volatility term that N(d2) does not.
The calculator prints the gap. On the numbers loaded above, a 195 call on a 182.50 stock at 28% IV with 45 days left, delta reads 0.2814 while the true ITM probability is 24.9%. Three points. Small on one trade. Over a hundred trades a year, not nothing, and worth knowing which direction the error runs.
Probability of touch is the number that ruins your week
Finishing in the money and touching the strike are different questions with very different answers. That same strike, 24.9% likely to finish ITM, has close to a 50% chance of trading through at some point in the next six weeks. Twice the number, and the one nobody looks at.
That is the number to look at if you know you will not sit still while a position goes against you. Plenty of traders close a perfectly good trade at a loss because the stock tagged the strike three weeks before expiration and then came back. The touch probability is a measure of how likely you are to be tested, not of how likely you are to lose.
What these numbers are not
They are risk-neutral probabilities from a lognormal model with constant volatility. They are not forecasts. They do not know the company reports earnings in eleven days, they do not know the stock has gapped 8% on each of the last four prints, and they assume price moves in a way real stocks do not, with thinner tails than reality.
They also say nothing about early assignment. Early exercise is driven by dividends and by extrinsic value collapsing on a deep in-the-money contract, and none of the inputs on this page will predict it. If you are short a call into an ex-dividend date and the extrinsic value is under the dividend, assume you will be assigned.
Questions people actually ask
Is delta the probability of assignment?
Close enough for strike selection, not correct. Delta overstates the odds of finishing in the money by a small and predictable amount. The exact figure is N(d2), which this calculator prints next to delta so you can see the gap on your own inputs.
Why is the probability of touch so much higher?
Because it counts every path that reaches the strike, even if the stock retreats before expiration. Finishing in the money requires the price to be there on one specific day. Touching requires it on any day. For an out-of-the-money strike, touch runs at roughly twice the ITM probability.
What delta should a premium seller target?
Most sellers live between 0.15 and 0.30. Lower delta means higher odds of keeping the premium and less premium to keep. There is no free point on that curve, and any source that tells you one delta is correct for everyone is not describing a trade-off.
Does OptionsKing use these probabilities?
The app computes the same figures from the same math, and applies a fixed confidence bar before anything is shown to you: nothing scoring below 60 is ever surfaced, and 75 is the recommended setting. How OptionsKing works covers what that guarantee does and does not mean.