August 30, 2026
Delta is a rate of change. It also doubles as a rough probability, which is the most useful shortcut in options selling and the one that quietly misleads the most people.
Delta is how much an option's price changes when the underlying moves one dollar. A put with a delta of −0.30 gains about $0.30 per share, $30 per contract, when the stock falls a dollar. A call with a delta of 0.60 gains $60 per contract when the stock rises one.
Deep in-the-money options approach a delta of 1.00 and behave nearly like the stock itself. Far out-of-the-money options approach zero and barely respond. At the money sits around 0.50.
For a seller the sign flips: short a 0.30-delta put means you lose roughly $30 per contract for each dollar the stock falls. Same number, other side.
Delta also approximates the probability the option finishes in the money. A 0.30-delta put is roughly a 30% chance of assignment; a 0.16-delta put is roughly 16%.
This is the reason delta is the unit most sellers actually think in. Choosing "the 20-delta put" is a statement about acceptable assignment odds that transfers across stocks, strikes and expirations, in a way that "the $47 strike" never does.
Two caveats on the approximation. It's a risk-neutral probability, derived from prices rather than from any forecast, so it embeds the market's pricing assumptions rather than a real-world frequency. And it runs slightly high — the true probability of finishing in the money is a bit below the delta, more noticeably on longer-dated options. As a working number it's excellent. As a precise one it isn't.
A one-standard-deviation move covers about 68% of outcomes, leaving 16% in each tail. So the 16-delta strike sits approximately one standard deviation away, and selling it means selling roughly at the edge of the expected range.
The expected-move calculation from the implied volatility piece and the 16-delta strike are pointing at the same place from different directions. If a $50 stock with 30% IV has a 30-day expected move of about $4.30, the 16-delta put should sit near $45.70.
Which is a reason to run the arithmetic occasionally rather than always reaching for the delta column. They agree, and seeing them agree makes both more legible.
This is the part the probability reading hides. Delta is a snapshot at current conditions, and it moves as the stock moves. The rate at which it moves has its own name, gamma.
A 0.16-delta put on a stock that then falls 8% isn't a 16-delta put anymore. It might be 0.35 or 0.45, meaning it now loses value roughly three times as fast per dollar as when you sold it. Your risk didn't scale with the stock's decline; it accelerated.
Time compounds this. Gamma is largest near the money and near expiration, so a position that drifts toward the strike in its final week behaves very differently than the same delta did with 40 days left. It's the specific reason many sellers close positions early rather than holding into expiration week — not the remaining premium, which is small, but the rate at which the remaining premium can change.
Selling a 16-delta put is often described as an 84% chance of success, and that number is roughly right and completely uninformative on its own.
What it describes: 84% of the time you keep a small premium, and 16% of the time you're assigned, having sold the strike precisely because it was far away, on a stock that had to move more than one standard deviation to get there. The wins are capped at the premium. The losses aren't capped at anything.
Win rate and expected value are different measurements, and a strategy can have a very high one and a poor other. The delta tells you the frequency. It says nothing at all about the size of what happens in the remaining cases, which is the half that determines whether the trade is any good.
One extension worth knowing. Delta aggregates: multiply each position's delta by 100 and by the number of contracts, add the shares you hold outright, and you get the account's equivalent share exposure.
Short five 0.30-delta puts is +150 deltas, or the directional equivalent of owning 150 shares. That's often a surprise to people who think of premium selling as market-neutral. It isn't; it's a long position expressed in an unusual way, and the aggregate is worth calculating before deciding whether to add another one.