Vega

Vega measures how much an option’s price reacts to a change in implied volatility — the part of the price that moves on fear and expectation, not on the stock itself.

Vega is how much an option’s price changes when implied volatility (IV) changes by one point, holding the stock price and time constant. A vega of 0.10 means the option gains or loses about 10 cents per share for each 1-point move in IV. Long options have positive vega — rising IV makes them more valuable. Short options have negative vega — rising IV works against the seller, because the option they are short becomes more expensive to buy back.

Implied volatility is an expectation, not a movement

The crucial idea is that implied volatility is the market’s expectation of how much the stock will move, priced into the option — not the stock actually moving. A stock can sit perfectly still while its options get more expensive, because traders are bracing for a coming event. IV is the temperature of that anticipation. When the market expects a wild ride, IV is high and every option on that stock costs more; when the market is calm, IV is low and options are cheap. Vega is your sensitivity to that mood shifting.

Worked example: the earnings IV crush

The clearest place to see vega at work is around an earnings report. In the days before a company reports, uncertainty about the result drives implied volatility up, and options — both calls and puts — get expensive. The stock may not have moved at all; the options are pricey purely because IV has spiked ahead of a known event.

Then the report comes out. The uncertainty resolves, IV collapses the next morning, and the options lose value fast — this sudden drop is called the IV crush. A buyer can be right about the direction of the move and still lose money, because the IV they paid for evaporated faster than the stock’s move could compensate. Picture buying a call before earnings for 5.00 when IV is elevated; the stock rises modestly on the news, but IV crashes, and the call is worth only 3.50 the next morning. Right on direction, down on the trade. A seller of that inflated premium, by contrast, benefits as the IV crush deflates the option they are short.

This is why sellers are short vega and often prefer to sell when IV is already elevated: there is more premium to collect, and more room for volatility to revert back down in their favor. Selling into high IV is selling something the market has temporarily marked up.

The danger built into short vega

There is a sharp edge here. A spike in implied volatility usually does not happen in a vacuum — it typically arrives alongside a market drop, because fear and falling prices travel together. For a cash-secured short put, that means a single event can hurt you on two fronts at once. The falling stock price moves against your position through delta, and the simultaneous jump in IV moves against you through vega. The two losses stack in the same direction at the worst possible moment.

Vega rounds out the four Greeks that matter most: delta for the stock’s direction, gamma for how fast that exposure changes, theta for time, and vega for volatility expectations. For how they read together on a single position, see The Greeks Explained. The takeaway: vega is your exposure to the market’s mood, and for a seller that mood is most generous and most threatening at the very same time.

Educational only — not investment advice. Options involve a substantial risk of loss and are not suitable for every investor.