Quick answer

The strike price is the fixed price written into an option contract: the price at which a call holder may buy, or a put holder may sell, the underlying shares.

Moneyness describes where the stock sits relative to the strike. An option is in the money if exercising it right now would have value, at the money if the strike is at or very near the stock price, and out of the money otherwise.

For calls, lower strikes are in the money when the stock trades above them. For puts, the logic flips: higher strikes are in the money when the stock trades below them.

A closer look

Take a stock trading at $100. The option chain lists many strikes for each expiration; here is how moneyness reads across three of them:

• $90 call: in the money by $10, because the right to buy at $90 is $10 better than the market price.
• $100 call: at the money; the strike matches the stock price.
• $110 call: out of the money by $10; the right to buy at $110 has no exercise value while the stock sits at $100.

For puts the labels reverse: the $110 put is $10 in the money, the $100 put is at the money, and the $90 put is out of the money.

Moneyness shows up directly in the premium. Suppose the $90 call trades at $11.20. Of that, $10.00 is intrinsic value (stock price $100 minus strike $90) and the remaining $1.20 is extrinsic value. The $110 call, quoted at $0.85, is all extrinsic; every cent of its price reflects time and the possibility of a move, not value that exists today.

Exchanges list strikes at set intervals, commonly $1, $2.50, or $5 apart depending on the stock's price, and add new ones as the stock moves, which is why active names show long, dense chains. How to read those chains is covered in reading the chain.

The formal detail

Moneyness has several formal measures beyond the in/at/out labels. Practitioners use the simple ratio of stock to strike (S/K), the log ratio ln(S/K), or, most commonly on trading desks, delta as a standardized coordinate; a 25-delta put identifies a strike by its risk profile rather than its dollar level, which makes comparisons across stocks and dates possible.

Strike listings are governed by exchange rules and adjusted by the clearing house for corporate actions. A 2-for-1 split, for instance, halves the strike and doubles the deliverable so holders are left economically whole; adjusted contracts are marked distinctly on the chain.

A common misconception treats out-of-the-money options as cheap in a meaningful sense because their dollar premiums are small. Price per contract is not the same as price relative to probability. What an out-of-the-money strike costs relative to the chance the stock reaches it is a pricing question, discussed under probability.