Theta

Theta measures time decay: how much an option's price is expected to fall each day as expiration approaches, if nothing else changes. An option with a theta of minus 0.05 loses about $0.05 per share, or $5 per contract, each day. Theta is the cost of holding options for buyers and the income of selling them for sellers. Understanding how it behaves is essential for choosing expirations and managing positions.

Theta basics#

Theta only affects extrinsic value. At expiration, an option is worth its intrinsic value only, so all extrinsic value decays to zero along the way. See Intrinsic and Extrinsic Value.

Decay is not linear#

The value of an at the money option is roughly proportional to the square root of time left. Halving the time left reduces the value by about 29%, not 50%. As a result, the daily decay grows as expiration approaches.

Theta and moneyness#

• At the money options have the largest theta in dollars because they have the most time value.
• Out of the money options have small dollar theta but can lose a large percentage of their value each day near expiry.
• Deep in the money options have little time value and therefore little theta.

Theta and volatility#

Higher implied volatility means more extrinsic value, so more to decay: high volatility options have larger theta. That is why option sellers are drawn to high volatility periods, and why buying options at high implied volatility is expensive in time decay. See Implied Volatility (IV).

The theta and gamma trade off#

You cannot collect theta without taking on negative gamma. Option sellers earn time decay day after day but lose when the underlying makes big moves. Option buyers pay theta for the right to benefit from big moves. Over time, whether buyers or sellers come out ahead depends on whether actual moves turn out larger or smaller than implied volatility predicted. See Gamma and Volatility Trading.

Weekends and holidays#

Option prices decay over calendar time, but markets are closed on weekends. In practice, much of the weekend decay is priced in before Friday's close, so selling options on Friday to "collect weekend theta" is less profitable than it sounds. Different pricing tools also handle weekends differently, so quoted theta can vary.

Using theta in strategy#

Many option sellers open trades with 30 to 45 days to expiry and close them with 1 to 3 weeks left, aiming to capture decay while avoiding the very high gamma of the final days. See Theta Harvesting.

Common mistakes#

• Buying short dated options and watching them decay before the move happens.
• Thinking theta is easy income for sellers. It is payment for taking on gamma risk.
• Ignoring theta on long option positions held for weeks.

Frequently asked questions#

What is theta in options?#

The expected daily loss in an option's value from the passage of time, assuming price and volatility stay the same.

Why does time decay accelerate near expiration?#

Because an option's time value is roughly proportional to the square root of time left, so each day removes a larger share of what remains.

Do option sellers always profit from theta?#

No. They collect theta, but large moves in the underlying can cause losses that exceed the decay earned.

Next, learn how volatility changes option prices in Vega.