The option’s theta measures the modeled change in its value as time passes with other inputs held fixed. For a long option it is often negative, reflecting time decay. Quote conventions differ; this page uses change in premium per share per day. Actual market prices need not fall each day, and some option configurations can have positive theta.
Example
A call priced at $2 with daily theta of −0.05 has an estimated one-day time effect of −$0.05 per share. If theta and all other inputs stayed fixed, a two-day approximation would be $2 − (2 × $0.05) = $1.90. Theta changes over time and other market movements can offset the decay.
Passage of time and its effects on the theta
Longer-dated options can lose time value each day; their theta is not automatically zero. At-the-money options often show faster daily erosion as expiration approaches. Compare the absolute size of theta rather than calling a more negative number “higher.”
Near expiration, the absolute daily decay of an at-the-money option can become large. Deep in-the-money and far out-of-the-money options can behave differently, so there is no single decay schedule for all contracts.
Changes in volatility and its effects on the theta
Higher implied volatility can increase the amount of time value and the absolute size of negative theta, particularly near the money with other inputs fixed. Rates, dividends, moneyness and exercise style also affect the result.
The chart above illustrates the relationship between the option's theta and the volatility of the underlying security which is trading at $50 a share and have 3 months remaining to expiration.
The charts illustrate model relationships with selected inputs held fixed. They are not current market quotes or universal curves for every option.
Using the estimate
Greeks are local model sensitivities, not guaranteed price changes. They change as the stock, time and implied volatility change. Check whether a quoted value is per share or per contract, which volatility increment is used, and whether theta is measured per day or per year. Selling an option reverses the position’s Greek signs.
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References: OIC options basics; OIC assignment; OCC contract adjustments. Editorial standards.
Short-term options applications
Explore Short-Term Options Trading to see how weekly, 1DTE and 0DTE expirations affect timing, price sensitivity and expiration risk. Availability and settlement depend on the selected product and series.