Theta Explainer
Theta estimates the effect of one day passing on an option’s theoretical value when other inputs do not change. Long options usually have negative theta because less time remains for favorable movement, while option writers generally have positive theta from the same decay.
Time decay is not linear. It often accelerates for near-the-money options as expiration approaches, while deep in- or out-of-the-money contracts can follow different paths. Quoting conventions may allocate decay across calendar days even when markets are closed.
A positive-theta position is not automatically low risk or profitable. The premium collected compensates the writer for exposure to underlying moves, volatility changes, gaps, and assignment. One adverse move can exceed many days of expected decay.
Theta depends on the pricing model and shifts when price or implied volatility changes. Traders should evaluate theta beside delta, gamma, vega, liquidity, and scenario losses. A calendar spread can even contain both long and short time exposures across different expirations.
Theta estimates one day of value change with other inputs fixed.
Long options usually lose time value.
Near-expiration at-the-money contracts can erode rapidly.
The path differs by strike and volatility.
Writers accept price, volatility, and assignment risk.
Scenario loss matters more than daily carry alone.
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