Theta Decay: What Time Really Does to Options

Theta estimates the passage-of-time effect on an option’s theoretical value with other inputs held constant.

Long options and time

Long calls and puts generally have negative theta. If the underlying and volatility do not move enough, time passing can reduce value.

Short options and time

Short options generally have positive theta, but premium collection is compensation for accepting risk. A single large move can overwhelm many days of decay.

Decay is not linear

Time value often changes faster as expiration approaches, especially near the money. At the same time, gamma risk can increase, making the position more sensitive to underlying movement.

Do not isolate theta

Theta is one sensitivity among several. Volatility changes, price gaps, liquidity, dividends, and assignment can matter more than one day of theoretical decay.