Theta Decay

The rate at which the value of an options contract declines as it approaches its expiration date.

Theta decay, commonly referred to as time decay, represents the daily loss in value of an options contract due to the passage of time. Because options have a fixed lifespan, the probability of the option expiring in-the-money (profitable) decreases with each passing day, assuming the underlying asset's price and market volatility remain constant. Theta is typically expressed as a negative number, indicating how much the option's premium will decrease daily. This decay is non-linear. It starts slowly months before expiration but accelerates rapidly in the final 30 to 45 days leading up to the expiration date. This acceleration occurs because the remaining time value of the option is shrinking at an exponential rate as the final deadline approaches. For buyers of options (long positions), theta decay is an ongoing headwind that erodes their investment daily, meaning they need a swift, favorable move in the underlying stock to offset this loss. For sellers of options (short positions), theta decay is a tailwind, as they profit from the option losing value over time, allowing them to buy it back cheaper or let it expire worthless.

An investor buys a call option for a premium of $5.00 when the option's theta is -0.10. Assuming the underlying stock price and volatility do not change, the option's premium will drop to $4.90 the next day ($5.00 - $0.10 = $4.90). If the investor holds the option for 5 days under the same conditions, the cumulative decay will reduce the premium to approximately $4.50.