Theta is an option greek that measures how much an option's price is expected to decrease due to the passage of time, assuming all other factors remain unchanged.
Theta helps traders understand the impact of time decay on an option's premium. Theta is usually expressed as the amount an option is expected to lose in value each day.
Theta can be compared to the melting of an ice cube. Even if nothing else changes, the ice cube gradually melts over time. Similarly, an option gradually loses value as it approaches expiry.
Example
A call option has:
- Premium = ₹100
- Theta = -2
This means the option is expected to lose approximately ₹2 in value each day if all other factors remain unchanged.
If:
- Today: Premium = ₹100
- After one day: Premium ≈ ₹98
- After two days: Premium ≈ ₹96
This decline occurs because there is less time remaining for the option to become profitable before expiry.
Why is theta important?
Theta helps traders:
- Estimate the impact of time decay on option positions.
- Compare options with different expiries.
- Understand how quickly option value may erode.
- Assess whether a trade requires a significant price move to offset time decay.
- Evaluate the benefits and risks of option buying and selling strategies.
For option buyers, theta represents a cost because options lose value over time. For option sellers, theta can be beneficial because time decay works in their favour.
What are the limitations of theta?
Some limitations include:
- Theta assumes all other factors remain unchanged.
- Time decay is not linear and accelerates as expiry approaches.
- Changes in volatility can outweigh the effects of theta.
- Theta values change as market conditions and option characteristics change.
- Actual premium changes may differ from theta estimates.
Therefore, theta should be considered alongside delta, gamma, and vega when analysing options.