How theta works
Every option's price has two parts: intrinsic value, what it would be worth if exercised now, and time value, the extra the market pays for what might happen before expiry. Theta eats only the time value, and the time value falls to zero at expiry.
- Bought options have negative theta. The position loses value every day the market does nothing.
- Sold options have positive theta. The seller gains what the buyer loses.
- At-the-money options have the most time value, so they lose the most in rupees per day.
- Decay speeds up near expiry. The last week takes a larger share of the value than the first week of a month.
A worked example on one Nifty lot
Nifty is at 25,000 with implied volatility of 14%. The at-the-money 25,000 call at different days to expiry, if Nifty stays at 25,000:
| Days to expiry | Call price | Theta a day | Loss a day on one lot |
|---|---|---|---|
| 30 | ₹469.35 | −₹9.01 | ₹586 |
| 12 | ₹280.50 | −₹12.85 | ₹835 |
| 5 | ₹174.70 | −₹18.62 | ₹1,210 |
| 2 | ₹107.85 | −₹28.10 | ₹1,827 |
| 1 | ₹75.35 | −₹38.79 | ₹2,521 |
From 30 days to 12 days, the call loses ₹188.85, about ₹10.50 a day. From 5 days to 1 day it loses ₹99.35, about ₹25 a day. The same strike decays more than twice as fast in its last days. The pattern follows the square root of time: halving the time left removes about 30% of the time value, not half.
Theta for buyers and sellers
A buyer of the 12-day call pays ₹18,232.50 a lot and loses about ₹835 of it each day that Nifty stays still. To break even by expiry, Nifty must rise above 25,280.50. The buyer needs a move large enough, and soon enough, to beat the decay. A seller of the same call collects ₹18,232.50 and earns about ₹835 a day while Nifty stays near 25,000, but pays the full move if Nifty rallies. Theta rewards the seller for the risk that gamma measures: options with the fastest decay also have the sharpest change in delta.
What changes theta
| Factor | Effect on theta |
|---|---|
| Time to expiry | Decay speeds up as expiry nears |
| Moneyness | Highest at the money; in-the-money and out-of-the-money options decay less in rupees |
| Implied volatility | Higher IV means more time value and a larger theta in rupees |
| Interest rates | A small effect: calls and puts at the same strike have slightly different thetas, −₹12.85 and −₹8.40 for the 12-day at-the-money pair |
Weekends and holidays
Theta is quoted per calendar day, and the market is closed on weekends and holidays. In practice, part of a weekend's decay is priced in before Friday's close, and the rest shows up at Monday's open. Traders who sell options on Friday for the weekend decay often find that the Friday afternoon prices already reflect much of it. Expiry-week decay is the sharpest: a Nifty weekly option that is still at the money on expiry morning loses most of its remaining value by the afternoon.
When theta matters most
- Holding bought options through quiet days. The decay is certain; the move is not.
- Weekly options. Nifty weekly contracts spend their whole life in the fastest part of the decay curve.
- Option selling. Theta is the seller's income and the reason to sell when time value is rich.
- Spreads. A spread's theta is the difference between its legs, so a bought spread decays more slowly than a bought option alone.
- Choosing an expiry. A view that needs two weeks should not be traded in an option that expires in five days.
Related terms
- Delta meaning: how much an option moves per point of the underlying
- Gamma meaning: how fast delta changes
- Vega meaning: how an option responds to implied volatility
- Intrinsic value meaning: the part of the price theta does not touch
Learn more
- Option greeks: theta with the other Greeks on Nifty examples
- Option selling strategies: trades built to collect theta
- Option buying strategies: entries and exits that respect decay
Frequently asked questions
What does negative theta mean?
That the position loses value as time passes. Bought calls and puts have negative theta; sold options have positive theta.
When is theta highest?
For at-the-money options in the last few days before expiry. The 25,000 call loses about ₹9 a day with 30 days left and about ₹39 a day with one day left.
Does theta decay over weekends?
Yes, in calendar days. Part of the weekend's decay is priced in on Friday afternoon and the rest appears at Monday's open.
Can I avoid theta as an option buyer?
Not fully. Buying a longer expiry slows the daily decay, and a debit spread offsets part of it, but every bought option loses time value until expiry.
Where can I see theta?
In an option chain that shows Greeks, such as QuantPower's option trading app, and in the black scholes option pricing calculator.