Every option has an expiry date, and as that date approaches, the option loses the part of its value that comes from time. This erosion is called time decay, and theta is the Greek that measures how much value an option loses per day, all else equal.
This article explains how theta decay works, why it is not steady but accelerates towards expiry, how it differs across strikes, and what it means in practice for those who buy options and those who sell them.
Time value and what theta measures
An option's premium has two parts. Intrinsic value is what it would be worth if exercised now: for a call option, the amount by which the underlying is above the strike. Time value is everything else: the price of the possibility that the option becomes more valuable before expiry.
Out-of-the-money and at-the-money options are entirely time value. In-the-money options are part intrinsic, part time value.
At expiry, time value is zero. Every option ends worth only its intrinsic value, which may also be zero. So time value must disappear between now and then; the only question is how quickly.
Theta expresses that rate. A theta of minus 8 means that, if nothing else changes, the option loses about 8 in value over one day. For the buyer, it is a daily cost; for the seller, a daily income, earned in exchange for taking the risk of a move.
Why decay accelerates near expiry
Time value does not fall in a straight line. For an at-the-money option, it falls roughly in proportion to the square root of the time remaining, which means slowly at first and very quickly at the end.
Illustrative figures, with the index at 24,000 under ordinary volatility, for the 24,000 call option, assuming the index stays exactly where it is:
| Days to expiry | Approximate premium | Lost since previous row |
|---|---|---|
| 14 | 170 | — |
| 7 | 120 | 50 over 7 days |
| 2 | 64 | 56 over 5 days |
| 1 | 45 | 19 in one day |
| 0 (expiry) | 0 | 45 in the final day |
In the first week, the option loses about 7 a day. In the final day, it loses everything that is left. This is why options in their final days behave so differently from those with weeks to run, and why weekly options are dominated by decay.
How it differs across strikes
At-the-money options have the most time value, so they lose the most in rupee terms each day.
Out-of-the-money options have less time value and lose fewer rupees per day, but a larger share of their value. A far out-of-the-money option with a few days left can lose most of its remaining premium in a day or two if the underlying does not move towards it.
In-the-money options carry intrinsic value that does not decay, so the time-value part erodes while the intrinsic part follows the underlying.
Example: with the index at 24,000 and two weeks left, the 24,300 call option might be about 60. If the index stays flat, both it and the 24,000 call option at 170 end at zero. The at-the-money option loses more rupees, 170 against 60, but the out-of-the-money one loses its value faster in percentage terms in the final days, because nothing is left once the chance of reaching the strike fades.
Decay is only one force
Theta describes what happens if nothing else changes. In reality, other forces act at the same time.
Price movement (delta and gamma). A move in the underlying can easily outweigh a day's decay. Near expiry, gamma is high, so even small moves cause large swings in at-the-money options.
Volatility (vega). A rise in implied volatility can lift premiums despite decay; a fall can crush them faster than decay alone, as often happens after events.
Weekends and holidays. Calendar days pass without trading. Market prices usually anticipate this, so much of the weekend's decay is reflected in prices before the market closes on Friday.
So an option buyer is not doomed by decay, and an option seller is not paid for free. The buyer needs a move large enough, soon enough, to outrun decay. The seller collects decay but carries the risk of a move large enough to wipe out many days' income.
What it means in practice
For buyers: time is the main enemy. Buying options with very little time left means paying for a move that must happen almost immediately. Buying more time costs more upfront but decays more slowly per day. Have a time-based exit: if the expected move has not happened by a certain date, close rather than watch the premium shrink.
For sellers: decay is the source of income, and it is largest near expiry. But that is also when gamma is highest, and a sharp move can turn a small, steady income into a large loss. Defined-risk structures, such as credit spreads, cap that damage.
For spreads: combining bought and sold options lets the decay on one leg offset the other. Calendar spreads, for example, aim to profit from the near option decaying faster than the far one.
Reading it on the chain: many option chain screens show theta for each strike. Compare it with the premium: an option losing 8 a day on a premium of 40 is shedding a fifth of its value daily if nothing moves, which tells you how quickly the expected move must arrive.
Costs: near-expiry option premiums are small, so bid-ask spreads and charges are a large share of each trade. Count them.
An account with an option chain that shows theta
Check decay, delta and implied volatility before choosing an expiry. Free to open
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Frequently Asked Questions
What is theta decay in options?
The loss of an option's time value as expiry approaches. Theta measures roughly how much value the option loses per day if nothing else changes.
Why does theta decay accelerate near expiry?
For at-the-money options, time value falls roughly with the square root of time remaining, so it erodes slowly with weeks left and very quickly in the final days.
Which options lose the most to time decay?
At-the-money options lose the most rupees per day. Out-of-the-money options lose fewer rupees but a larger percentage of their value, especially near expiry.
Does time decay happen on weekends?
Calendar time passes, but market prices usually anticipate weekends and holidays, so much of that decay is reflected before the market closes.
Is selling options for theta a safe income?
No. Sellers collect decay but carry the risk of a large move, especially near expiry when gamma is high. Defined-risk structures limit the damage.
Related Reading
- The hidden cost on every option trade
- All the Greeks together
- What happens to options over a weekend
- Using decay across two expiries
- Buyer and seller trade-offs
- Where time value comes from
Disclaimer: TheFinBaba provides educational content only - this is not investment advice. Trading involves risk of loss.
