Theta decay is the value an option loses as time passes, with everything else unchanged. An option’s price is partly a charge for the chance that the index moves before expiry, and every day that passes leaves less time for that to happen, so the charge shrinks. Theta is the daily size of that shrinkage, in rupees. It is the cost of holding an option for a buyer and the income of a writer. Two things about theta decay surprise people: it is not steady, it speeds up sharply in the last days and hours, and weekends cost far less than a calendar suggests. This guide explains both, with data from 13 NIFTY expiries and six weekends in 2026.
What theta decay means
An option’s premium has two parts. Intrinsic value is what it would be worth if it expired now: for a call, how far the index is above the strike; for a put, how far below. Time value is the rest, the price of the chance of a further move. At expiry, time value is zero and only intrinsic value is left. Theta decay is the journey of time value to zero.
Theta measures the speed of that journey: the premium lost per day from time passing alone, as the Option Chain defines it when its Greeks switch is on. A theta of −12 means the option loses about ₹12 a day if the index and implied volatility stay where they are. Buyers pay it; writers collect it.
Why theta decay speeds up near expiry
For an at-the-money option, time value is roughly proportional to the square root of the time left. That one fact explains why theta decay accelerates. Each day removes a larger share of what remains:
| Days to expiry | Share of remaining time value lost in the next day |
|---|---|
| 7 → 6 | about 7% |
| 5 → 4 | about 11% |
| 3 → 2 | about 18% |
| 2 → 1 | about 29% |
| Last day | all of it |
The same square-root rule works inside the expiry day, which is where most index option trading now happens: in SEBI’s study of individual traders, published in August 2026, 59 percent of index options turnover in FY26 was in contracts on their expiry day. In the model, with 25 percent of the session left, an at-the-money option still holds about half of the time value it had at the open, because the square root of a quarter is a half. On a NIFTY expiry that point comes at about 14:00, and the other half disappears in the last ninety minutes, which is why expiry afternoons feel so different from mornings. Real sessions, as the section below shows, hold on to their premium even longer.
Options away from the money decay differently. Far out-of-the-money options are cheap and lose a smaller amount in rupees each day, but a larger share of their price, and on the last day they head to zero unless the index reaches them. Deep in-the-money options are mostly intrinsic value and have little time value left to lose.
Theta decay on expiry day: what 13 NIFTY expiries show
OIData records the at-the-money straddle, the call plus the put at the strike nearest the index, through every NIFTY expiry session. Across the 13 weekly expiries from 9 June to 29 September 2026, the median straddle had lost 15.5 percent of its opening value by 10:00, 26 percent by 12:00, 44 percent by 14:00 and 52 percent by 15:00. By the last reading of the session it had lost about 85 percent; most of what remained was the distance from the index to the nearest strike, which is intrinsic value rather than time value. So about half of the opening premium was still there at three o’clock, and most of it went in the final half hour.
Set against the square-root model, the real sessions differ in a telling way. The morning ran ahead of the clock: 15.5 percent gone by 10:00 against about 8 percent in the model, as the premium paid for the uncertainty of the open drained away. The afternoon ran behind it: 52 percent gone by 15:00 against about 80 percent, because the index usually moved enough in the afternoon to keep the options near the money valuable until the end. The spread between days was wide. By 14:00 the decay ranged from 9 percent to 65 percent, and on 15 September, the day NIFTY slid 474 points from its high, only 18 percent of the straddle had decayed by 14:00.
Theta decay on Saturday and Sunday
A calendar has seven days a week; the market has five. Does an option lose time value over the weekend? The textbook answer depends on the clock you use. Count calendar days, and the time from Friday’s close to a Tuesday expiry is about four days, while from Monday’s open it is about one and a quarter, so an at-the-money straddle should open Monday more than 40 percent cheaper on an unchanged index. Count trading time, and nothing changes over the weekend at all.
The prices say the second clock is much closer to the truth. We looked at the six weekends between May and September 2026 before a Tuesday NIFTY expiry on which the index opened Monday within 40 points of Friday’s close, and compared the at-the-money straddle’s Friday closing price with its Monday opening price. The change ranged from 11 percent lower to 3 percent higher, and averaged about 2.5 percent lower. Six weekends is a small sample, but the pattern is clear: the weekend is not charged as two full days of decay. Writers do earn something over a weekend, and long weekends and holidays work the same way, but the cost of holding through a Saturday and Sunday is a fraction of what a calendar-day model suggests. The market holidays guide covers holiday weeks.
Theta is not free money
Writers collect theta decay because they take the other side of every large move. On 24 September 2026 NIFTY fell 1.64 percent, and the at-the-money straddle, which a writer would have expected to lose value with a day of decay, instead rose from about 225 at the open to 259.05 at the close: the move outran the decay, as the straddle premium chart shows. On expiry days the same arithmetic is sharper. Gamma, the sensitivity that makes an option’s delta change, is at its highest exactly when theta is, so the session that pays writers the most decay is also the one in which a sustained move hurts them most. The gamma blast guide shows such a day from the buyer’s side.
Reading theta decay in OIData
- The Expiry Day page plots the at-the-money straddle against a square-root-of-time model through the session, with the call and put decay so far, and lists premium decay strike by strike; its context card ranks today’s decay against past expiries at the same time of day.
- Straddles & Strangles charts the combined premium of the at-the-money straddle, or any legs you choose, through every session, not only on expiry days.
- The Option Chain shows theta for every strike when its Greeks switch is on, beside delta and vega.
- The Holidays page shows the weeks in which a holiday shortens the time left before an expiry.
Theta decay FAQ
What is theta decay? The loss of an option’s time value as expiry approaches, with the index and implied volatility unchanged. Theta is its daily rate in rupees.
Is theta decay linear? No. For an at-the-money option, time value falls roughly with the square root of the time left, so decay speeds up in the final days and hours.
Does theta decay happen on weekends? Only a little. In six 2026 weekends before a NIFTY expiry, the at-the-money straddle opened Monday about 2.5 percent below its Friday close on average, far less than two days of decay.
When is theta decay highest? On expiry day, and especially at its end: across 13 NIFTY expiries in 2026, the median at-the-money straddle still held about half of its opening value at 15:00, and most of that went in the final half hour.
Do option writers always profit from theta decay? No. They earn it only if the index does not move by more than the premium they collected. A large move can cost them many days of decay at once.
Takeaways
- Theta decay is time value draining to zero by expiry; theta is the daily rupee rate.
- It accelerates: about 7 percent of the remaining time value goes from seven days to six, all of it on the last day.
- On expiry day the morning decays faster than the clock and the afternoon slower: in 13 NIFTY expiries the median straddle had lost 44 percent by 14:00, 52 percent by 15:00 and about 85 percent by the last reading.
- Weekends cost far less than a calendar suggests: in six 2026 cases, about 2.5 percent of the straddle on average.
- Theta pays writers only while the index stays inside what they were paid for; on expiry day gamma is at its peak too.