How much an option's value decays with one day's passage of time.
Quick answer: Theta measures how much value an option loses each day purely from the passage of time — the daily 'rent' an option buyer pays and an option seller collects.
Simple explanation
Every day that passes, an option loses a little value even if nothing else changes, because there is less time for it to move into profit. A Theta of −8 means the option loses about ₹8 per share per day. Buyers fight Theta; sellers earn it. Time decay is slow far from expiry and accelerates sharply in the final week.
Theta — visual
How Theta behaves
An at-the-money option's value bleeds slowly at first, then decays faster and faster as expiry nears — the classic non-linear time-decay curve.
Measures
How much an option's value decays with one day's passage of time
Sign
Long options −Θ (buyers pay) · Short options +Θ (sellers earn)
Typical range
Most negative for long ATM options; accelerates as expiry nears
Order
First-order
Detailed explanation
Why time has value
An option's premium is intrinsic value plus time value. Time value exists because, while the contract is alive, the underlying might still move favourably. As expiry approaches, that window shrinks, so time value melts toward zero — the option converges to pure intrinsic value. Theta quantifies that daily melt. For an ATM Nifty weekly option, Theta is the dominant force in the last two or three sessions.
Non-linear decay
Theta is not constant. Time value decays roughly with the square root of time remaining, so an option loses value slowly when expiry is far away and rapidly in the final days. A monthly option barely decays in its first week but can lose a third of its remaining value in the last three days. This is why sellers favour the back end of the expiry cycle and why buyers of weekly options are racing the clock.
Theta is highest at-the-money
At-the-money options have the most time value to lose, so they carry the largest Theta. Deep ITM options are mostly intrinsic value and decay little; far OTM options have little value left to decay. This is why short straddles and Iron Condors — which sell ATM-ish premium — are built to harvest Theta.
The weekend and holiday effect
Time decay does not pause for weekends or NSE holidays. Sellers often like to be short premium over a weekend to collect two or three days of Theta while the market is closed. Sharp buyers avoid holding low-Delta weekly longs over a long weekend, where two sessions of decay can gut the premium with no chance of a move.
Formula
Theta formula
Θ_call = −(S·n(d₁)·σ)/(2√T) − r·K·e^(−rT)·N(d₂) (per year; ÷365 per day)
Theta is usually quoted per calendar day. It is negative for long options (value lost) and positive for short options (value gained).
Practical example (Nifty)
Illustrative — Nifty spot 24500, lot size 75
Nifty at 24,500, five days to weekly expiry. You buy the 24,500 CE for ₹120 with Theta −18. If Nifty sits still for one day, the option is worth about ₹120 − 18 = ₹102 — a ₹18 × 75 = ₹1,350 loss per lot from time alone. Two flat days and you are down ~₹2,600 before any move. Now flip it: sell that call and you collect that decay instead. But remember the trade-off — collecting ₹18/day of Theta means carrying the Gamma risk of a sudden move.
Practical trading impact
Time decay is the option buyer's enemy and the seller's income — pick your side deliberately.
Theta accelerates in the final days, so weekly option buyers need the move to happen quickly, not eventually.
ATM options carry the most Theta; sellers target them, buyers of ATM weeklies must respect the daily bleed.
Decay runs over weekends and NSE holidays — factor two to three days of Theta into any position held across them.
Common mistakes
Buying cheap OTM weekly options and holding them flat, watching Theta erase the premium while waiting for a move that never comes in time.
Selling options for Theta without sizing for the Gamma/volatility risk that can wipe out weeks of collected decay in one move.
Forgetting that Theta accelerates — a position that decayed slowly all month can collapse in the last three sessions.
Holding long premium over a long weekend or holiday and being surprised by the multi-day decay.
Professional usage
Experienced sellers treat Theta as a yield they harvest while actively managing the Gamma bill that comes with it — they roll or close before the final Gamma-heavy sessions, avoid over-sizing, and prefer selling elevated IV so decay works alongside a volatility drop. Skilled buyers minimise Theta drag by buying slightly ITM options (more Delta, less time value at risk) or by choosing expiries with enough time for the thesis to play out.
Key takeaway
Theta is the daily rent of an option. Buyers pay it and need a timely move to win; sellers collect it and must survive the Gamma and volatility risk that comes attached. Decay is slow early and brutal near expiry.
Frequently asked questions
What is Theta in options trading?
Theta measures how much value an option loses per day from time passing alone. A Theta of −8 means the option loses about ₹8 per share each day if nothing else changes.
Is Theta good or bad?
It depends on your side. Theta is negative (bad) for option buyers, who lose value daily, and positive (good) for option sellers, who collect that decay.
Why does Theta accelerate near expiry?
Time value decays roughly with the square root of time left, so it melts slowly when expiry is far and very fast in the final days as the option converges to its intrinsic value.
Which options have the highest Theta?
At-the-money options, because they have the most time value to lose. Deep ITM and far OTM options decay much more slowly.
How do option sellers profit from Theta?
By selling premium and letting time decay reduce the option's value, buying it back cheaper or letting it expire worthless — while managing the Gamma and volatility risk they take on.
Does Theta decay over weekends?
Yes. Time passes over weekends and holidays, so options lose value across them even though the market is closed. Sellers often like holding premium over weekends.
What is Theta for weekly vs monthly options?
Weekly options have much higher daily Theta relative to their price because they are always near expiry, whereas monthly options decay slowly until their final week.
How is Theta related to Gamma?
They are a trade-off: collecting Theta means being short Gamma (accelerating risk on moves), and being long Gamma means paying Theta. You cannot have both positive at once.
Can Theta be positive for a long option?
Almost never in Indian markets. Long options have negative Theta; positive Theta comes from being net short options in a position.
How do I reduce Theta decay when buying options?
Buy slightly in-the-money options (more intrinsic, less time value at risk), choose longer expiries so decay is slower, or avoid holding low-Delta weeklies through flat periods.
How do I read Theta from my broker's option chain?
Theta is shown as a per-share number, almost always negative for the option's own value, so a Theta of −12 on a Nifty option means it loses about ₹12 per share per day. To get the rupee impact on your position, multiply by the lot size of 75, so −12 becomes roughly ₹900 of decay per lot per day.
How do I convert Theta into a daily rupee figure for my position?
Multiply the option's Theta by the lot size (75) and by the number of lots. A 2-lot short strangle with a combined Theta of +30 earns about 30 × 75 × 2 = ₹4,500 a day if nothing else moves. Track this daily 'Theta harvest' against the Gamma losses when the market does move.
How much higher is Bank Nifty Theta than Nifty Theta?
In absolute rupees, Bank Nifty options usually carry larger Theta because their premiums are bigger and the index is more volatile, so an ATM Bank Nifty weekly can decay ₹40–₹80 per share a day versus ₹15–₹30 for Nifty. Always compare Theta relative to premium and against the same lot size before assuming one is 'better' to sell.
How does Theta behave on 0DTE (expiry-day) options?
On expiry day, remaining time value is tiny, so an ATM option's entire premium is essentially Theta that will vanish by close. The decay is no longer a smooth daily number; it collapses through the session, which is why expiry-day sellers can profit fast but face brutal Gamma if the index swings into their strike.
Should I sell weekly or monthly options to collect Theta?
Weeklies offer the highest Theta per day relative to premium, so they suit active sellers who manage positions closely, while monthlies decay slowly and demand less babysitting but tie up more margin per rupee of daily decay. Many Indian sellers split the difference by selling weeklies for pace and monthlies for stability.
Does Theta increase around events like the Budget or RBI policy?
Not directly — Theta itself does not spike for events, but the inflated implied volatility before an event pumps up premium and therefore the absolute Theta number. After the event, IV crush and time decay hit together, which is why sellers who survive the event often see premiums melt quickly the next day.
Is a higher Theta always better for an option seller?
No. A large Theta usually comes bundled with large Gamma and Vega, so the fat daily decay you collect is exactly the premium at risk from a sharp move or an IV spike. Chasing the biggest Theta strikes without sizing for that risk is how sellers give back weeks of gains in one session.
How does Theta compare with Vega for a monthly option buyer?
For a monthly buyer, early on Vega often dominates — a shift in IV moves the premium more than one day of slow decay. As expiry approaches, Theta takes over and accelerates while Vega shrinks, so the same long option that was a volatility bet becomes a race against time decay in its final week.
Why do two options with the same premium have different Theta?
Because Theta depends on how much of the premium is time value and how close the option is to expiry, not just the rupee price. A near-expiry ATM option and a far-dated deep-ITM option can cost the same but the ATM weekly will bleed far faster, since almost all its value is decaying time value.
Does Theta hurt a debit spread as much as a naked long option?
No. In a debit spread you buy one option and sell another, so the short leg's positive Theta partly offsets the long leg's negative Theta, giving a much smaller net daily decay. This is a key reason spreads are gentler on time decay than outright long calls or puts.
Can Theta work in my favour while I am net long options?
Only through structure, not a single long option. A calendar spread is net long a far option and short a near one; the near option decays faster, so the position can have net positive Theta even though you are 'long' volatility. A standalone long option always has negative Theta.
How does rising India VIX change the Theta I collect as a seller?
When India VIX rises, option premiums inflate and the absolute Theta number grows, so the daily decay you collect looks larger. But that richer Theta is compensation for higher Vega and Gamma risk — you are being paid more per day precisely because the odds of a damaging move have gone up.
Is Theta decay linear through the trading day or overnight?
It is not evenly spread. A large chunk of daily decay is booked over the non-trading hours, so options often open a bit cheaper than Friday's close reflecting weekend decay, then bleed more slowly intraday. Sellers holding overnight capture that gap; intraday-only sellers capture comparatively less pure Theta.
Voice search & related questions
Natural-language questions people ask about Theta.
What is Theta in simple words?
Theta is the amount an option loses in value every day just because time is passing. Buyers lose it; sellers earn it.
Why does my Nifty option lose money even when the market doesn't move?
That is Theta, the daily time decay. An option is worth less each day because there is less time left for it to move into profit.
How much does an option decay per day in rupees?
Take the option's Theta number and multiply by the lot size of 75. A Theta of −20 means about ₹1,500 lost per lot per day if nothing else changes.
When is Theta decay the fastest?
In the final few days before expiry, especially for at-the-money weekly options, where the premium can melt very quickly.
Should I buy weekly options and hold them for several days?
Usually not if the market is flat, because weekly Theta is high and will eat the premium fast. Buy weeklies only when you expect a quick, sizeable move.
Can I earn Theta while staying safe?
You can collect Theta by selling premium, but it is never risk-free — every rupee of Theta you earn comes with Gamma and volatility risk. Defined-risk spreads keep the danger sized.
Does Theta keep working over the weekend?
Yes. Time passes on Saturday and Sunday too, so options lose value across the weekend even though the market is closed.
Why do sellers like holding positions into a long weekend?
Because they collect two or three days of time decay while the market is shut and cannot move against them, so long as no gap-risk event is looming.
Educational content only — not investment advice. Greek values are illustrative and computed from a Black-Scholes model. Options trading involves substantial risk. See our Risk Disclosure and SEBI Disclaimer.