Time decay is the steady loss of an option's value simply because time is passing. Every day that goes by is one less day for the underlying to make the move you paid for, so the time-value part of the premium shrinks — even if the stock price, volatility and everything else stay exactly where they were. Theta is the Greek that measures this decay, as the amount of premium an option is expected to lose per day.
Why does an option lose value just because time passes?
An option's premium is intrinsic value plus time value, and time value is the price of possibility — the chance that the underlying moves favourably before expiry. If that split is new to you, start with What makes up an option's premium?
Possibility has an expiry date. Say Kaveri Motors trades at ₹520 and Anjali buys the ₹550 call for ₹4 with two weeks left. All ₹4 is time value — the option has no built-in worth, only a chance. With fourteen days ahead, that chance is worth something. With seven days, less. With one day, very little. If Kaveri Motors just drifts sideways, the premium bleeds to ₹3, ₹2, ₹1 and finally zero, without the stock falling at all. Nothing went wrong with the trade except the clock.
What is theta, exactly?
Theta puts a daily number on the melt. If Anjali's ₹4 option shows a theta of ₹0.30, then — all else unchanged — it should be worth roughly ₹3.70 tomorrow. Theta is quoted as a negative number for a bought option, because the passage of time takes value away from the holder.
Two things about that number are worth knowing:
- It's an estimate, not a debit. The premium you actually see also moves with price and volatility all day; theta is only the time component, isolated.
- Theta itself changes. As expiry approaches, the daily decay on at-the-money and near-the-money options gets larger, not smaller. The last few days are the steepest part of the melt.
You can see the live theta of any contract on Rupeezy — see How to check Options Greeks on Rupeezy?
Is the decay the same every day?
No — and this is the part that catches beginners. Decay is slow when expiry is far away and accelerates sharply as it nears. A rough mental picture: an option loses time value gently for most of its life, then sheds the bulk of what remains in the final week, with the fastest melt in the last day or two.
Two practical consequences follow:
- Cheap options near expiry are cheap for a reason. A few rupees of premium with two days left looks affordable, but it sits on the steepest part of the decay curve. The underlying now has to move fast and far — merely being right about direction, slowly, still loses.
- Decay follows the calendar, not trading hours. Weekends and holidays count. An option doesn't pause melting because the market is shut; sellers price those non-trading days into the premium.
With weekly index options, this effect dominates the whole product: the contract's entire life is the steep end of the curve. That is also what makes expiry day behave the way it does — see What happens on expiry day?
Who does time decay help, and who does it hurt?
Every day works against the option buyer and for the option seller. The buyer paid for time and watches it evaporate; the seller collected the time value upfront and keeps whatever melts away. This is the structural reason option selling is often described as earning "theta income" — and the equally structural catch is that the seller carries the large-loss risk described in What are the risks of trading Futures and Options (F&O)?
Decay also compounds with volatility. A buyer who overpays when implied volatility is high can lose to the IV collapse and to theta at the same time — being right on direction and still losing money. The volatility half of that story is in What is implied volatility (IV)?
Can time decay ever work in my favour?
Yes — whenever you are the one holding the collected premium, or when you treat it as a known cost:
- Selling options turns theta into a tailwind: each quiet day moves the trade in the seller's favour. The risk profile is the mirror image, so this is not a free income stream.
- Spreads pair a bought option with a sold one, so the sold leg's decay offsets part of the bought leg's — one reason defined-risk strategies are popular among traders who want direction without paying full theta.
- Hedging reframes decay as an insurance premium. When you buy a protective put, theta is the cost of the cover, melting away exactly like a policy premium — acceptable, because protection was the point. See How do options help in hedging?
For buyers who want to fight decay rather than switch sides: buy more time than you think you need, and remember that exiting an option early keeps whatever time value remains — holding to expiry surrenders all of it.
Things to keep in mind
- Time decay never pauses and never reverses. Direction can rescue a trade; time cannot.
- The melt accelerates near expiry — the cheapest-looking options carry the fastest decay, which is exactly when many beginners buy them.
- Theta is one number in the Greeks family and interacts with the others; a favourable price move can outrun decay, and an IV drop can worsen it.
- Sellers earn decay but carry the tail risk. Collecting theta is a strategy with obligations, not a savings account.
- All figures here are illustrative, not live quotes or predictions.
Read next
What happens on expiry day? — Where every option's time value finishes its journey to zero.