Theta (Time Decay)

Theta is the options Greek that puts a number on time decay: how much an option's premium erodes each day simply because expiry is one day closer. It is quoted as a negative figure, because for the buyer time is always working against you.

How it works

An option's price has two parts: intrinsic value (real exercise value) and time value (the extrinsic premium you pay for the *possibility* of a favourable move). Theta eats away only the time value. As the clock runs down, the chance of a big move shrinks, and so does the premium.

Crucially, decay is not linear. It follows a curve that accelerates sharply in the final days and becomes brutal in the last 24 hours, especially for at-the-money options. A weekly contract loses time value far faster per day than a monthly one at the same strike.

In India

India's market is dominated by short-dated weekly options on Nifty and Bank Nifty, so theta is front and centre. After SEBI's 2025 derivatives reshuffle, Nifty weekly options now expire on Tuesday, and 'expiry day' trading is essentially a battle over how fast premium decays.

This is why option sellers (who collect premium upfront) often profit from theta, while the crowd of retail option buyers chasing cheap out-of-the-money weeklies watch their premium melt even when the index barely moves. SEBI's own studies have repeatedly shown the large majority of individual F&O traders lose money, and time decay is a big reason why.

Why it matters

If you buy options, theta is your enemy: you need the underlying to move enough, and soon enough, to beat the daily bleed. Holding a long option through a quiet expiry week is a slow loss.

If you sell options, theta is your edge, but it comes with potentially large risk if the market moves sharply against you. A practical takeaway: never buy an option without asking whether the expected move can outrun theta before expiry. On expiry day, time value can vanish within hours.