Theta (Θ)
Time decay — what you pay (or earn) simply for holding the position another day.
The change in an option’s price as one day passes, with everything else unchanged.
Overview
Theta measures time decay: how much value an option loses each day as expiry draws nearer, assuming the underlying and volatility stay put. An option is a wasting asset, and theta is the size of that daily erosion.
Buyers of options pay theta — their position bleeds a little every day. Sellers of options collect theta — that daily decay is their income. This is the core trade-off of options: buyers rent time and convexity; sellers earn time but carry the tail risk.
Theta is not linear. It accelerates as expiry approaches, especially for at-the-money options, where the last week can decay faster than the previous month.
A plain-language example
You are long a NIFTY 24,000 call with theta −40.
If NIFTY and implied volatility are unchanged overnight, the option is worth about ₹40 less tomorrow — that is the rent you paid to keep the position alive.
The seller of that same call has theta +40: they pocket roughly ₹40 a day for taking the other side, in exchange for accepting the risk of a large move.
Long options: negative theta (you decay) · Short options: positive theta (you earn) · ATM theta is largest and accelerates into expiry.
Buyers vs sellers
If you buy the option
Long options carry negative theta — time works against you.
If you sell the option
Short options carry positive theta — time works for you.
What moves Theta
| When… | Effect on Theta |
|---|---|
| Approaching expiry | Decay accelerates, sharply so for ATM options in the final week |
| Option at-the-money | Theta is largest — most extrinsic value to lose |
| Deep ITM / OTM | Theta is smaller — little time value remains to decay |
| Rising implied volatility | More extrinsic value ⇒ more absolute theta to bleed |
Theta at a glance
How traders use it
- Premium-selling strategies (short straddles/strangles, iron condors, credit spreads) are built to harvest theta while defining or hedging the risk.
- Option buyers should size for decay: if you are right on direction but slow, theta can still make the trade a loser.
- Theta and gamma pull in opposite directions — the positive theta a seller earns is compensation for the negative gamma they carry.
Watch out for
- Theta is only “free money” on quiet days; one gap can erase weeks of collected decay for a short-option seller.
- Decay is not evenly spread across the day or the week — a large chunk of weekly-option theta lands in the final sessions.
See Theta on a live position
Open the Strategy Builder, add a leg, and hover the Theta row in the Greeks tab to watch it update in real time.
Educational content only — not investment advice. All values are illustrative and do not reflect live quotes. Options carry significant risk; consult a registered adviser before trading.