Theta: how options decay with time
An option is a wasting asset. Every day that passes, with nothing else changing, an option is worth a little less — because there is less time left for the move the buyer is hoping for. Theta puts a number on that decay.
What theta measures
Theta is the amount of value an option loses from the passage of one day, holding everything else constant. It is quoted as a negative number for option owners: a theta of −0.05 means the option loses about $0.05 per share ($5 per contract) each day, purely from time. Only extrinsic value decays — intrinsic value is immune — so theta bites hardest on at-the-money options, which are all extrinsic value.
Decay is not linear
Time decay accelerates as expiration approaches. An option with 90 days left loses value slowly; the same option in its final week sheds value rapidly, and in its final day the decay is steepest of all. The classic picture is a curve that starts gently and then plunges — extrinsic value falling faster and faster until it hits zero at expiration. This is why the choice of expiration date is really a choice about how much theta you are paying or collecting.
An ATM option worth $3.00 with 30 days left might carry a theta of −$0.05/day early on. But decay is not even: of that $3.00 of extrinsic value, perhaps $1.00 melts over the first three weeks and the remaining $2.00 melts in the final week — a theta near −$0.30/day at the very end. A buyer who is right about direction but early can still watch the position bleed out; a seller in the same window is collecting that accelerating decay.
Who pays and who collects
Theta is a transfer from option buyers to option sellers. If you are long options, theta is a headwind — you need the underlying to move enough, soon enough, to outrun the decay. If you are short options, theta is a tailwind — you profit simply from time passing, as long as the underlying stays contained. Income strategies (covered calls, cash-secured puts, credit spreads, iron condors) are fundamentally theta-harvesting trades.
The theta–gamma trade-off
Theta and gamma are inseparable. The same at-the-money, near-expiration option that has the richest gamma also has the fastest theta. A long holder rents gamma (helpful acceleration) and pays theta (time decay) for it; a seller collects theta and is short gamma (exposed to big moves). You cannot have positive gamma and positive theta on the same option — the market prices them as opposites.
Common pitfalls
Buying short-dated options for a slow thesis. If your view needs weeks to play out, a weekly option’s theta can kill the trade before you’re proven right. Match the expiration to the expected timing of the move.
Assuming weekend decay hits on Monday. Many models bleed theta over the weekend, so an option can open Monday already lighter even though the market never traded. Sellers plan around this; buyers get surprised by it.
What to do with this
Before you buy an option, ask whether the expected move is large enough and soon enough to beat the theta you’re paying. Before you sell one, remember theta is your income — but it comes bundled with short gamma. And note what sets the size of theta in the first place: implied volatility, the subject of vega.
Theta is priced from implied volatility — and vega measures how the premium reacts when that volatility changes.