Theta
Theta measures how much value an option bleeds away each day simply because time is passing — it is the clock the seller collects and the buyer pays.
Every option has an expiration date, and as that date approaches, the option’s value erodes even if nothing else changes. Theta puts a number on that erosion: it is how much value the option loses per day to the passage of time, holding the stock price and volatility constant. Theta is negative for buyers, who pay it — their option is worth a little less each morning — and positive for sellers, who collect it, because the option they are short gets cheaper to buy back over time.
Reading the number
Theta is quoted per share, and option contracts cover 100 shares, so the per-contract figure is 100 times the quote. A theta of −0.05 means the option loses about 5 cents per share per day — roughly 5 dollars a day per contract. For the buyer that is 5 dollars a day melting away; for the seller it is 5 dollars a day of value drifting in their favor, assuming everything else stays put.
Decay is not linear
A common beginner mistake is to imagine time value draining at a steady rate. It does not. For an at-the-money option, decay accelerates as expiration nears. The option loses value slowly when there are months left and increasingly quickly in the final weeks, with the steepest drop in the last stretch before expiration. Decay is also continuous: the clock does not stop on weekends or holidays. Value erodes over a Saturday and Sunday just as it does on a trading day, which is why an option can be worth noticeably less Monday morning than it was Friday afternoon even though the market was closed the whole time.
Worked example: selling a 45-day put
Suppose you sell a put with 45 days to expiration and collect a premium of 2.00 per share — 200 dollars for the contract. If the stock simply sits still, theta hands a little of that 200 dollars back to you each day as the option’s time value erodes. You do not have to wait until expiration to realize the gain. Many sellers buy the put back once they have captured around half the premium — here, roughly when it can be repurchased for about 1.00 — rather than holding to the very end. The reason they do not hold to the end is not theta; it is gamma. The remaining decay is small, but the jumpiness of the position climbs sharply into the final days, so the last dollar of premium is the most uncomfortable to earn.
Theta is not free money
It is tempting to look at positive theta and see a position that “makes money every day.” That framing is dangerous. Theta is payment for bearing a real risk: while you are short the option, you are exposed to a large or fast move in the stock that can erase weeks of collected decay in a single session. The decay is your compensation for standing in front of that possibility, not a reward for doing nothing.
There is a genuine reason sellers can come out ahead over many trades. On average, options tend to be priced a little richer than the moves that actually arrive — the market charges slightly more for protection and leverage than the realized movement justifies. That gap is known as the volatility risk premium, and it is the real edge behind selling options. (See why be the seller for a fuller treatment.) Theta is the daily mechanism through which that premium is collected, but the edge lives in the premium being a touch too high, not in time passing by itself.
Time decay works quietly in your favor day after day, but one large adverse move can wipe out weeks of it at once. The defense is not cleverness about entry timing — it is sizing the position so that a bad day is survivable. The Greek that giveth (theta) and the Greek that taketh away (gamma) are bound together; respect both.
For how theta fits with delta, gamma, and vega in a single position, see The Greeks Explained.
Educational only — not investment advice. Options involve a substantial risk of loss and are not suitable for every investor.