What is theta decay in options?
Theta is the amount an option's price is expected to fall each day as time passes, with the stock price and implied volatility unchanged. It is the cost of holding an option and the income of selling one. Theta decay is not steady: it accelerates as expiration approaches, which shapes almost every decision about which expiry to buy or sell.
What theta actually measures
An option's price has two parts: intrinsic value, the amount it is in the money, and time value, everything else. Time value is what you pay for the possibility that the option becomes more valuable before it expires. As time runs out, that possibility shrinks, and time value falls toward zero at expiry.
Theta is the daily rate of that fall. An option with a theta of minus 0.08 loses about eight cents of value per day if nothing else changes. Multiplied by 100 shares per contract, that is eight dollars per contract per day. Option sellers have positive theta: they gain that eight dollars a day while the stock stays put.
Only time value decays. A deep in-the-money option that is almost entirely intrinsic value has little theta, because there is little time value left to lose.
Why decay accelerates
In standard option models, the time value of an at-the-money option is roughly proportional to the square root of the time remaining. That one fact explains the familiar decay curve. Halve the time remaining and the time value falls not by half but to about 71 percent of where it was, because the square root of one half is about 0.71.
Put concretely: an at-the-money option with 30 days left loses about 29 percent of its time value over the first 15 days and the remaining 71 percent over the last 15. The last week is steeper still. That is why short-dated options lose value so quickly and why sellers often favour the final weeks before expiry.
Out-of-the-money options follow a different shape. Their time value is the entire price, and it can decay steadily for weeks and then collapse in the final days as the chance of reaching the strike disappears.
Which options decay fastest
In dollars, at-the-money options have the largest theta, because they carry the most time value. In percentage terms, out-of-the-money options lose value fastest near expiry, because almost all of their price is time value and it can vanish in days.
Implied volatility raises theta. A higher volatility means more time value to begin with, so there is more to lose each day. That is why options on volatile stocks, or options priced ahead of an event, can show very large thetas.
Theta and gamma are tied together. The options with the most gamma, short-dated and at the money, also have the most theta. Owning that sensitivity to price moves costs money every day, and selling it pays every day in exchange for the risk of a large move.
Weekends and holidays
Option models generally count calendar days, so time value decays over weekends and holidays even though markets are closed. In practice, market makers anticipate the weekend and tend to lower implied volatility on Friday, so a good share of the weekend's decay shows up in prices before the close rather than as a gap on Monday morning.
This is why selling an option on Friday to collect weekend decay is often less profitable than it looks on paper. The decay is usually already priced in, and the seller still carries the risk of weekend news.
Holidays work the same way. A three-day weekend adds an extra calendar day of decay, and market makers generally anticipate it in their pricing beforehand. Short-dated options that span a holiday are, in effect, priced for one fewer trading day than their calendar life suggests.
Using theta without misreading it
Theta is an estimate for one day with everything else frozen. In reality, the stock moves and volatility changes, and those effects often swamp a day of decay. A seller who collects ten dollars of theta can lose a hundred on a modest move against them.
The right way to read theta is as a rate of payment for an exposure. Buyers pay theta to own gamma and vega; sellers collect theta for carrying them. Whether that trade is worth it depends on how much the stock actually moves compared with what the options priced in.
That framing also explains why theta is never free money. A position with large positive theta is short gamma by construction. Its daily income is the market's payment for accepting the risk that the stock moves more than priced, and in a fast market that risk can arrive all at once.
Common questions
When does theta decay accelerate?
In the final weeks before expiry, and most sharply in the last few days, because time value is roughly proportional to the square root of the time remaining.
Do options lose value over the weekend?
Yes, in model terms, because time passes. In practice much of the weekend decay is priced in before Friday's close.
Does theta affect in-the-money options?
Only their time value decays. Deep in-the-money options have little time value, so their theta is small.
Theta is the daily decay of an option's time value. It accelerates with the square root of time, is largest in dollars for at-the-money options, and is the price buyers pay for gamma and sellers collect for carrying it.
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