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What is Theta Decay?

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Used in a sentence

The Daily Ledger · Markets

With two weeks to expiration, theta decay was eating the position faster than the stock could climb.

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Explained in three depths

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Overview

Theta decay is the value an option loses each day just because time passes. A bet with a deadline is worth less with each day that goes by. Buyers pay this cost daily; sellers collect it.
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Overview

Theta decay is your option losing value every single day just from the calendar. It is a melting bet with a deadline. Buyers bleed it daily, sellers farm it. 😎

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Detail

An option's price has two parts: real value it would have if used today, and time value, the extra you pay for the chance that the stock still moves your way before expiration. Theta measures the daily shrink of that second part: a theta of -0.05 means the contract loses about $5 a day even if the stock never moves, because each passing day removes chances for the move to happen. That is why an option gets cheaper at the same stock price: what you bought was possibility, and there is less of it left every morning. The melt is not a straight line, gentle months out, faster in the final 30 days, steepest in the last week, when almost no chances remain. The melting applies only to that time-value part. Take a $50 call with the stock at $56: about $6 of its price is solid, because the option already lets you buy $56 shares for $50, and only the extra above $6 drains away as days pass. Sit the stock at $56 for two weeks and the option's price slides toward $6: nothing about the stock changed, but two weeks of remaining chances are gone, and that slide is exactly the income an option seller collects.
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Detail

An option's price is real value plus hope: the extra you paid for the chance the stock still makes its move before the deadline. Theta is the hope leaking out: theta -0.05 means about $5 a day gone with the stock doing absolutely nothing, because every sunrise deletes one more day for the miracle to happen. Same stock price, cheaper option, that is all theta is. The leak is slow months out, faster in the last 30 days, a waterfall in the final week when the calendar is nearly empty. The leak only eats the hope part. Stock at $56 with a $50 call: $6 of that price is concrete, you can buy $56 shares for $50 today, and everything above the $6 is hope on a timer. Park the stock at $56 for two weeks and the price grinds toward $6: the stock did nothing, the calendar did everything, and the seller who wrote the option pockets exactly what you leaked. 😎

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Analogy

Concert tickets as the show date approaches. A ticket for a show three months out holds its price, one for next week gets harder to sell each day, and at showtime an unused ticket is worth exactly nothing. The seat never changed; the time to use it ran out, and the value was the time.
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Analogy

Milk in the fridge, sale rack edition. Fresh cartons sell at full price, but the one dated this Friday gets marked down harder every morning, because each day that passes leaves fewer days to actually drink it. Nothing about the milk changed; what you are buying is the time left to use it, and that keeps shrinking until it is worth nothing.
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AI explanations may contain errors · Not professional advice

Formal definition — The same term, explained the usual way

Theta is the partial derivative of an option's value with respect to time, conventionally expressed as the daily erosion of extrinsic value. Decay is nonlinear, accelerating as expiration approaches, with at-the-money options exhibiting the greatest absolute theta. Time decay accrues to short option positions and constitutes the principal return source of premium-selling strategies.

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