Time Decay

What Is Time Decay?

Time decay describes how an option loses value simply because less time remains until it expires. An option gives its holder the right, but not the obligation, to buy or sell an underlying asset at a set price before a set date. The price paid for that right, called the premium, has two parts: intrinsic value and extrinsic value. Intrinsic value is the amount an option would be worth if exercised immediately. Extrinsic value reflects the chance that the option could become more valuable before it expires, and time is a major part of that chance. Time decay is the process by which extrinsic value shrinks.

How Time Decay Works

Time decay is usually measured by the option Greek called theta, which estimates how much an option's price is expected to fall for each day that passes, holding other factors constant. The rate of decay is not steady. With more time left, the chance of a favorable price move is spread over a longer period, so value falls slowly. As expiration approaches, that remaining opportunity narrows and decay tends to speed up. At expiration, extrinsic value reaches zero, and only intrinsic value, if any, remains.

Time decay affects buyers and sellers differently. Buyers pay for time and generally lose ground as it passes. Sellers collect premium and generally benefit from decay, though they take on other risks.

An Example

Consider two options on the same stock with the same strike price, one expiring in a month and one expiring in a year. If the stock price stays unchanged, the one-month option usually has much less extrinsic value than the one-year option, because it has less time for a favorable move. Its price will tend to fall faster as the weeks pass, even though the stock itself has not moved.

Why It Matters to Investors

Time decay is one reason option prices can change even when the underlying stock price does not. It also explains why a long option position can lose value while the stock trades sideways. Other factors, especially implied volatility, also affect option prices and can offset or amplify time decay. Understanding decay helps readers interpret option quotes and the risks of both buying and selling options.