What is Theta (decay) in options?
Daily rent — what buyers pay and sellers collect, just for time passing.
Theta (Θ) measures how much an option's price is expected to decay per day, all else held equal. It's almost always shown as a negative number for long options — a Theta of -0.08 means the position loses about eight cents in value overnight from time decay alone, independent of what the stock does. For option sellers, that same decay works in reverse: it's income earned simply by the calendar advancing.
Why options lose value just from time passing
An option's premium is made of intrinsic value (what it's worth if exercised right now) plus extrinsic value (everything else — the time and uncertainty still priced in). Extrinsic value only exists because there's time left for the stock to move favorably, so every day that passes with no move is one less day for that to happen, and the market prices that in by shrinking the premium. Theta is the daily rate of that shrinkage.
Why the decay isn't a straight line
Theta is small when there's a lot of time left and accelerates as expiration nears — extrinsic value decays roughly with the square root of time remaining, not evenly by day. A 60-day option loses relatively little value in its first week; that same option with 5 days left can lose a large chunk of its remaining premium in a single session. This nonlinear shape is why the final two weeks of an option's life feel completely different from the first six.
A worked example
Say an AAPL at-the-money call trades for $3.20 with 45 days to expiry and a Theta of -0.04. All else equal, it should be worth roughly $3.16 tomorrow. Fast-forward to 5 days left on a similar at-the-money contract, where Theta might be -0.15 or steeper on the same stock — now that option can lose $0.15 or more in a single session with the stock completely flat. That's the same underlying mechanism, just measured at a much steeper point on the decay curve.
Common mistakes
Assuming Theta decay is constant. Using today's Theta to estimate value a month from now overstates near-term decay and badly understates decay in the final weeks.
Buying short-dated, at-the-money options and holding through a quiet period. This is the single fastest way to feel Theta's bite — maximum decay concentrated in minimum time.
Selling premium without respecting Gamma. Collecting Theta is the reward for short options, but Gamma risk is the reason that income isn't free — a sharp move can erase weeks of collected Theta in one session.
Try it live
The Theta below is real — pulled from a live option contract (strike nearest spot, ~30–45 days to expiry) on OptionScope's real CBOE-fed chain. Look up any symbol:
See the 0DTE Theta Timeline — free →
Related: All 5 Greeks explained · Gamma · Vega · Theta decay, deep dive