By the FinLingo Team | Capital markets practitioner, front office experience at a major European investment bank. FinLingo covers 342 lessons from bonds to exotic derivatives. About · Last updated:
Theta (Θ) measures the rate at which an option loses value as time passes, all else being equal. A theta of −0.05 means the option loses $0.05 per day. Time decay is the silent cost of holding options — every night, whether the stock moves or not, the option bleeds a little value.
An option's value has two components: intrinsic value (what it is worth if exercised now) and time value (the extra premium for the possibility of a favourable move before expiry). As expiry approaches, time value shrinks because there is less time for favourable moves to occur. At expiry, time value is zero and only intrinsic value remains.
Theta is not constant. It accelerates as expiry approaches. An ATM call with 90 days to expiry might lose $0.03 per day. At 30 days: $0.05 per day. At 7 days: $0.12 per day. At 1 day: $0.25 or more. This acceleration follows roughly a square-root-of-time pattern — the last 30 days erode more value than the first 60. Holding an ATM option into the last week of its life is expensive.
This is the fundamental equation of options trading. Long options are long gamma and short theta: you benefit from stock moves but pay time decay every day. Short options are short gamma and long theta: you collect time decay but suffer when the stock moves. The break-even question is always the same: will the gamma gains from stock movement exceed the theta paid? That depends on whether realised volatility exceeds implied volatility.
You buy an ATM call for $5.00 with theta of −$0.08 and 60 days to expiry. After 10 days with no stock movement, the option is worth roughly $5.00 − (10 × $0.08) = $4.20. You have lost $0.80 to time decay alone. If the stock moved enough during those 10 days to generate $1.20 of gamma profit from rebalancing, your net P&L is +$0.40. If the stock was calm and gamma generated only $0.30, you are down $0.50. Theta is the rent you pay for the right to be long gamma.
Theta decay is the daily loss in an option value due to the passage of time. Each day, the option has less time for a favourable move, so its time value decreases. Theta is always negative for long option holders (they pay decay) and positive for short option holders (they collect decay). It is the cost of holding an option position.
Because time value erodes on a square-root-of-time basis, not linearly. With 60 days left, the option has significant time value and each day removes a small fraction. With 5 days left, the remaining time value is small but must reach zero at expiry, so each day takes a larger percentage. ATM options lose roughly half their remaining time value in the final quarter of their life.
They are opposite sides of the same trade. Long gamma means you profit from stock movement but pay theta daily. Short gamma means you collect theta but lose when the stock moves sharply. The breakeven is determined by realised vs implied volatility: if realised vol exceeds implied, gamma profits exceed theta costs (long gamma wins). If realised vol is lower, theta collection exceeds gamma costs (short gamma wins).
FinLingo covers theta and all Greeks in Level 3 — 10 interactive units. Watch theta acceleration in The Lab pricer. Level 1 is free.
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