Theta: time decay accelerates near expiry
Define theta as daily option price decay, and recognize that decay accelerates as expiry approaches, especially for ATM options.
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Options, Risk Math, and Psychology
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Define theta as daily option price decay, and recognize that decay accelerates as expiry approaches, especially for ATM options.
Theta, the time-decay tax
Theta is the daily price you pay for owning an option. Every day that passes — even if the stock doesn't move at all — your option loses a little value. That bleed is theta. It's usually quoted as a negative number for buyers: theta of -0.05 means the option loses about $0.05 a day from time decay alone. Sellers see theta as a positive — they collect that decay as profit, day after day, as long as the option doesn't move against them.
Here's the punchline that catches beginners off guard: theta is not linear. It accelerates. An option with 60 days to expiry bleeds slowly. The same option with 10 days to expiry bleeds fast. With 2 days left, theta is brutal. Picture the decay curve — it slopes gently downward for most of the option's life, then drops off a cliff in the final two weeks. This is why buying short-dated options is a race against the clock.
Where is theta highest? At-the-money options have the biggest theta because they have the most extrinsic value to lose. Deep ITM options have most of their value baked in as intrinsic (which doesn't decay), so theta is small. Deep OTM options don't have much value to lose either. The ATM option sits on the most premium per dollar of move expected — and the most premium to bleed.
Pros use theta to think about the cost of waiting. If you buy a call that costs $5.00 with theta of -$0.10, you're paying $0.10 a day for the right to wait. If the stock doesn't move in 10 days, you've burned $1.00 in time alone — 20% of the option's value gone. That's the trade you accept when you buy. The flip side: theta is exactly what option sellers harvest. They're paid to wait.
Theta gets sneaky over weekends. The calendar keeps ticking even when the market is closed. An option held from Friday close to Monday open eats three days of theta — Saturday, Sunday, and Monday's allotment. Long-option holders sometimes get a nasty surprise opening their broker Monday morning. Sellers, meanwhile, smile at every weekend they're short premium. This is one reason short-term option sellers love taking positions on Friday — they collect weekend theta with the market closed and limited risk of intraday moves against them.
Recap: theta = daily decay. Negative for buyers, positive for sellers. Accelerates near expiry. Highest for ATM options.
Knowledge check
Answer before moving on.
1. An option has theta of -0.08. What does that mean for the buyer?
2. Where is theta highest in absolute value?
3. Why does theta 'accelerate' near expiry?
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