What it means
An option's price has two parts: intrinsic value, which is what it would be worth if exercised today, and time value, which is what a buyer pays for the possibility that the market moves further in their favour before expiry. Theta measures the daily erosion of that second part, because every day that passes leaves less time for the favourable move to happen.
This matters because time decay is the one variable in options that is completely certain in direction. Share prices may rise or fall and volatility may go either way, but the calendar only moves forwards, so an option buyer starts each day slightly behind and needs the underlying to move enough to cover the loss.
Theta is quoted per share, and standard equity option contracts cover 100 shares, so the daily dollar effect on a position is theta multiplied by 100 and then by the number of contracts. Traders track this as the daily cost of carrying a long option position, or the daily income from a short one.
Decay is not spread evenly. Theta is largest for at-the-money options and accelerates sharply in the final weeks before expiry, so an option with a month left loses time value far faster per day than one with a year left.
Deep in-the-money and deep out-of-the-money options have less time value to lose, so their theta is smaller. The nuance for anyone selling options to earn theta is that the income is real but the risk is not symmetrical.
Collecting a few dollars of decay each day works until the underlying moves sharply, at which point the loss from delta and volatility can wipe out months of accumulated theta in a single session. Theta is a reward for taking risk, not a free income stream.
In practice
Real-world examples.
Example
A corporate treasurer buying three-month currency options to protect a supplier payment sees the hedge lose value each week even though the exchange rate has barely moved. The finance team explains in the monthly report that this is theta, the cost of holding optionality, not a trading loss.
Example
An investor runs a covered call strategy, selling one-month calls against shares already owned to collect premium. Theta works in the investor's favour, delivering steady decay income in flat markets, but a takeover bid sends the share price above the strike and the shares are called away.
Example
A trading desk holds a long options book into a quiet holiday week and calculates that the position will bleed roughly $4,000 in time value over the break. The desk reduces the position before the holiday rather than paying decay on a market that is unlikely to move.
Think of it
“Theta is time decay-how much value the option loses each day just from time passing.
Formula
Calculation
Daily dollar decay = theta per share x 100 shares per contract x number of contracts
A trader buys 20 call option contracts on a listed share. Each option is priced at $4.00 per share and the model shows a theta of -0.05, meaning the option is expected to lose 5 cents of value per share per day if the share price and volatility stay flat.
One contract covers 100 shares, so it is worth $4.00 x 100 = $400, and its daily decay is $0.05 x 100 = $5. Across 20 contracts, the position is worth 20 x $400 = $8,000 and decays by 20 x $5 = $100 a day.
After 10 calendar days with no movement in the share price, the option has lost 10 x $0.05 = $0.50 per share, falling from $4.00 to $3.50. The position is now worth 20 x $3.50 x 100 = $7,000, a loss of $1,000, which matches $100 of decay a day for 10 days. To break even over that fortnight, the share price would have to rise enough to add $0.50 per share of value back into the option.Case study
Seen in the real world.
Kestrel Vale Capital is a fictional, illustrative boutique fund that ran a strategy of buying at-the-money index calls a fortnight before each quarterly earnings season, expecting volatility to lift prices. The thesis was reasonable and the fund was frequently right about direction.
The problem was the entry timing. In the final two weeks before expiry, theta on at-the-money options is at its steepest, and the fund was routinely paying around $9,000 a day in decay across the book. In one illustrative quarter the index moved 1.8% in the fund's favour, yet the position still closed down because eleven days of decay had cost more than the move was worth.
Kestrel Vale rewrote its rules to buy options with three months to expiry rather than two weeks, accepting a higher upfront premium in exchange for much slower daily decay, and to roll positions before entering the final month. In this fictional case the strategy's win rate barely changed, but its returns improved because it stopped fighting the calendar.
Watch out
Common mistakes.
- Treating theta as a fixed daily amount, when it accelerates steeply as expiry approaches and is largest for at-the-money options.
- Forgetting the contract multiplier and reading a theta of -0.05 as five cents a day, when it is $5 a day per 100-share contract.
- Selling options purely to collect theta while ignoring that a single sharp move can erase many months of accumulated decay income.
Questions
People also ask.
Is theta always negative?
It is negative for bought options, since the holder loses time value each day, and effectively positive for the seller, who benefits as that value erodes.
Does theta apply over weekends?
Yes, decay is measured in calendar days rather than trading days, so an option typically loses roughly three days of time value between Friday close and Monday open.
How does theta relate to the other option greeks?
Theta measures sensitivity to the passage of time, while delta measures sensitivity to the underlying price and vega to volatility, and a full position report shows all of them together.
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