Theta decay calculator: what time costs an option each day
Every day an option loses a little value, even if the stock doesn't move. How much is today worth? And why does the last week cost so much more? Type in your option and watch the clock run down to expiry, day by day.
| Days left | Value a share | Your position | The next day costs | Time value left |
|---|
How it works: the page prices your option with the Black-Scholes model for every day left until expiry. The stock price, the implied volatility and the rate stay exactly as you typed them; only the days change. One day's decay is the value with today's days left minus the value with one day fewer. Time value is the price minus what the option would be worth if it expired right now (its in-the-money amount). Time value always reaches $0 at expiry. One contract is 100 shares.
Why the last weeks cost the most
An option with 60 days left still has lots of time for the stock to move, so each day is a small slice of its value. With 5 days left, each day is a big slice. That is why the table's "next day costs" column grows as expiry gets close, and why the final week holds a big share of an at-the-money option's time value. Sellers like that speed-up. Buyers pay for it. Out-of-the-money options are different. Most of their value is gone before the last week, so their decay peaks earlier. Type a strike further from the price and watch the fastest day move. The speed-up has a flip side too: near expiry, a small stock move swings the price far more than a day of decay does. See theta decay explained.
What the table leaves out
Real stocks don't sit still. This table holds the stock price and IV fixed so you can see time on its own. In a real trade, a $2 drop in the stock can undo a week of decay on a put you sold, and a jump in IV can make the option cost more even as days pass. In the wheel strategy, decay is the part you collect. The stock move is the part you carry. Pick the strike for the stock move first, and let decay be the bonus.
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