It is important to know how to calculate stock basis for exercised options in order to be able to determine the amount of profit or loss generated on a given trade. Basis is the technical term for an investor's ultimate cost in a stock. For example, when an investor buys a share of stock for $10, his basis in that stock is $10. If he then sells someone an option to buy the stock from him for $12.50 and he collects an option premium of $1 for the option, that $1 he collected lowers his overall cost in the stock to $9 per share.
Receive notification from your brokerage that an option has been exercised. This will most likely come in the form of a trade confirmation the day after option expiration. If an option is “In-The-Money” by even one cent, the option will be exercised.
Determine your initial cost in the stock. This will be the share price you paid to buy the stock for the first time.
Adjust your cost basis by calculating the total option premiums you have collected against the stock. Keep in mind that options which expired previously without being exercised also reduce your basis in the stock.
For example, let's say you bought 100 shares of XYZ in January for $10 per share. You then sold the February $12.50 call option and collected a $1 premium, lowering your basis in the stock to $9 per share. On option expiration day in February, the stock is $11 per share, so the option expires worthless. You decide to sell a March $12.50 call, and this time you collect a $2 premium, lowering your overall basis to $7 per share.
Calculate your profit or loss. If an call option is exercised at a strike price higher than your basis in the stock, you have made a profit. To calculate the profit, you must subtract your basis in the stock from the strike price of the option.
To use our earlier example, on option expiration day in March the stock is $13 per share. The option you sold is exercised at its strike price ($12.50). Your basis in the stock is $7. Therefore, $12.50-$7=$5.50 profit per share on the trade.
If an option is exercised at a strike price below your cost basis, you have a loss. To calculate the loss, subtract the strike price from your basis in the stock.
Calculate your tax basis. For tax purposes, your basis in a stock also includes all the commissions and fees you incurred during the trade. The easy way to calculate that is to add up all the commissions and fees and divide the total by the number of shares you own.