As a stock market investor, you purchase shares of each company you want to invest in. No matter how many shares you purchase, you can calculate your gain or loss on an individual share basis. To do so, you need to know the price you paid for the stock as well as either the current price, if you still own the stock, or the price at which you sold the stock. Your gains and losses per share can be measured as a raw number or as a percentage of your per share purchase price.

Subtract the price per share you paid for your investment from the current price per share of the stock to find the gain or loss. A negative result means a loss, while a positive result means a gain. For example, if you bought a stock at $78 and sold it at $82, subtract $78 from $82 to find that you gained $4 per share.

Divide your gain or loss per share by the purchase price per share to find the rate of gain or loss. In this example, divide $4 by 78 to get 0.0135477399666667.

Multiply the rate of gain or loss per share by 100 to find the percentage gain or loss per share. Completing this example, multiply 0.0135477399666667 by 100 to find you gained 1.35 percent per share.

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