Stock prices rise and fall with regularity -- it's just the nature of the beast. If you've spent much time investing in the stock markets, you've probably had your share of gains and losses. Most investment statements calculate a stock's overall gain or loss, but not all break this down on a per-share basis. Calculating a per-share gain or loss begins with the cost basis of the stock, which is subtracted from the per-share proceeds of a sale.

Divide the total purchase price of the stock by the number of shares purchased. Be sure to include any brokerage fees in the purchase price. For example, if you purchased 50 shares of stock XYZ for $1,000 and paid $50 in transaction fees, add $50 to $1,000 to get $1,050, then divide the total by 50 to calculate a per-share cost basis of $21.

Repeat this calculation for the sale of your stock, subtract any brokerage fees from the total sales price. Continuing with the example, if you sold half of the shares for $700 and incurred another $50 transaction fee, subtract $50 from $700 to get $650, then divide the total by the 25 shares sold. This calculates an average sales price of $26 per share. The actual stock value would have been $28 ($700 divided by 25), but you are discounting this price by spreading the brokerage fee across each share.

Subtract the per-share cost basis from the average sale price to calculate gain or loss. A negative figure is a loss, while a positive figure is a gain. In the example, subtract the $21 cost basis from the $26 sale price to arrive at a gain of $5 per share.

Divide the $5 figure by the per-share cost basis and multiply by 100 to calculate your percent return on investment. In the example, $5 divided by $21 calculates a decimal return of 0.238. Multiply by 100 to convert this figure to the percentage format of 23.8 percent. Because this figure is positive, it describes the actual gain of 23.8 percent over your original investment. This figure differs from the stock value increase, because it also factors in your brokerage expenses.

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