Share prices for stocks fluctuate from buying and selling pressures. It's this fluctuation that allows profit or loss when investing in stocks. If a stock price increases, the value of your investment also increases. Likewise, if a stock price drops, the total value decreases. However, stock prices don't necessary fluctuate in the same direction. One stock may rise while another falls. To calculate the change in all your shares, you must calculate the current portfolio value and compare it to the initial investment.

Look up the number of shares you own of each stock, the original purchase price and the current stock prices. This information is listed in your investment statement. Most recent stock prices can be found on online investment sites, such as money.cnn.com, finance.yahoo.com or google.com/finance.

Multiply the number of shares by the current price of the stock. For example, if you have 30 shares of stock AAA priced at $100 per share, the total current value is $3,000. Repeat this calculation to derive the total current value of each stock.

Add the total current values of each stock. To extend the example, if the current value of your shares of stock BBB is $5,000 and the current value of your shares of stock CCC is $2,000, your total portfolio is worth $10,000.

Subtract the current value from your initial investment cost. If the result is negative, then you lost value. If it's positive, you gained value. If it's zero, then there is not change at all. If you can't remember your total investment cost, you can use the same procedure as before, but multiply the number of shares by the original share price. In the example, if you originally invested $8,000, you would subtract that from $10,000 to calculate a gain of $2,000.

Divide the gain by the original investment to calculate the rate of return. In the example, $2,000 divided by $8,000 gives you a 0.25 rate of return. Multiply by 100 to convert the rate to a percentage. Your portfolio has increased by 25 percent.

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