Most brokers send investors an account statement at the end of each month. The account statement tells you how much you made or lost over the month. It also usually tells you how much you've lost or gained year to date (YTD). While you can always find this information on your account statement, it is important to be able to compute your own portfolio return so you can verify that your statement is correct.

Use the formula for percentage gains or losses: "purchase price or beginning price - current price" / "purchase price or beginning value of the asset". This formula works whether you are calculating the gain or loss for a single stock or a portfolio.

Determine the current value of your portfolio ($10,000, for example) and the purchase price (for example, at the beginning of the year it was valued at $8,000).

Calculate the difference between the purchase price and the current price: $10,000 - $8,000 = $2,000.

Divide the difference by the starting value for the gain or loss. Using the above example: $2,000 / $10,000 = 25 percent. A decrease in value results in a negative value or loss.