The easiest way to calculate your gains or losses on the stock market is using the raw numbers. But, though this is useful for knowing how much money you've made or lost, it doesn't give the full picture. For example, if you make $50 in one day, that'd be a great return if you had only invested $1,000. However, if you'd invested $50,000, that $50 gain is rather small. Measuring your gains as a percentage allows you to compare the stock performance relative to the initial investment amount.
Add any dividends paid to the value of the stock at the end of the period to figure the actual closing value. Dividends reduce the value of the stock, but the shareholders receive the funds. For example, say a stock is worth $25 per share before it pays you a $2-per-share yearly dividend. If the price goes down to $23 as a result of the dividend you haven't actually lost any money: You have $2 in cash from the dividend and stock worth $23, keeping your total value at $25.
Subtract the initial stock price from the actual ending stock value. For example, if you wanted to figure the change in the stock value during one week, you'd subtract the price at the start of the week from the price at the end of the week. If it was worth $25 at the start of the week and $27 at the end of the week, the value increased by $2.
Divide the increase by the starting value to figure the rate of increase. In this example, divide the increase of $2 by the initial value of $25 to get 0.08.
Multiply the rate of increase by 100 to convert it to a percentage increase. Finishing the example, multiply 0.08 by 100 to get an 8-percent increase in stock value.
Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."