Changes in stock price can be measured not only in dollars, but also by percentage change. Measuring the percentage change helps you compare the amount of the decline relative to how much the stock was previously worth. For example, a drop of $1 per share is a lot more significant if the stock was worth $10 to start than if it were worth $100. But, you need to figure out the new stock price if you want to know how it affects your bottom line — or if you want to buy more shares because you expect the price to rebound.
Divide the percentage decrease by 100 to find the decrease as a decimal. For example, if the stock went down by 4 percent, divide 4 by 100 to get 0.04.
Multiply the decimal by the prior stock price to find the amount of the decrease. For this example, if the stock was worth $30, multiply $30 by 0.04 to find the stock decreased by $1.20.
Subtract the amount of the decrease from the prior stock price to calculate the new price. In this example, subtract the decrease of $1.20 from the original price of $30 to find the stock price is now $28.80.
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."