Calculating Your Portfolio's Beta

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Calculating your portfolio's beta will give you a measure of its overall market risk. To do so, find the betas for all your stocks. Each beta is then multiplied by the percentage of your total portfolio that stock represents (i.e., a stock with a beta of 1.2 that comprises 10% of your portfolio would have a weighted beta of 1.2 times 10% or .12). Add all the weighted betas together to arrive at your portfolio's overall beta.
Source: http://www.investorguide.com/igu-article-823-stock-basics-measuring-a-stocks-risk.html