a simple google could have solved your question ... https://www.google.be/webhp?sourceid=chrome-instant&rlz=1C1RNKB_enBE484BE513&ion=1&espv=2&ie=UTF-8#q=google+beta+stock+calculation
http://quant.stackexchange.com/questions/15797/how-does-yahoo-finance-calculate-beta