I now and then execute trades based on the market cap of a company. But for at least 1 company (IBKR) I know the market cap should be 10 times the number which is displayed on several major sites: http://finance.yahoo.com/q?s=ibkr According to Yahoo the MC of IBKR is $1b while it should be around $10b (I think the $1b is only the free float). This mistake is already there since IBKR began trading (more than a year ago). With other shares you see often that the MC stays the same while they have issued new shares (for instance AMD). Does someone know why this number is so often incorrect and where I can find good data?