The prospectus mentions 5% of the stock, without voting rights, for $500 million. That works out at $10 billion market cap. http://www.sec.gov/Archives/edgar/data/1381197/000104746906014485/a2174744zs-1.htm Page 99 of the prospectus compares IB trading volumes with Schwab. IB customers trade much more frequently than Schwab customers but there are less of them and they pay less in commissions. IB 77,000 customers make 124 trades a quarter at an an average of $4.49 equals $42,870,520 Schwab 676100 customers trade twice a quater for an average of $12.89 equals $174,289,580. So IB has 24.6% of the revenue of Schwab a quarter but IB customer funds are growing at over 10% a quarter. So Schwabs market cap is $23.21 billion and IB is worth 24.6% of this or $5.7 billlion. I am assuming that Schwab sold it's market making arm to Knight capital, correct me if I am wrong. On page 2 of the prospectus though it mentions that IB is now doing 196,000 trades a quarter, divided by 77,000 accounts and multiplied by 63 trading days a quater equals 160 average trades per quarter multipled by $4.49 equals $720 per account or $55,442,520. This is 31.81% of Schwabs market cap or $7.6billion.