From IBs website: Short Positions: maximum (50% * marginable stock value, short sale maintenance margin) or maximum (100% * non-marginable stock value). Short sale proceeds are applied to cash and the short position is subtracted from equity. So If I have $2000 in an IB account, how many shares of DIA at 100 can I short? 13? BTW, IBs customer support on this was horrible "It is IBs policy not to calculate margin for customers".