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".
Click on sell in tws on the item you want to check margin for, change the quantity, then click order/check margin from the choices at the top of the page.