I am trading USA and European spreads with IB. I can calculate the margin requirements for USA options, it is clearly explained. But for European options IB uses the "SPAN" methodology. IB says this about it: we are using the PC SPAN model for calculating our margin, we have some algorithms written in house to add an extra risk margin for a volatile market and as well for options going closer to expiration. These algorithms are confidential and we do not give customers access to our algorithms. If you need more information about the PC SPAN margin model you can go to the CME webpage. http://www.cme-ch.com/pcspan/ No explenation there, only a download for 500 dollars for some XP program. I found a similar program here: http://www.lch.co.uk it is free but from 2007 and I could not get it to work. Anybody here on this forum that can clarify this ?