There are many different formulas used to calculate the margin requirement on options. Which formula is used will depend on the option type or strategy determined by the system. There are a significant number of detailed formulas that are applied to various strategies. To find this information go to the IBKR home page at www.interactivebrokers.com. Go to the Trading menu and click on Margin. From the Margin Requirements page, click on the Options tab. There is a table on this page which will list all possible strategies, and the various formulas used to calculate margin on each.