I want to understand when writing a naked put how much the price of the underlying would have to change in order to face a margin call. I use Interactive Brokers. Do they have any type of margin simulation where I would be able to see how much margin is required under different conditions? Assuming the best route is to calculate it manually, I'd like to post an example here and have my math double checked. From the IB website: Short Naked Put Index Options Put Price + Maximum((15% * Underlying Price - Out of the Money Amount), (10% * Strike Price)) Let's use a SPY put leap as an example: http://finance.yahoo.com/q?s=SPY121222P00020000 It's trading at $.21 with a strike of $20 and the underlying trading at $110. So would the formula be: $21 + Maximum((.15 * $110 - $90), (10% * $20)) = $21 + Maximum(-$73.5, $2) = $23 So I would need $2 to cover the margin in addition to the $21 generated by the sale of the option. Is that right? IB tells me N/A for commission and margin if I preview an order (screenshot attached). Any idea what's up with that?