I'd like to share my experience with portfolio margin on short options and solicit yours. What I learned recently, to my surprise, is that PM is very bad for option writing. With RegT, the rule is clear: 100% * option market value + (20% * underlying market value - out of the money amount or 10% * underlying market value, whichever is greater) or $2.50 * multiplier * number of contracts, whichever is greater. With PM, I could get no useful information from my broker (interactive brokers) - I was told that it is risk-based and impossible to calculate manually. In practice, however, I discovered that in my PM account, the margin is at least the value of the underlying stock when I short naked calls, regardless of how far the strike is OTM. For example, the stock trades at $5, a call@7 sells at $1. Selling 10 contracts raised my margin by $5000. I wonder if there is any rule-of-thumb to estimate the impact of call-writing in PM.