It is bad that I had to do this but IB keeps the workings of portfolio margin 'proprietary'. I can't find anything useful in the documentation and the reps are trained not to tell. The first bad thing is that the margin can change fast. One evening I saw 20% excess liquidity. The next morning I positions liquidated. When I check my account, I still had 10% excess. So a swing in the first hours bit me off guard. After a few wounds, I started analyzing the workings of PM. This is what I figured out, mainly for option portfolios. The margin calculation of a single position is based on exposure (delta*100*sharePrice), and volatility. In my account the margin is roughly Mgn = exposure*Vol/2 In the case of a group of positions on same underlying, the exposures are calculated on both sides, positive and negative, and the higher absolute value is used. So if you do a straddle or strangle, the side with higher delta determines the margin. This is friendly for delta-neutral trades -- you get one side free. When it comes far OTM and long maturity options, high deltas (0.3) are used in calculating the exposure, resulting in unreasonably high exposure, the hence margin. The margin policy forces you to use near-the-money spreads. Shorting naked calls can cost more than shorting the stock itself if the vol is high. Do you guys have similar realizations?