Sorry for bringing back an old thread, but isn't interest only charged if you have a negative cash balance? Cash balance is the amount of excess cash you hold in your account. It is one component of your Net Liq which is the total account value. Margin is a risk metric. At IB, it will only directly cost you if IB decides to assess a "Net Exposure Fee" which seems arbitrary and they basically are saying that you are taking on too much risk. Is that correct?