http://individuals.interactivebrokers.com/en/accounts/fees/interest.php?ib_entity=llc Basically it's a tier-based system, and interest is paid on the account balance that is above the $10k level. It's tied to the benchmark Fed Funds rate, at various discounts based on account size, as explained on the site. edit: the benchmark is dependant on your currency ... the Fed Funds I quoted above is for USD-based accounts.