I don't know much. But maybe it is (partly) due to its pro-rata calculation. It is explained here: http://en.wikipedia.org/w/index.php?title=Interactive_Brokers&oldid=114578306#Commission_and_fees You don't get discounts for all contracts. Each tier of contracts are charged differently. For example, a customer executes 1,500 US futures contracts in a month, the IB execution costs would be: * the first 300 contracts at $0.90 * the second 700 contracts at $0.70 * the third 500 contracts at $0.45 Fees are charged on a monthly basis, not pro-rata basis. For example, a customer opens its account at 20th. The whole sum of any monthly charges will still be applied.