When a trader has no usable/free margin, he or she is subject to a margin call. A larger amount of funding is needed for the account. It usually occurs when trading losses reduce the usable margin below an acceptable level determined by the broker.

In order to avoid margin calls, traders should avoid committing a large portion of equity to margin, which leaves very little room for losses to be absorbed. Managing and reducing broker risks effectively is a necessary mechanism from the broker's perspective.