A fixed rate mortgage has the same payment for the entire term of the loan. An adjustable rate mortgage (ARM) has a rate that can change, causing your monthly payment to increase or decrease. LIBOR, which stands for the London InterBank Offered Rate, is an index set by a group of London based banks, and sometimes used as a base for U.S. adjustable rate mortages. This calculator compares a fixed rate mortgage to a LIBOR ARM.