Interest rate on an adjustable rate mortgage based on the total of the current value of an index and margin applicable to the mortgage. The rate is the basis for the computation of monthly loan payments. Assume a mortgage is indexed to the consumer price index, and it has a value of 5%. The margin on the loan is 1.5%.The fully indexed rate will be 6.5% which is the basis for the initial year's interest rate. The interest rate will change each year depending on the change in the related index.