DEFINITION of 'Mortgage Index'

The benchmark interest rate an adjustable-rate mortgage's fully indexed interest rate is based on. An adjustable-rate mortgage's interest rate, known as the fully indexed interest rate, is comprised of an index value plus a margin. The margin tends to be constant, but the index's value is variable. Several benchmark interest rates serve as mortgage indexes.

BREAKING DOWN 'Mortgage Index'

Some common mortgage indexes include: the prime lending rate, the one-year constant maturity treasury (CMT) value, the one-month, six-month and 12-month LIBORs, as well as the MTA index, which is a 12-month moving average of the one-year CMT index.

The index that an adjustable-rate mortgage is tied to is an important factor in the choice of a mortgage. For example, if a borrower believes that interest rates are going to rise in the future, the MTA index would be a more economical choice than the one-month LIBOR index because the moving average calculation of the MTA index creates a lag effect.

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