How do adjustable rate mortgages work? Unlike fixed-rate mortgages, adjustable rate mortgages (ARMs) are based on shorter term securities that fluctuate upward or downward based on today’s leading indexes (e.g., Constant Maturity Treasury (CMT), London Interbank Offering Rate (LIBOR) or Treasury Bill. A margin is added on top of the index rate by the lender to calculate the interest rate.
Because ARMs are adjustable, they go up and down at pre-set intervals during the duration of the loan. Some offer a low teaser rate to qualify potential buyers which accelerates to a higher rate thereafter. ARMs can adjust once a year, every month, or three to five years, and are typically amortized over a 30 year period. Some offer a lifetime cap which sets the maximum rate that can be charged during the life of the loan with some states having their own percentage limits.
