ARM vs Fixed Rate Mortgage Comparison

An ARM is a bet, and this tool prices both sides

An adjustable-rate mortgage offers you a lower rate for an initial fixed stretch, commonly five, seven, or ten years, after which the rate floats with an index and can climb. A 7/1 ARM, for instance, is fixed for seven years and then adjusts annually. The lender hands you that lower starting rate because you, not they, are absorbing the risk that rates rise later. So the real question is never "which rate is lower today," it is "will I be gone, or refinanced, before the cheap period ends, and can I survive it if I am not." This calculator answers that by pricing the full range: the best case where the intro rate holds, and the worst case where the rate jumps to its lifetime cap.

It computes a standard 30-year payment at each rate, then sums the actual payments you would make over your stated holding period. For the ARM it charges the intro rate during the fixed years and the worst-case post-adjustment rate afterward, so the total reflects the genuine downside, not a rosy assumption that rates stay put.

A 7/1 ARM held three years too long

Consider a $400,000 loan with a 6.5% fixed option against a 7/1 ARM starting at 5.5%, and assume a worst-case post-adjustment rate of 9%, which is roughly the 5.5% start plus a typical 5-point lifetime cap. Now suppose you expect to hold ten years, three years past the ARM's fixed window. Here is how the two paths compare over that decade.

This is the whole tension in one table. If rates behave and the ARM never adjusts upward, you pocket about $30,854 over ten years. If they spike to the cap right after year seven, the worst case costs $304,613, roughly $1,220 more than just taking the fixed loan. You are risking a modest loss for a meaningful gain, but only if you stay. Exit at year seven and the ARM wins outright, because you only ever paid the cheap rate.

Read the caps before you trust the intro rate

Every ARM carries caps that bound how far the rate can move, and they are the difference between a manageable adjustment and a payment shock. They are usually written as three numbers, such as 2/2/5: the first is the most the rate can rise at the first adjustment, the second is the cap on each later adjustment, and the third is the lifetime cap above your start rate. A 5.5% start with a 5-point lifetime cap can reach 10.5% at the absolute worst, which is why the worst-case input on this tool matters so much. Set it to your actual lifetime cap, not a hopeful guess. The honest stress test is simple: compute the monthly payment at the fully capped rate and ask whether you could still afford the house. If the answer is no, the ARM is a risk you cannot actually carry, regardless of how good the intro savings look.

Are today's ARMs the same as the ones that caused 2008?

No, and this matters. The toxic loans of the mid-2000s were often option ARMs and subprime products with teaser rates, negative amortization, and no real underwriting. Since the 2010 Dodd-Frank reforms and the ability-to-repay rule, lenders must qualify you at a higher rate, negative amortization on these products is largely gone, and the structures are far more transparent. A modern conforming ARM is a legitimate tool for the right borrower, it is just still a rate bet.

What if I plan to refinance before the rate adjusts?

That is a reasonable plan, but treat it as a hope, not a guarantee, because you cannot control where rates or your finances will be in seven years. Refinancing requires qualifying again and paying closing costs, and if rates are high when your fixed period ends, the refi that was supposed to rescue you may not be cheaper than the adjustment itself. Model the worst case here as if you cannot refinance, and only choose the ARM if you can live with that outcome.