Adjustable Rate Mortgage Calculator
ARMs offer lower initial rates than fixed mortgages, but payments can rise when the rate adjusts. This calculator shows your starting payment, expected adjustment, and maximum possible payment.
How Adjustable Rate Mortgages Work
The ARM structure is simple: a low fixed rate for the initial period, followed by periodic adjustments tied to a market index. A 5/1 ARM has a fixed rate for five years, then adjusts annually. A 7/6 ARM is fixed for seven years, then adjusts every six months.
The adjusted rate equals the index (usually SOFR, the Secured Overnight Financing Rate) plus a margin set at loan origination. If SOFR is 4.2% and your margin is 2.5%, your new rate is 6.7%—unless caps prevent it from rising that high.
Rate caps protect you from runaway increases. A 2/2/5 cap structure means your rate can rise no more than 2% at the first adjustment, 2% at each subsequent adjustment, and 5% total over the loan's life. Even if the index skyrockets, your rate can only climb 5 percentage points from the initial rate.
When ARMs Make Financial Sense
ARMs shine for borrowers with short holding periods. If you plan to sell or refinance within five years, a 5/1 ARM's lower initial rate saves money you'll never give back when the rate adjusts. Why pay 7% on a fixed-rate loan when you can pay 5.5% on an ARM and refinance before year five?
They also appeal to buyers expecting income growth. A doctor finishing residency or an entrepreneur building a business might accept payment uncertainty now, confident future earnings will absorb any payment increase. This strategy is risky but rational for high earners with strong income trajectories.
ARMs become dangerous when borrowers barely qualify at the initial rate and can't afford the adjusted payment. Loan officers sometimes steer marginal buyers toward ARMs to make the debt-to-income ratio work, ignoring the inevitable payment shock. If you can't comfortably afford a 2% rate increase, don't gamble on an ARM.
ARM Payment Shock and How to Avoid It
Payment shock hits when your rate adjusts and your monthly bill jumps $400, $600, or more. On a $400,000 loan at 5.5%, you pay $2,271 monthly. If the rate adjusts to 7.5% (a 2% increase, well within cap limits), your payment rises to $2,797—a $526 increase overnight.
Lenders used to qualify ARM borrowers at the initial rate, which led to foreclosure waves when rates adjusted upward in the mid-2000s. Post-crisis regulations now require lenders to qualify borrowers at the fully-indexed rate (index plus margin) or the initial rate plus 2%, whichever is higher. This protects you from approving a loan you can't afford after adjustment.
Smart ARM borrowers treat the initial payment as a bonus and bank the difference between an ARM and a fixed-rate payment. If a fixed loan costs $2,500 and your ARM costs $2,271, save that $229 monthly. After five years you'll have $13,740 plus interest—a cushion to absorb payment increases or a down payment for your next home.
Frequently Asked Questions
What is an adjustable rate mortgage (ARM)?
A mortgage with an interest rate that stays fixed for an initial period (typically 3, 5, 7, or 10 years), then adjusts periodically based on a market index plus a margin.
How do ARM rate caps work?
ARMs have three caps: initial adjustment cap (often 2% or 5%), periodic adjustment cap (usually 2%), and lifetime cap (typically 5%). These limit how much your rate can increase.
What is the margin on an ARM?
A fixed percentage added to the index rate to determine your new rate at adjustment. If the index is 4% and your margin is 2.5%, your adjusted rate is 6.5% (subject to caps).
Should I choose an ARM or fixed-rate mortgage?
ARMs make sense if you plan to move or refinance before the rate adjusts, or if you expect rates to fall. Fixed rates win for long-term ownership or if you need payment certainty.
What happens at the end of the fixed period?
Your rate adjusts to the index rate plus margin, subject to caps. After that, it typically adjusts annually. Your lender notifies you 60 days before each adjustment.