This ARM Mortgage Calculator estimates the monthly payment on an adjustable-rate mortgage through its fixed period, first adjustment, and an eventual peak at the lifetime rate cap.
Estimate Payment Changes on a 5/1, 7/1, or Other Adjustable-Rate Mortgage
This calculator projects the monthly payment on an adjustable-rate mortgage through its fixed period, its first rate adjustment, and a worst-case scenario at the lifetime rate cap. It’s built for homebuyers comparing an ARM against a fixed-rate loan, or anyone who already has an ARM and wants to see what happens once the introductory rate expires.
What to Enter and What the Results Show
Enter home price, down payment percent, and loan term in years. Pick an ARM type such as 5/1 or 7/6, then set the initial rate, expected rate increase per adjustment, and lifetime cap. Results show your initial fixed payment, the payment after the first adjustment, the worst-case peak payment, and total lifetime interest.
How the Adjustable Rate and Payment Recast Are Calculated
The initial payment uses the standard mortgage amortization formula:
$$M = L \times \frac{i(1+i)^n}{(1+i)^n – 1}$$
L is the loan amount (home price minus down payment), i is the monthly interest rate (the annual rate divided by 12), and n is the total number of monthly payments (loan term in years times 12).
When the fixed period ends, the payment gets recalculated using the current balance, the new rate, and the remaining term. The CFPB’s Consumer Handbook on Adjustable-Rate Mortgages calls this a “recast.” This calculator repeats that recast at every adjustment: add the expected increase, cap it at your lifetime maximum, then re-amortize what’s left.
The most common input mistake is treating Expected Adjustment Rate as a one-time total instead of a per-adjustment amount. Enter 0.5%, and a 5/1 ARM adds that 0.5% at year 6, then again at year 7, year 8, and onward, not once across the whole loan.
Down payment has to stay under 100%, since 100% down leaves nothing to amortize. Loan term and home price must be greater than zero. An initial rate of 0% is valid math, an interest-free loan repaid in equal installments, though not a real mortgage product.
The lifetime cap should sit at or above your initial rate: set it lower, and the first adjustment will pull your rate down to the cap instead of up, which isn’t how a real cap works. And because this model only adds the expected increase and never subtracts, it estimates a rising-rate scenario. It can’t show what happens if your index falls.
These figures are a projection based on the rate assumptions you enter, not a lender’s quote or a prediction of where rates will actually go, and this calculator isn’t tax, legal, or financial advice.
Why Real ARMs Often Cap the First Adjustment Differently Than Later Ones
Most ARMs don’t use a single adjustment limit. Lenders typically publish a three-number cap structure, such as 2/2/5: the first number caps the initial adjustment, the second caps every adjustment after that, and the third caps the total increase over the life of the loan.
This is industry convention, documented by the CFPB and used in Fannie Mae’s standard ARM offerings, not a fixed statute, and the exact numbers vary by lender and loan type.
This calculator simplifies that into one flat per-adjustment increase and a single lifetime cap. If your real loan’s initial cap is larger than its periodic cap, common on 7/1 and 10/1 ARMs, your actual first jump could be bigger than this tool estimates.
Payment at Each Stage of the Loan
The jump from the first bar to the second is the initial shock when the fixed period ends. The jump to the third bar only happens if rates keep climbing every adjustment until they hit the cap, the scenario this tool treats as worst-case, not most-likely.
Common Questions About This ARM Mortgage Calculator
What does “5/1 ARM” mean?
The first number is how many years the rate stays fixed. The second is how often it adjusts afterward: in years for 5/1, in months for 5/6. A 5/1 ARM holds its rate for 5 years, then adjusts once a year.
Does the Expected Adjustment Rate track a real index?
No. It’s a flat estimate you supply, not tied to SOFR, a Treasury index, or any real benchmark. Actual ARMs move with an index plus a margin, which can rise, fall, or hold steady.
Can this calculator model a rate that goes down?
No. It only adds your expected increase at each adjustment, capped at the lifetime maximum. It estimates a rising-rate scenario and won’t show what happens if the index falls.
Why might my real first adjustment differ from this estimate?
Many ARMs cap the first adjustment differently than later ones, such as a 2/2/5 structure. This tool applies one flat increase throughout, so a larger initial cap on your actual loan could mean a bigger first jump.
Can the down payment be 100%?
No. At 100% there’s no loan left to amortize, so the calculator blocks that input. Down payment has to stay below 100%.