ARM Mortgage Calculator
An ARM is quoted on the rate for the first few years. What matters is the rate afterwards — so this models every adjustment under a scenario you choose, and shows the highest payment the contract would ever allow.
On this page
Reading the notation
A 5/1 ARM is fixed for five years, then adjusts once a year. A 7/1 is fixed for seven. A 10/6 is fixed for ten and adjusts every six months. The first number is years of fixed rate; the second is how often it moves afterwards.
After the fixed period the rate becomes the index plus the margin. The index is a published benchmark that moves with the market. The margin is a fixed number written into your note that never changes. Together they are the "fully indexed rate", and the caps limit how quickly the rate can travel towards it.
The caps are the real protection
Three caps do the work, usually written as something like 2/2/5:
The initial cap limits the first adjustment. The periodic cap limits every adjustment after it. The lifetime cap limits how far the rate may ever rise above where it started.
The lifetime cap is the number that matters most, because it defines the worst case. An ARM starting at 5.875% with a 5% lifetime cap can reach 10.875% and no further. The payment at that rate is the figure you should be able to afford before signing — not the initial payment, which is the one being advertised.
Why we do not forecast the index
Nobody knows where the index will be in five years. A calculator that filled in a projection would be presenting a guess in the same typeface as the contract terms, and readers reasonably assume that everything on a results panel is equally solid.
So the index and how it moves are yours to enter. The useful way to use this is to run three scenarios: the index holding steady, the index rising at a quarter point per adjustment, and the worst case the caps permit. If the loan is affordable in all three, the risk is priced. If it fails the third, the caps are the whole story and you should look at them closely.
How the payment is recalculated
At each adjustment the new rate is set, and the payment is re-amortized over the remaining term on the balance at that moment. A 5/1 ARM adjusting after five years re-amortizes 25 years of payments on whatever is left.
One consequence is worth noting: the same rate rise moves the payment less later in the loan than early on, because there is less balance and less time. Rate shock is largest at the first adjustment, which is also when the initial cap is the only thing standing between you and the fully indexed rate.
A worked example
A $420,000 5/1 ARM at 5.875% for five years, index at 4.3%, margin 2.75%, caps of 2/2/5, index rising a quarter point at each adjustment, against a 6.5% fixed quote.
The initial payment is $2,484 against $2,655 on the fixed loan — $171 a month cheaper, or about $10,300 over the fixed period. At the first adjustment the fully indexed rate would be 7.30%, but the 2% initial cap holds it to 7.875%, taking the payment to roughly $2,900. That is a $416 increase in one step.
The lifetime cap allows 10.875%, at which the payment reaches about $3,700 — half again as much as the initial payment. Whether that is acceptable depends entirely on whether you would still be in the loan, and on what your income looks like if it happens.
When an ARM makes sense
When you are confident you will be gone before the fixed period ends, and when you could absorb the worst case if you are wrong. Both conditions, not either.
The second is where plans fail. People who intended to move in five years often do not, and a loan that only works if a life plan holds is a fragile loan. Run the worst case against your actual budget rather than against the salary you hope to have.
What this does not cover
- Negative amortization. Some older ARMs allowed payments that did not cover the interest. Not modelled here, and rare in current products.
- Payment caps. Distinct from rate caps and their own source of trouble.
- Interest-only periods. Not modelled.
- Conversion options. Some ARMs allow conversion to a fixed rate for a fee.
- Escrow. Property tax and insurance move independently of the rate.
Compare against a fixed loan with the mortgage calculator, and see what a later refinance would cost with the refinance calculator.
Frequently asked questions
What does 5/1 ARM mean?
Fixed for five years, then adjusting once a year for the rest of the term. A 7/1 is fixed for seven years; a 10/6 is fixed for ten and adjusts every six months.
The adjustments continue for the whole remaining term — a 5/1 on a 30-year loan has 25 years of adjustments ahead of it.
How high can my ARM payment go?
As high as the lifetime cap permits, and no higher. That is the initial rate plus the lifetime cap — 10.875% on a loan starting at 5.875% with a 5% cap.
The worst-case payment on this page is that figure. It is the number to check against your budget before signing, rather than the initial payment.
What is the margin and does it change?
The margin is a fixed number added to the index at every adjustment, written into your note when the loan is made. It never changes.
It matters more than the initial rate over the life of the loan: the index is out of everyone's control, but a lower margin permanently lowers every future rate.
Is an ARM a bad idea?
Not inherently. It is a rate risk transferred from the lender to you in exchange for a lower initial payment, and that can be a good trade when the fixed period comfortably covers how long you will hold the loan.
It becomes a bad idea when the initial payment is what makes the house affordable. If the worst case would not be manageable, the loan is priced on an assumption rather than a plan.
Can I refinance out of an ARM before it adjusts?
Usually yes, subject to qualifying at the time and paying closing costs again. It is the common exit, and it is also the assumption that fails when rates rise or circumstances change.
The refinance calculator prices what that exit would cost.
What happens if the index falls?
The rate falls too, subject to the periodic cap limiting how fast and to any floor in your note. Many ARMs have a floor equal to the margin, which sets a lower bound.
Enter a negative index change in the scenario field to model a falling-rate path.
Related calculators
This calculator is provided for general educational and estimation purposes only and is not lending or financial advice. Future index values cannot be known and nothing here predicts them. The scenario is one you supply; the caps are the only real limit and they come from your note.