ARM Calculator
Find out what happens to your payment when the fixed period ends: the first adjustment, the highest the rate can ever go, and what that would cost you.
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What an adjustable-rate mortgage really promises
An ARM trades certainty for a lower starting rate. You get a discount for the fixed period, and in exchange you accept that the rate will move afterwards. Whether that trade is good depends almost entirely on two things: how long you will actually keep the loan, and how bad the worst case turns out to be.
The mechanics are worth understanding because the marketing hides them. A 5/1 ARM is fixed for five years and then adjusts annually. The new rate is the index plus the lender's margin, but it cannot move more than the cap allows. On the figures here — 300,000 dollars at 6 percent, an index of 4.5 percent and a margin of 2.75 — the fully indexed rate is 7.25 percent, and the payment rises from 1,798.65 to 2,017.81 dollars at the first adjustment. That is an increase of 219 dollars a month, arriving all at once.
The cap structure is what limits the damage. The initial cap restricts the first move, the periodic cap restricts every move after that, and the lifetime cap sets an absolute maximum — here 11 percent, which is the 6 percent start plus the 5 point lifetime cap. If every adjustment went against you, the payment would reach 2,717.83 dollars. That is 51 percent more than you started with, on a loan whose headline rate looked like a bargain.
The term never changes, and this is the part borrowers most often get wrong. A rate rise does not extend the loan; it makes the payment larger so the loan still ends on its original date. That is worth stating plainly because it means a cap that sounds small can move a payment a long way. The honest way to judge an ARM is to ask whether you could afford the ceiling payment, not the teaser.
How the ARM is calculated
The loan is simulated month by month. At each adjustment the rate moves under the cap rules, and the payment is recalculated on the remaining balance and remaining term.
| Symbol | Meaning |
|---|---|
Index |
The benchmark rate the loan follows, such as SOFR or a Treasury average |
Margin |
The lender's fixed markup, set at closing and never changing |
Initial cap |
The most the rate can rise at the first adjustment |
Periodic cap |
The most it can rise at each later adjustment |
Lifetime cap |
The most it can ever rise above the starting rate |
Two scenarios are shown. The first uses your index assumption and applies the caps. The second forces every adjustment upward as far as the caps allow, because the index is a guess and the ceiling is a contract.
Worked example: 300,000 dollars, 5/1 ARM at 6 percent
Index 4.5 percent, margin 2.75 percent, 2 point caps and a 5 point lifetime cap.
| Figure | Result |
|---|---|
| Payment for the first 5 years | $1,798.65 |
| Fully indexed rate today | 7.25% |
| Rate after the first adjustment | 7.25% |
| Payment after the first adjustment | $2,017.81 |
| Increase at that point | $219.16 |
| Highest rate possible | 11.00% |
| Payment at that rate | $2,717.83 |
| Worst-case increase | $919.18 (51%) |
| Total interest expected | $413,262.05 |
| Extra interest versus a fixed rate | $65,747.48 |
You save on the rate for five years and then pay 219 dollars more each month. If the index rises further and the caps allow the rate to reach 11 percent, the payment becomes 2,717.83 dollars — 51 percent above where it started. The question that decides whether an ARM is right for you is not whether you can afford 1,798 dollars. It is whether you could afford 2,718.
Estimates only. Your lender's figures may differ because of fees, escrow and rounding.
Deciding whether an ARM is right for you
Plan against the ceiling, not the teaser
The starting rate is temporary by design. The number that should decide the loan is the payment at the lifetime cap — here 2,717.83 dollars against a teaser of 1,798.65. If the ceiling payment would be unaffordable, the ARM is a bet on rates rather than a mortgage.
Be honest about how long you will keep it
An ARM's discount is real only while the fixed period lasts. If you will certainly sell or refinance within five years, a 5/1 ARM is often the cheapest borrowing available. If there is a real chance you stay past the fixed period, you have taken on risk for a discount you may never need.
Ask what index it tracks and how it is published
The margin is fixed forever, but the index moves. A loan tracking a volatile index will adjust more sharply than one tracking a smooth average. Ask which index, where it is published, and what its highest value has been over the last twenty years.
Compare the discount against the fixed-rate loan honestly
A 5/1 ARM at 6 percent against a 30-year fixed at 6.5 percent saves roughly half a point. Over five years on 300,000 dollars that is worth about 8,000 dollars — real money but not life-changing. Weigh that against 919 dollars a month of possible downside.
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Frequently asked questions
What does 5/1 ARM mean?
Fixed for five years, then adjusting once a year. The number before the slash is the fixed period in years and the number after it is how often the rate adjusts, in years. So a 7/1 ARM is fixed for seven years and adjusts annually after that, and a 5/6 ARM adjusts every six months.
How much can my ARM payment increase?
It depends on the cap structure. On the example here the first adjustment adds 219 dollars, and the absolute worst case is 919 dollars more than the starting payment. The lifetime cap limits the rate, but the payment rise is larger than the rate rise in percentage terms because the whole remaining balance is repaid over the remaining term.
What is the fully indexed rate?
The index plus the lender's margin. It is what your rate would be if the loan adjusted today. On the figures on this page the index is 4.5 percent and the margin 2.75 percent, so the fully indexed rate is 7.25 percent. It is the number to compare against your current rate.
Can an ARM rate go down?
Yes, and it usually can move by the same capped amount in either direction. A falling index brings the rate down at each adjustment rather than instantly. This calculator models that: if your index assumption is below the starting rate, the rate steps down under the same caps.
Is an ARM a good idea right now?
It depends on your horizon and your tolerance for the ceiling payment, not on a forecast. An ARM suits a borrower who will certainly move or refinance before the fixed period ends, or who can comfortably afford the lifetime cap. If neither is true, a fixed-rate loan buys certainty that the discount rarely compensates for.