My ARM is adjusting. How do I work out the new payment myself?

Take the index value from your notice, add your margin, and you have the fully indexed rate. Then compare it against your current rate plus the step cap, and against your original rate plus the lifetime cap. Whichever of those three is lowest is your new rate. The payment is then recalculated on your current balance over the months remaining — not the original loan amount and not the original term, which is why the jump is often larger than the rate change alone suggests.

Why did my rate rise when the index barely moved?

Almost certainly because a previous step cap is unwinding. If the index rose faster than your cap allowed at an earlier reset, the shortfall does not disappear — it sits there waiting, and arrives at subsequent adjustments until your rate catches up with the fully indexed rate. On the worked example the rate climbs from 6.75% to 7.05% with the index completely static. Check whether your current rate is still below index plus margin; if it is, more rises are already locked in.

What is the most my payment can change at one adjustment?

The rate can move by your step cap, which is commonly one or two points, but the payment change is larger than that implies. Because the payment is recalculated over a shorter remaining term each time, the same rate rise produces a bigger payment jump later in the loan than earlier. A one-point rise with twenty-five years left cost $254.64 on the worked example; the same rise with ten years left on the same balance would cost noticeably more per month, though for fewer years.

When is my servicer required to tell me?

Well in advance, and the rules distinguish between the first adjustment and the rest. For the initial rate change on a loan coming off its fixed period, the notice must arrive several months ahead — long enough to give you time to refinance or make arrangements. For each subsequent adjustment that alters the payment, the window is shorter but still gives you a couple of months. If you receive a payment change with no prior notice at all, that is worth querying rather than accepting.

My notice does not match what I calculate. What should I do?

Ask the servicer to show the calculation in writing, quoting the index value and the date it was taken from. Discrepancies are usually one of a handful of things: the wrong index date, a margin that does not match the note, a cap applied from the current rate when it should run from the original, or a rounding convention you were not expecting. Servicing transfers are a common trigger, because the loan terms have to be re-keyed by someone who was not there at origination. Raise it before the new payment takes effect rather than after.

Should I refinance now or ride out the adjustments?

It turns on where your rate is heading rather than where it is today. If the cap is still unwinding, your rate is going to the fully indexed level whatever happens next, so compare a fixed offer against that figure and not against your current one. If you have already arrived at the fully indexed rate, you are genuinely floating and the decision is a view on where the index goes. Remember that refinancing costs several thousand and locks you out of any fall, while staying costs nothing and captures one automatically.

What if I cannot afford the new payment?

Contact the servicer before missing anything, because the options available to a current borrower are considerably wider than those offered to one already behind. Depending on the loan there may be a modification, a repayment arrangement, or a conversion option written into your original note that lets you switch to a fixed rate without a full refinance — worth checking, since many borrowers do not know they have one. A HUD-approved housing counsellor can advise free of charge and has no interest in which route you take.

Does the lifetime cap count from my start rate or my current one?

From the rate you started on, which is why it is worth knowing that figure years later. A five-point lifetime cap on a loan that began at 5.75% means the rate can never exceed 10.75%, however long you hold it and however many adjustments occur. It does not reset when your rate moves, and it does not stack — if you have already risen four points you have one point of headroom left, not five. Some notes express the ceiling as an absolute rate instead of a rise, so read the wording rather than assuming.

My rate went down. Will it keep falling?

Only until it reaches the fully indexed rate, and it will get there in steps rather than at once. The same cap that slowed your rises slows your falls, so if the index has dropped a long way you may need several adjustments to feel the benefit. Below that, the note usually sets a floor equal to the margin — though in practice that is nearly unreachable, since the fully indexed rate is the index plus that same margin and would only fall below it if the index turned negative. What genuinely limits a falling rate is the step cap, not the floor.

Does paying extra reduce my adjusted payment?

Yes, and more effectively than on a fixed loan. Extra principal reduces the balance, and because the payment is recalculated on that balance at every reset, the reduction is captured at the next adjustment rather than only shortening the term. On a fixed mortgage overpaying shortens the loan but leaves the monthly figure alone; on an ARM it lowers the payment itself from the next reset onward. If you are facing a rise and have cash available, paying down before the adjustment date is unusually well rewarded.

Adjustable Rate Mortgage Calculator

Your adjustment notice arrives, and it tells you the new rate without showing how it got there. This page reconstructs the arithmetic so you can check it. Three separate limits compete at every reset, and your new rate is the lowest of them. The fully indexed rate is today index plus your margin. T...

This page is for people who already hold an adjustable rate mortgage and want to know what the next reset does. If you are still deciding whether to take one, the ARM versus fixed calculator is the one you want.

YOUR LOAN NOW

BALANCE ($)

RATE NOW (%)

YEARS LEFT

FROM YOUR ADJUSTMENT NOTICE

INDEX (%)

MARGIN (%)

RESETS IN (MO)

EVERY (MO)

STEP CAP (%)

LIFE CAP (%)

START RATE (%)

The lifetime cap counts from the rate you started on, not from where you are today — which is why the start rate matters even years later. Your ceiling is 10.750%.

YOUR PAYMENT AFTER THE NEXT RESET

$2,845.33

Up $253.41 from $2,591.92. The rate moves from 5.750% to 6.750%, set by the step cap — the lowest of the three limits that apply. Recomputed when the reset lands in 2 months, on the $410,762 you will owe by then over 298 months.

NEW RATE

6.750%

SET BY

the cap

CHANGE

+$253.41

The cap has not protected you — it has spread the rise out. Today the index justifies 7.050%, but the step cap only allows 6.750% this time. The rest arrives at the next reset. Even if the index never moves again, your rate climbs to 7.050% and your payment to $2,922.42 after 2 adjustments. That rise is already locked in by where the index sits today.

Reset	In	Rate	Payment	Set by
#1	2 months	6.750%	$2,845.33	the step cap
#2	8 months	7.050%	$2,922.42	the index
#3	1y 2m	7.050%	$2,922.42	the index
#4	1y 8m	7.050%	$2,922.42	the index
#5	2y 2m	7.050%	$2,922.42	the index
#6	2y 8m	7.050%	$2,922.42	the index

Check this against the notice your servicer sends. They are required to tell you well in advance of an adjustment that changes the payment, and the notice states the index value used, the margin, the new rate and the new payment. Errors happen — a wrong index date, a margin that does not match the note, a cap applied from the wrong starting point. If the figure here differs materially from theirs, ask them to show the calculation before the payment changes.

THE RESET

Knowing which limit binds tells you what happens next. If the index is what set your rate, you have arrived and it moves only when the index does. If the cap set it, the rise is not finished — the remainder is already justified by today index and simply arrives at the following reset.

Live calculation · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Take the index value and margin straight from your adjustment notice rather than from a rate website. The notice states which index date was used, and servicers use a specific published value rather than whatever is current today.
2
Enter the rate you originally started on, not just your rate now. The lifetime ceiling is measured from the start rate, so it matters even years into the loan.
3
Check which of the three limits the page says is binding. If it is the cap, your rate has further to climb even with the index frozen, and the table shows where it settles.
4
Compare the figure here against the one on your notice. If they differ materially, ask your servicer to show the calculation before the new payment takes effect.

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Last updated: July 31, 2026 · For borrowers already holding an ARM · Always check against the notice from your servicer.