ARM planning scope This worksheet estimates a teaser-period payment, the first reset, and a capped
worst-case path using your index, margin, and rate-cap assumptions. It is not a
lender quote and does not replace the official loan disclosures for your ARM.
Common ARM presets
How to read these inputs
The fully indexed rate is the assumed index plus the contractual margin. The
first-reset rate cannot rise above the first-adjustment cap, and later resets
cannot rise faster than the periodic cap or above the lifetime cap.
Teaser-period payment
$2,042.50
Estimated monthly principal-and-interest payment during the first
5 years before the ARM resets.
Payment-shock check If the loan resets to 6.75%, the estimated
monthly payment becomes $2,243.17,
which is a payment shock of $200.66 per month.
Balance at first reset
$324,667.65
Fully indexed rate
6.75%
First adjusted payment
$2,243.17
Maximum possible rate
10.75%
Base reset total interest
$445,500.17
Worst-case capped payment
$3,089.07
Capped worst-case reset path
Assumes the loan rises by the allowed cap until the lifetime ceiling is reached
Reset
Starting balance
Rate
Payment
Interest in period
Ending balance
First reset (5/1 ARM)
$324,667.65
6.75%
$2,243.17
$21,757.35
$319,507.01
Reset 2
$319,507.01
8.75%
$2,657.68
$27,795.14
$315,410.02
Reset 3
$315,410.02
10.75%
$3,089.07
$33,746.02
$312,087.21
Reset 4
$312,087.21
10.75%
$3,089.07
$33,370.68
$308,389.06
Reset 5
$308,389.06
10.75%
$3,089.07
$32,952.95
$304,273.18
Reset 6
$304,273.18
10.75%
$3,089.07
$32,488.03
$299,692.37
Reset 7
$299,692.37
10.75%
$3,089.07
$31,970.59
$294,594.13
Reset 8
$294,594.13
10.75%
$3,089.07
$31,394.70
$288,920.00
Reset 9
$288,920.00
10.75%
$3,089.07
$30,753.76
$282,604.93
Reset 10
$282,604.93
10.75%
$3,089.07
$30,040.42
$275,576.52
Reset 11
$275,576.52
10.75%
$3,089.07
$29,246.51
$267,754.19
Reset 12
$267,754.19
10.75%
$3,089.07
$28,362.92
$259,048.28
Reset 13
$259,048.28
10.75%
$3,089.07
$27,379.51
$249,358.95
Reset 14
$249,358.95
10.75%
$3,089.07
$26,285.03
$238,575.15
Reset 15
$238,575.15
10.75%
$3,089.07
$25,066.91
$226,573.22
Reset 16
$226,573.22
10.75%
$3,089.07
$23,711.19
$213,215.58
Reset 17
$213,215.58
10.75%
$3,089.07
$22,202.34
$198,349.09
Reset 18
$198,349.09
10.75%
$3,089.07
$20,523.05
$181,803.31
Reset 19
$181,803.31
10.75%
$3,089.07
$18,654.07
$163,388.55
Reset 20
$163,388.55
10.75%
$3,089.07
$16,573.98
$142,893.70
Reset 21
$142,893.70
10.75%
$3,089.07
$14,258.92
$120,083.79
Reset 22
$120,083.79
10.75%
$3,089.07
$11,682.36
$94,697.32
Reset 23
$94,697.32
10.75%
$3,089.07
$8,814.75
$66,443.24
Reset 24
$66,443.24
10.75%
$3,089.07
$5,623.23
$34,997.64
Reset 25
$34,997.64
10.75%
$3,089.07
$2,071.20
$0.00
Base reset scenario total cost
$795,500.17
Assumes the first adjusted rate applies for the rest of the term.
Worst-case capped total cost
$1,033,943.58
Assumes every future reset rises by the allowed cap until the lifetime ceiling is reached.
Important limits
This page does not model lender-specific floors, negative-amortization ARM
structures, escrow, taxes, insurance, or refinance decisions after the
teaser period. Always compare the result with your official ARM disclosure,
especially the index, margin, and cap structure written into the loan note.
ARM mortgage calculator: teaser payment, first reset, and worst-case capped payment path
An ARM mortgage calculator is most useful when it shows more than the low teaser payment. This page estimates the initial payment, the balance at the first reset, the fully indexed follow-on rate, and a capped worst-case path so you can judge payment shock before taking an adjustable-rate mortgage seriously.
What an ARM mortgage calculator needs to show
A basic adjustable-rate mortgage starts with an introductory fixed rate for a set number of years, then resets periodically according to the loan's index, margin, and cap structure. That means the teaser payment is only one part of the decision. The more important planning question is how high the rate and payment could go after the fixed period ends.
That is why this ARM mortgage calculator reports the balance at the first reset, the fully indexed rate implied by your index-plus-margin assumption, the first adjusted payment after the teaser period, and a capped worst-case path after later resets. Searchers often compare 5/1 ARM, 7/1 ARM, and 10/1 ARM options, but the real risk depends on the caps as much as the label.
An ARM can still be reasonable in some situations, especially when the borrower expects to move, refinance, or pay down the loan before the reset risk becomes dominant. But for an honest comparison against a fixed-rate mortgage, you need to understand the maximum payment path as well as the starting payment.
How teaser rates, indexes, margins, and caps fit together
The first payment is calculated like any other amortizing mortgage payment: the loan amount, teaser rate, and full loan term are converted into a monthly principal-and-interest amount. During the fixed period, the balance still amortizes, so the first reset happens on a smaller balance than the original loan amount.
At the first adjustment, many lenders look at the current index and add the contractual margin to find the fully indexed rate. The contract then applies caps. A first-adjustment cap limits how much the rate can rise immediately after the teaser period, periodic caps limit each later increase, and the lifetime cap limits how far the rate can rise above the original teaser rate across the full loan term.
This calculator uses those caps directly. The first-reset scenario compares the fully indexed rate with both the first-adjustment cap and the lifetime cap, then uses the lowest permitted ceiling. The worst-case table then assumes future resets keep rising by the periodic cap until the lifetime ceiling is reached. That creates a planning stress test rather than a prediction.
M = P × r / (1 − (1 + r)^−n)
Standard amortization formula used for the teaser-period payment and for each reset-period payment on the remaining balance and remaining term.
Fully indexed rate = index + margin
The contractual follow-on rate basis before the ARM's cap structure limits the actual reset rate.
First reset rate = min(index + margin, teaser rate + first cap, teaser rate + lifetime cap)
Simplified capped first-reset rule used by this calculator to model the first payment shock scenario.
Worked example: a 5/1 ARM with 2/2/5 caps
Suppose a borrower takes a 350,000 mortgage over 30 years with a 5.75% teaser rate fixed for 5 years. If the assumed index at the first reset is 4.5% and the loan margin is 2.25%, the fully indexed rate would be 6.75%. Under a common 2/2/5-style cap structure, the first reset is still allowed to move to 6.75% because it stays inside both the first-adjustment and lifetime caps.
The first-reset payment can therefore be materially higher than the teaser payment even though the loan has already amortized for 5 years. That difference is the payment shock. A borrower who only qualifies comfortably at the teaser payment and ignores the reset path may be underestimating the affordability risk.
The worst-case capped path matters too. If later annual resets continue rising by the periodic cap until the lifetime ceiling is reached, the payment can climb further even when the balance is falling. The balance decline softens the increase, but it does not remove the risk of a much higher monthly obligation later in the term.
What this ARM estimate does not cover
This page does not model every ARM feature used in live loan contracts. It does not apply lender-specific floors, negative-amortization options, payment-option ARM behaviour, escrow, taxes, insurance, recast rules, or refinance decisions before the reset period ends. It is focused on standard amortizing ARMs with rate caps and a conventional reset structure.
It also treats the index path as an assumption rather than a forecast. Real indexes can fall, remain flat, or rise by less than the maximum cap. The worst-case table is therefore a stress test, not a prediction of what will happen. A borrower should compare it with the loan estimate, the promissory note, and their own refinance or move plans.
Finally, this page is not a lender quote or underwriting approval. Mortgage affordability depends on debt-to-income, reserves, credit score, taxes, insurance, and lender overlays that sit outside the amortization math shown here.
CFPB mortgage key terms — Official CFPB glossary covering ARM, 5/1 ARM, amortization, and related mortgage terms.
Frequently asked questions
What does 5/1 ARM or 7/1 ARM actually mean?
The first number is the length of the initial fixed-rate period in years. The second number is how often the rate can adjust after that fixed period ends, usually every year. So a 5/1 ARM has a fixed rate for 5 years, then can reset once per year after that.
What is the fully indexed rate on an ARM?
The fully indexed rate is the index plus the loan's contractual margin. It is the uncapped follow-on rate basis the lender would use before applying the ARM's first-adjustment, periodic, and lifetime caps. The actual reset rate may be lower than the fully indexed rate if the caps block a larger jump.
Why can the monthly payment jump even though the balance is lower?
Because the payment is recalculated on the remaining balance using a higher interest rate and a shorter remaining term. Even though the balance has fallen since origination, the new rate can still raise the monthly payment sharply if the teaser rate was materially lower than the follow-on rate.
Should I compare only the teaser payment when shopping ARM loans?
No. A teaser payment is useful for understanding the first few years, but it can understate the real risk of the loan. You should compare the teaser payment, the first-reset payment, the cap structure, and the worst-case capped path together, then judge whether you could still manage the loan if rates rise before you refinance or move.
