Transparent method
The formula
Worked example
Example inputs
How to interpret the result
An ARM's adjustment cap limits how much the rate can move at any single reset, but that cap says nothing about the lifetime cap governing the total possible increase over the loan's full life. Modeling only the first reset shows part of the risk while leaving the loan's eventual worst-case rate, still meaningfully higher, out of the picture entirely.
Before you rely on it
What to check
Check the loan's lifetime cap in addition to the first-reset adjustment cap, since the first reset alone doesn't show the loan's full eventual worst-case rate.
The common error
Where people go wrong with ARM adjustment cap calculator
Treating the first reset's capped increase as the loan's full worst-case scenario. Later resets can continue raising the rate up to the separate lifetime cap, well beyond what the first adjustment alone shows.
Sensitivity evidence
How loan amount changes the estimated reset payment
| Loan amount | Estimated reset payment | Payment increase |
|---|---|---|
| $280,000 | $1,845 | $299 |
| $315,000 | $2,076 | $336 |
| $350,000worked example | $2,306 | $374 |
| $385,000 | $2,537 | $411 |
| $420,000 | $2,768 | $449 |
Every input, tested
Which input moves the estimated reset payment most
| Input | Tested from | To | Estimated reset payment at each end | Swing |
|---|---|---|---|---|
| Introductory rate | 4.3% | 6.3% | $2,072 to $2,552 | $480 (21%) |
| Loan amount | $315,000 | $385,000 | $2,076 to $2,537 | $461 (20%) |
| First adjustment cap | 1.0% | 3.0% | $2,116 to $2,505 | $389 (17%) |
| Total term | 22.0 years | 38.0 years | $2,542 to $2,200 | $341 (15%) |
| Initial fixed period | 5.0 years | 9.0 years | $2,331 to $2,280 | $51 (2.2%) |
| Index at reset | 5.5% | 7.5% | $2,306 to $2,306 | none |
| Loan margin | 1.5% | 3.5% | $2,306 to $2,306 | none |
Two variables at once
Estimated reset payment by loan amount and introductory rate
| Loan amount \ Introductory rate | 3.3% | 5.3% | 7.3% |
|---|---|---|---|
| $280,000 | $1,479 | $1,845 | $2,201 |
| $315,000 | $1,663 | $2,076 | $2,477 |
| $350,000 | $1,848 | $2,306 | $2,752 |
| $385,000 | $2,033 | $2,537 | $3,027 |
| $420,000 | $2,218 | $2,768 | $3,302 |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Loan amount | $350,000 |
| Introductory rate | 5.3% |
| Initial fixed period | 7.0 years |
| Index at reset | 6.5% |
| Loan margin | 2.5% |
| First adjustment cap | 2.0% |
| Total term | 30.0 years |
| Estimated reset payment | $2,306 |
| Payment increase | $374 |
Where the inputs come from
Introductory rate, fixed period, margin and term come from the ARM disclosure. Index at reset is your assumption for the benchmark at the first adjustment. The first adjustment cap limits the increase at the first reset, and separate periodic and lifetime caps limit later ones, usually written as three numbers like 2/2/5. Enter the first number here.
When the result stops applying
The estimate models the first reset only, on the assumption of a single index value. Subsequent resets can raise the rate further up to the lifetime cap, and the payment is recalculated over the remaining term. Some ARMs allow interest-only periods or negative amortization, which change the picture, so check the loan type.
What to compare it with
Read the note for three items: the index, the margin and the caps. Then test the payment at the first reset, at the second reset and at the lifetime cap. If the worst case is unaffordable, choose a longer fixed period or a fixed loan. Compare with the fixed-versus-ARM calculator to see how much the introductory discount is worth. The disclosure that comes with an ARM, the CHARM booklet, explains the terms in plain language.
People also ask
Frequently asked questions
How do the cap numbers 2/2/5 work?
The first number limits the first adjustment, the second limits each later adjustment, and the third limits the total rise over the life of the loan.
Does the cap protect me from the index?
Only in part. If the index plus margin exceeds the cap, the cap holds the rate down for that reset, but the rate can catch up at the next one.
What if rates fall?
If the index falls, the rate can drop to the new index value plus the margin, subject to any floor written in the note. Some ARMs set the floor at the starting rate, which limits how much you benefit from a decline.