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ARM Calculator
An adjustable rate looks cheap until it adjusts. This ARM calculator plots best, expected and worst-case payment paths for a 5/1 ARM or any fixed period straight from your rate caps — including the worst-case number most lender tools leave out.
See how this works on a $320,000 5/1 ARM — 3 real examples
Worst-case monthly payment (at the 11.00% lifetime cap)
$0
Initial payment
$0
Expected max payment
$0
Amount financed
$0
Payment shock: in the worst case your payment jumps $0 at the first adjustment (year 5) — a 0% increase over the initial payment.
Monthly payment paths
Lines show principal & interest only. Taxes, insurance and HOA add $0/mo to every payment.
Rate reset scheduleBest, expected & worst-case P&I by period
Each row is one rate period. Payments are principal & interest, re-amortized over the remaining balance and term at each reset.
| Period | Best rate | Best P&I | Exp. rate | Exp. P&I | Worst rate | Worst P&I |
|---|
How your ARM rates and payments are calculated
During the fixed period the payment is the standard fully-amortizing mortgage payment on the loan amount at the initial rate:
M = P · i / (1 − (1 + i)−n)
where P is the loan amount, i the monthly rate (annual rate ÷ 12) and n the number of months. At every reset the rate changes and the remaining balance is re-amortized over the remaining term at the new rate, so the payment jumps in step with the rate. The three paths differ only in how the rate moves:
- Expected — the rate rises by your “expected change per adjustment” at each reset, clamped by the periodic and lifetime caps.
- Worst case — the rate rises by the full periodic cap at every reset until it reaches the lifetime cap, then holds there. This is the highest payment your contract allows.
- Best case — the rate falls by the same expected step (floored at 0% and at the initial rate minus the lifetime cap).
Caps are applied as percentage points relative to your starting rate, matching the standard initial/periodic/lifetime cap structure (e.g. 2/2/5). Property tax accepts either dollars per year or a percent of the home price per year; it and insurance are then divided into equal monthly escrow amounts and, with any HOA dues, added on top of principal and interest. This tool does not model PMI or a specific index; the expected path is a planning assumption, not a rate forecast.
One 5/1 ARM, three futures
A $320,000 loan at 6% for the first five years, 2/2/5 caps. After year five the contract lets the rate move — in either direction.
The lucky break: rates fall every reset
per month at the floor$1,367.09
- Five years locked at 6% cost $1,918.56 a month before taxes and insurance.
- A quarter-point drop at every reset walks the payment down to $1,367.09 by the final year.
- Lifetime interest stops at $253,334, less than half of what the middle path collects.
This is the outcome ARM buyers are betting on; nothing in the contract promises it.
Load this example (opens in a new tab)The middle path: a quarter point a year
per month at its peak$3,150.99
- Even modest quarter-point steps eventually reach the 11% cap — at the twentieth reset, not the third.
- The payment tops out at $3,150.99, including $550 of monthly taxes and insurance.
- Interest over the full term comes to $509,331 — roughly double the falling-rate path.
A gentle slope ends up at the same ceiling as the cliff; it only takes longer to get there.
Load this example (opens in a new tab)The ceiling: caps hit at full speed
per month at the cap$3,449.02
- Two-point jumps take the rate from 6% to 11% in three resets, done by year eight.
- The very first adjustment adds $379.71 to the check — a 20% raise overnight.
- By payoff, $655,174 of interest has been charged — more than twice the $320,000 borrowed.
The contract allows this payment, so the budget has to — that is the whole point of the caps.
Load this example (opens in a new tab)Illustrations only, not financial advice. Your own rate, taxes and insurance will differ — check with a qualified financial advisor before acting on any of this.
5/1 ARM basics, in plain English
- The name is the schedule. A 5/1 ARM is fixed for five years, then re-prices once a year; 7/6 is fixed for seven and adjusts every six months.
- Caps set the ceiling. In a 2/2/5 structure the first and each later move cap at 2 points and the rate can never rise more than 5 above your start.
- Plan for the worst case. It assumes the rate climbs by the full periodic cap every reset until it hits the lifetime cap — the payment you must actually afford.
- Watch the first jump. Payment shock at the initial adjustment is what catches most borrowers off guard, so check that number before you sign.
- ARMs suit short horizons. They win if you’ll sell or refinance before the fixed period ends, or if you have strong reason to expect falling rates.
Related calculators
Frequently asked questions
How does an adjustable-rate mortgage work?
An ARM keeps one fixed interest rate for an introductory period — 5 years on a 5/1 ARM — then adjusts on a set schedule for the rest of the term. At each reset the rate becomes the current index plus a fixed margin, and the remaining balance is re-amortized at the new rate. Because the rate can move, so can your monthly payment: this calculator shows how far it can swing in each direction.
What is a 5/1 ARM, and what does the name mean?
A 5/1 ARM is an adjustable-rate mortgage named for its schedule. The first number is how many years the rate stays fixed (5), and the second is how often it adjusts after that (1 = once a year). So a 5/1 ARM is fixed for five years, then re-prices every year for the remaining 25 years of a 30-year loan. A 7/6 ARM is fixed for seven years and then adjusts every six months.
What are ARM rate caps (for example 2/2/5)?
Caps limit how much the rate can change. In a 2/2/5 structure the first adjustment can move at most 2 percentage points, each later adjustment at most 2 points, and the rate can never rise more than 5 points above your starting rate over the life of the loan. The worst-case line in the chart above rises by the periodic cap at every reset until it hits the lifetime cap — that ceiling is your true maximum payment.
What is the fully-indexed rate (index plus margin)?
When your ARM adjusts, the new rate is the current value of a published index — such as SOFR or the 1-year Treasury — plus a fixed margin set in your loan documents (often 2.5–3%). That sum is the fully-indexed rate, subject to the caps. If the index is 4.5% and your margin is 2.75%, the fully-indexed rate is 7.25%, capped by your periodic and lifetime limits.
How high can my ARM payment go?
The absolute ceiling is set by the lifetime cap. With a 6% start and a 5-point lifetime cap the rate can reach 11%, and the worst-case payment above shows the resulting monthly cost after the balance is re-amortized at that rate. The same cap math applies to a 5/1 ARM jumbo loan above the conforming limit — only the balances are larger. Many lender calculators hide this figure — modeling it is the whole point of the worst-case path here, because that is the payment you must be able to afford.
Is an ARM better than a fixed-rate mortgage?
An ARM usually starts with a lower rate, so it can win if you plan to sell or refinance before the fixed period ends, or if you expect rates to fall. A fixed-rate loan wins on certainty — the payment never changes. Compare the initial payment with the worst-case payment above: if the worst case would strain your budget and you might still be in the home after the reset, the fixed loan is often the safer choice.
Disclaimer: these calculators are educational tools, not financial advice. They model your inputs with published formulas, but they cannot know your full situation — for decisions with real stakes, talk to a qualified professional. Formulas and defaults last reviewed .