ARM Calculator

An adjustable rate mortgage is two loans in a trench coat: a fixed-rate loan for the opening years, then a floating-rate loan whose payment resets with the market. Enter the loan amount, the starting rate, how long it stays fixed, and the adjustment caps, and this ARM calculator prices all three acts: the starting payment, the balance when the fixed period ends, and the worst-case payment if rates climb to the loan’s ceiling.

How an ARM Is Built: Index, Margin and Caps

After the introductory period, an ARM's rate is not chosen by your lender's mood; it is a formula printed in your note: rate = index + margin. The index is a public benchmark (today usually SOFR, the Secured Overnight Financing Rate) that floats with the market; the margin is a fixed spread, commonly 2 to 3 points, set at origination and never changed. Around that formula sit the caps, the three numbers that define your risk and appear in disclosures as a string like 2/2/5: the initial adjustment cap (how far the rate may move at the first reset — 2 points in the common structure this calculator defaults to), the periodic cap (each later adjustment, often 1-2 points), and the lifetime cap (the absolute ceiling, often 5 points above the start — 10.5 percent on a 5.5 percent start). The product names encode the fixed period: a 5/6 ARM is fixed for five years, then adjusts every six months; a 7/1 ARM is fixed for seven, then yearly. Everything the loan can ever do to you lives inside those caps, which is why this page prices the ceiling as carefully as the start.

Worked Example: $400,000, 5.5% Start, 5-Year Fix

The defaults describe a typical 5/6 ARM: $400,000 over 30 years, starting at 5.5 percent — about a point under the comparable fixed rate (a 6.5 percent fixed loan on the same amount costs $2,528.27 a month). The ARM's starting payment is $2,271.16, saving $257 a month, $15,427 across the five fixed years. Then the honest chapters. The balance at the first adjustment is $369,842. If rates have risen and the first adjustment takes the full 2-point cap to 7.5 percent, the payment on the remaining 25 years becomes $2,733.10 — $462 above the starting payment and $205 above what the fixed loan would still be charging. If rates keep climbing to the 10.5 percent lifetime cap, the payment reaches $3,491.98: $1,221 a month above the start. Neither scenario is a prediction; both are contractually permitted outcomes, printed here because an ARM decision made without them is not a decision, it is a hope. The middle path deserves equal clarity: if rates stand still, the rate resets to index-plus-margin near today's level and the ARM keeps most of its early advantage — the product rewards the borrower whose horizon ends (sale, refinance, payoff) near the fixed period's end, and taxes the one who stays by accident.

ARM vs Fixed: Who Should Take Which

The comparison resolves into horizon and shock-absorption. The ARM is the rational instrument when the stay is shorter than the fixed period with margin — a known relocation, a starter home with a dated exit, a borrower who will refinance into a fixed loan from a stronger position — because then only Act One (the cheap years) is ever performed. It is also defensible for high-income households for whom even the capped payment is a small budget share: the worst case must be survivable, and for them it trivially is. The fixed loan wins when the home is long-term and the budget is tight at today's payment: its premium over the ARM start (here $257 a month) is the price of deleting interest-rate risk from a family's three decades, and most families should pay it gladly. The hybrid discipline the calculator enables: price both, require the ARM's capped payment to fit the budget before considering it, and bank the early monthly saving rather than spending it — the household that saves the $257 difference owns its own shock absorber when the reset arrives. The US Consumer Financial Protection Bureau's mortgage guides explain the disclosures (the Loan Estimate's ARM sections) that every offer must show you.

The Reset Date Is a Financial Event: Plan for It

Borrowers treat the adjustment date as weather; it is better treated as a scheduled negotiation with three prepared moves. Refinance before the reset if fixed rates then are at or below your adjusted rate — the refinance calculator on this hub prices the switch, and starting the process six months early costs nothing. Recast or prepay during the fixed years if you will keep the loan: extra principal paid while the rate is low shrinks the balance that the higher rate will later rent — $369,842 is the number every prepayment attacks. Stress-test annually: re-run this page each year with the current index value; if the projected reset payment has crossed your comfort line, the time to act is while the fixed rate still has years to run, not in the reset month's mail. And read the note's fine print once, at origination, for the two clauses that ambush people: any prepayment penalty (rare in modern conforming ARMs, worth confirming absent) and the floor rate (many ARMs never adjust below the margin, which caps your benefit in a falling-rate world just as the ceiling caps your pain in a rising one).

Related Tools

More from the Finance Calculators hub: the Mortgage Refinance Calculator — the exit most ARM borrowers eventually price — the Mortgage Amortization Calculator, and the Home Loan Calculator for the fixed-rate comparison.

Frequently Asked Questions

What does 5/6 ARM mean?

The first number is the fixed period in years; the second is how often the rate adjusts afterwards, in months. A 5/6 ARM holds its starting rate for five years, then resets every six months for the remaining twenty-five. A 7/1 ARM fixes seven years, then resets annually (the "1" predates the monthly convention and means one year). The second number matters more than it looks: six-monthly adjustments reach the caps twice as fast as annual ones in a rising market — the same caps, a steeper staircase — which is why two ARMs with identical starts and caps are not identical risks.

Can my payment go down at an adjustment?

Yes — if the index has fallen, the rate resets downward (subject to the floor, usually the margin itself) and the payment is recalculated on the remaining balance and term. ARMs are symmetric instruments priced as if asymmetric, because borrowers shop the start and fear the ceiling; in the falling-rate decades, ARM holders repeatedly reset downward while fixed borrowers had to refinance (and pay closing costs) to capture the same fall. The calculator prices the ceiling because that is the risk; the floor's benefit is the product's quiet second half.

What is a payment-option ARM, and should I avoid one?

A relic worth recognising: it let borrowers choose a monthly payment below even the interest due, with the shortfall added to the balance — negative amortisation by menu. These loans were a signature accelerant of the 2008 crisis and have effectively vanished from the mainstream market under modern ability-to-repay rules. If any modern offer lets the balance grow by design, walk away: this page's ARM arithmetic assumes every payment at least covers interest and amortises the balance over the remaining term, which is the only adjustable structure a household should sign.

Do ARMs have prepayment penalties?

Modern conforming ARMs generally do not, and US qualified-mortgage rules sharply restrict them; some non-QM and investor loans still carry them in the early years. The clause matters precisely because the standard ARM strategy is an early exit — a penalty taxes the exit the product design assumes. Confirm the absence in the Loan Estimate before signing, and treat any ARM with a prepayment penalty beyond the fixed period as a different, worse product than the one priced here.

What happens if I sell or refinance before the first adjustment?

Then the ARM performed exactly as its best case promises: you borrowed at the discounted starting rate for the whole of your tenure and the floating years belonged to someone else. This is the strategy the product is priced for — the lender accepts a below-fixed rate for the fixed years partly because many borrowers stay into the resets — and it is why the honest ARM analysis always starts from your expected tenure rather than from the rate. If there is a realistic chance the tenure extends past the fixed period, the capped payment must fit the budget; if the tenure is contractually short (a dated relocation, a planned sale), the ARM's saving over the fixed loan — $15,427 across five years in the worked example — is close to free money, and the calculator's first line is the only one you need.

Is the starting rate on an ARM a "teaser"?

Often the starting rate sits below the fully indexed rate (index plus margin) that would apply if the loan adjusted today — that gap is the teaser element, and the disclosure rules force it into the open: the Loan Estimate shows the fully indexed rate and the maximum payment in the first years. Compare offers by margin and caps, not by start: two ARMs starting at 5.5 percent with margins of 2 and 3 points are different loans from the first reset onward, and this page's worst-case line — the number the teaser never advertises — is the comparison that cannot be gamed.