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ARM Calculator

Compare an adjustable rate mortgage (ARM) against a fixed-rate mortgage. See how your payments could change when the initial rate period ends and the rate adjusts. Enter the initial rate, adjustment caps, and expected rate environment to project costs.

An Adjustable Rate Mortgage (ARM) is a hybrid loan: fixed for an initial period (commonly 5, 7, or 10 years), then floating with market rates for the rest of the loan term. The trade is straightforward — lower initial rate in exchange for rate uncertainty later. In moderate-rate environments, the ARM rate often comes in 0.5–1.0% below the comparable 30-year fixed, which can save real money for borrowers who don't intend to hold the loan to maturity.

The naming convention is "X/Y" — e.g., 5/1 ARM means 5 years fixed, then adjusts annually. 7/6 ARM means 7 years fixed, then adjusts every 6 months. After the fixed period, the rate resets to the chosen index (often the Secured Overnight Financing Rate, SOFR) plus a margin (typically 2–3 percentage points), subject to caps that limit how much the rate can move at each adjustment and over the life of the loan.

This calculator compares an ARM's lifetime cost against a fixed-rate alternative under a single scenario for the post-adjustment rate. The honest answer about whether the ARM saves money depends on what rates do — which no one knows. Use the calculator to test multiple post-adjustment scenarios (lower, same, higher) and assess whether you can afford the payment under the worst-case cap. If the worst case is unaffordable, the ARM is wrong for your situation regardless of how good the math looks under benign assumptions.

Inputs

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Compare against this fixed rate

Results

Initial ARM Payment

$1,987

Adjusted ARM Payment

$2,391

Fixed-Rate Payment

$2,270

ARM vs Fixed Savings

$-19,442

Fixed saves

ARM vs Fixed Balance Over Time

Total Interest Comparison

Year-by-Year Breakdown

YearRatePaymentInterestARM BalanceFixed Balance
15.50%$1,987.26$19,132.32$345,285.19$346,269.88
25.50%$1,987.26$18,866.37$340,304.42$342,280.04
35.50%$1,987.26$18,585.42$335,042.70$338,012.40
45.50%$1,987.26$18,288.62$329,484.18$333,447.61
55.50%$1,987.26$17,975.07$323,612.11$328,564.98
67.50%$2,391.46$24,115.53$319,030.06$323,342.39
77.50%$2,391.46$23,759.81$314,092.29$317,756.16
87.50%$2,391.46$23,376.48$308,771.19$311,780.97
97.50%$2,391.46$22,963.39$303,037.00$305,389.74
107.50%$2,391.46$22,518.23$296,857.64$298,553.50
117.50%$2,391.46$22,038.51$290,198.57$291,241.26
127.50%$2,391.46$21,521.55$283,022.54$283,419.90
Last updated: Reviewed by the CalcMountain editorial team

Formula

Initial monthly payment (during fixed period): M_initial = L × [ r_initial × (1 + r_initial)^n ] / [ (1 + r_initial)^n − 1 ] Where: L = Loan amount r_initial = Monthly initial rate (initial rate ÷ 12) n = Total loan term in months (typically 360) Remaining balance after the fixed period: B = L × [(1 + r_initial)^n − (1 + r_initial)^k] / [(1 + r_initial)^n − 1] Where k = number of months in the fixed period. After adjustment, monthly payment recalculates against remaining balance, new rate, and remaining term: M_adjusted = B × [ r_new × (1 + r_new)^(n−k) ] / [ (1 + r_new)^(n−k) − 1 ] Where r_new = Monthly post-adjustment rate. ARM caps (typical "2/2/5" structure): Initial adjustment cap: 2% (first adjustment can't move rate more than 2%) Periodic adjustment cap: 2% (each subsequent adjustment limited to 2%) Lifetime cap: 5% (rate can't exceed initial rate + 5% ever) So a 5.5% initial rate with 2/2/5 caps can become at most 7.5% at first adjustment, 9.5% at second, and 10.5% lifetime maximum. Example: $350,000 loan, 5/1 ARM at 5.5%, 30-year term, expected adjustment to 7.5%. Initial monthly P&I (years 1–5): $1,988 Remaining balance after 5 years: ~$324,000 Adjusted payment (years 6–30 at 7.5% on $324K over 25 years): ~$2,394 Payment shock: +$406/mo Lifetime cost ARM: ~ $809,200 ($119,300 in years 1–5 + $689,900 over years 6–30) Lifetime cost 6.75% fixed: ~$817,360 If post-adjustment rate is exactly 7.5%, ARM saves ~$8,000. If post-adjustment rate is 9.0%, ARM costs ~$45,000 more than the fixed alternative.

How to use this calculator

  1. Enter the loan amount — the loan principal, not the home price.
  2. Enter the initial fixed-period rate. Most ARM rate sheets show this prominently — it's the "teaser" rate that runs for years 1 through the end of the fixed period.
  3. Enter the fixed period (3, 5, 7, or 10 years). Longer fixed periods cost more on the initial rate but provide more time before any adjustment risk.
  4. Enter the expected rate after the first adjustment. Look up the ARM's "fully indexed rate" if available (current index + margin) — that's the rate that would apply if it adjusted today. Stress-test with worst case (initial rate + first adjustment cap, usually 2%).
  5. Enter the total loan term — 30 years is standard for ARMs.
  6. Enter a comparable 30-year fixed rate for the comparison. Get current quotes from the same lender so the comparison is honest.
  7. Review monthly payment in both phases, lifetime cost of each option, and the payment shock at adjustment.
  8. If the adjusted payment under the worst-case rate cap would be unaffordable, the ARM is not the right choice — pick the fixed rate. Affordability under the cap is the safety floor.

Worked examples

ARM holding for the fixed period only

$400,000 loan, 7/1 ARM at 6.25%, vs 6.875% fixed. Planning to sell in year 6. 7/1 ARM payment (years 1–7): $2,463 6.875% fixed payment: $2,627 Monthly savings: $164. Cumulative savings over 6 years: $11,800. Since you sell before any rate adjustment, the rate environment at year 7 doesn't matter. The ARM is the clear winner for short-horizon holders.

ARM held through one adjustment — rates flat

$350,000 loan, 5/1 ARM at 5.5%, vs 6.5% fixed. Post-adjustment rate also 5.5% (rates flat). Years 1–5 ARM payment: $1,988 Years 6–30 ARM payment: $1,988 (unchanged, if rate truly flat) Fixed 6.5% payment: $2,212 Monthly savings: $224 every month for 30 years. Lifetime savings: ~$80,600. The "rates stay flat" scenario is the best case for ARMs. In reality, rates fluctuate — but if your post-adjustment rate is close to or below the original fixed-rate alternative, you win.

ARM caught in a rising rate environment

$400,000 loan, 5/1 ARM at 5.5%, vs 6.5% fixed. Rate adjusts to 9.5% (worst case under 2/2/5 caps). Years 1–5 ARM payment: $2,271 Years 6–30 ARM payment at 9.5%: $3,177 (payment shock +$906) Fixed 6.5% payment: $2,528 Monthly cost in adjustment period: $649 more than fixed. Cumulative cost vs fixed: +$70,000 over the loan life. This is why ARMs are dangerous for long-term holders. The savings during the fixed period evaporate quickly if rates rise sharply. Always verify you can afford the worst-case capped payment.

When to use this calculator

Use this calculator before deciding between an ARM and a fixed-rate mortgage. The decision hinges on two questions: (1) How long will you actually keep the loan? and (2) Can you afford the worst-case payment if you keep it past the fixed period?

ARMs make the most sense when: you're confident you'll sell or refinance within the fixed period (military relocation, certain career change, planned move), you're in a high-rate environment where rates are likely to fall (so adjustment is downward), the ARM rate is meaningfully (0.5%+) below the comparable fixed, or you can comfortably afford the worst-case capped payment.

Fixed rates make the most sense when: you plan to stay in the home long-term (10+ years), you value payment certainty regardless of rate movements, the gap between ARM and fixed is small (under 0.25%), or you couldn't handle the worst-case capped payment.

Pair this with the mortgage-payment calculator (for fixed-rate scenarios), the mortgage-refinance calculator (since refinancing out of an ARM before adjustment is a common exit strategy), and the home-affordability calculator (to confirm you can support both the initial and capped payments).

A practical note: the post-2008 ARMs are far more borrower-friendly than the pre-2008 versions that caused the financial crisis. Today's ARMs are fully amortizing (no negative amortization), require qualified-mortgage underwriting (qualified at the fully indexed rate, not the teaser), and have transparent caps. The category is no longer the trap it once was — but it still requires careful analysis of the worst case.

Common mistakes to avoid

  • Qualifying at the initial rate without checking the capped rate. Under qualified-mortgage rules, lenders must qualify you at the fully indexed rate — but borrowers should also confirm they can afford the maximum capped payment (rate + lifetime cap), not just today's payment.
  • Assuming rates will fall by the time the ARM adjusts. They might. They might also rise. Plan for the rate environment that would make the ARM costly, not just the favorable scenario.
  • Ignoring the difference between fully indexed rate and teaser rate. The teaser is the marketing rate; the fully indexed rate (current index + margin) is what would apply if adjusted today. Always know both numbers.
  • Holding through the fixed period when refinancing was the original plan. If you plan to refinance before the fixed period ends, build a real plan (rate target, timeline, cost). Hoping it will work out without commitment leads to expensive surprises.
  • Choosing the shortest fixed period for the lowest rate. A 3/1 ARM saves a little more on rate than a 7/1 but exposes you to adjustment risk much sooner. The 7/1 often produces better expected outcomes for borrowers who can't guarantee an exit.
  • Overlooking prepayment penalties. Some ARMs (and some fixed rates) have prepayment penalties for the first 1–3 years. Always check the prepayment terms in the loan estimate — they affect both refinance decisions and accelerated payoff strategies.

Frequently Asked Questions

Sources & further reading

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