Free adjustable-rate mortgage (arm) calculator
See how a 5/1 or 7/1 ARM payment shifts when the rate adjusts — initial payment, post-adjustment payment, and worst-case scenario, updated live, as you type.
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The adjusted rate shown is a scenario. Actual future rates depend on market index (SOFR) + lender margin. ARM rates can rise or fall at each adjustment.
Results are estimates. Consult a professional.
How the adjustable-rate mortgage (ARM) calculator works
An adjustable-rate mortgage has two phases: a fixed-rate initial period (e.g., 5 years on a 5/1 ARM) and an adjustment phase where the rate resets periodically based on a market index plus a margin. The calculator computes your payment for both phases and shows you worst-case payment after caps are applied.
ARM rates are tied to a benchmark index — most modern ARMs use the Secured Overnight Financing Rate (SOFR) after LIBOR was retired in 2023. Your lender adds a fixed margin (commonly 2.75%) on top of the index. The sum is your fully indexed rate, subject to cap constraints at each adjustment.
CFPB Consumer Handbook on Adjustable-Rate Mortgages (CHARM booklet) — cap structures and index definitions.Worked example: $350,000 5/1 ARM at 6% initial
Borrower takes a $350,000 5/1 ARM at 6.0% initial rate with 2/2/5 caps. After 5 years the index + margin pushes the rate to 8%. We calculate both the initial payment and the adjusted payment on the remaining balance.
5/1 ARM vs 30-year fixed monthly payment comparison
Initial ARM rate assumes approximately 0.75% below the 30-year fixed rate (2024 Freddie Mac PMMS average spread). Adjusted payment assumes rate rises to match the fixed rate after year 5. Taxes and insurance excluded.
| Loan Amount | ARM Initial (6.25%) | Fixed 30yr (7.0%) | ARM Adjusted (8.0%) | Worst Case (11%) |
|---|---|---|---|---|
| $300,000 | $1,847 | $1,996 | $2,239 | $2,858 |
| $400,000 | $2,462 | $2,661 | $2,985 | $3,810 |
| $500,000 | $3,078 | $3,327 | $3,732 | $4,763 |
Source: CFPB ARM disclosure guide; Fannie Mae ARM product guidelines.
Tips for evaluating an adjustable-rate mortgage
ARMs can save money in the right circumstances, but they transfer interest rate risk from the lender to you. These five considerations help you decide whether the trade-off makes sense.
- Match the fixed period to your horizon — If you plan to sell or refinance within 5 years, a 5/1 ARM keeps your costs low during the period you'll actually own the home. Beyond that window, rate risk grows.
- Stress-test with the worst-case payment — Apply the lifetime cap (typically starting rate + 5%) and verify your budget can still absorb the payment. If not, a fixed rate is safer.
- Read the index and margin in your loan docs — Lenders may use different SOFR tenors. A lower margin matters more than a lower index, since the margin is fixed for the life of the loan.
- Understand adjustment frequency — A 5/1 ARM adjusts every year after year 5. A 5/6 ARM adjusts every 6 months. More frequent adjustments mean faster exposure when rates rise.
- Compare the break-even point — Calculate how many months of initial savings cover the potential cost of rate increases. If break-even is beyond your expected tenure, the ARM advantage disappears.
Accuracy and limitations
This calculator models the initial fixed period and one rate adjustment scenario. Real ARMs may have multiple adjustments over their life, each subject to their own caps. The actual adjusted rate depends on the index value at each adjustment date — which cannot be predicted. Taxes, insurance, and PMI are not included. Loan-level pricing adjustments (LLPAs) from Fannie Mae or Freddie Mac may affect the rate you're actually quoted based on credit score, LTV, and property type.
Not financial advice — consult a mortgage professional for your specific situation.
ARM mortgage terms defined
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