ARM vs. Fixed Rate Mortgage Calculator

Compare your ARM's initial rate to a fixed rate — see the break-even point, rate-adjustment risk, and which option saves more over your expected timeline.

ARM vs. Fixed Rate: What You Need to Know

  • An adjustable-rate mortgage (ARM) offers a lower fixed rate for an initial period — commonly 5, 7, or 10 years — then adjusts annually based on a benchmark index plus a market margin set by the lender.
  • Rate caps limit how much your ARM can increase: the initial cap (first adjustment), periodic cap (each subsequent year), and lifetime cap (maximum increase over the life of the loan). A 5/2/5 cap structure is common.
  • ARMs can save money if you sell or refinance before the fixed period ends. If you stay longer and rates rise, a fixed-rate mortgage provides the certainty of a payment that never changes.

Calculation details

Methodology

The calculator amortizes the fixed loan at one rate for the full term. The ARM uses its initial rate through the entered fixed period, then recalculates payment from the remaining balance and remaining term using the entered adjusted rate. It tracks annual balances, payments, cumulative interest, and the first year modeled ARM payments exceed fixed payments.

Assumptions and limitations

  • The ARM uses one entered adjusted rate for all remaining years rather than modeling an index, margin, caps, floors, or later resets.
  • Both options use the same purchase price, down payment, and fully amortizing term.
  • Taxes, insurance, PMI, points, closing costs, and refinance or sale proceeds are excluded.
  • Future ARM rates cannot be predicted; the adjusted rate is a user-selected scenario.
  • Review the official ARM disclosure for index, margin, adjustment frequency, and caps.

Worked example: 7/1 ARM versus fixed

  • $450,000 home with 20% down
  • $360,000 loan and 30-year term
  • 7.25% fixed rate
  • 6.0% ARM initial rate for seven years
  • 8.5% modeled ARM rate after the initial period

Initial P&I is about $2,456 fixed versus $2,158 ARM, a difference of about $297 per month. Over 84 initial-period payments, the modeled payment difference totals about $24,986.

The initial savings must be weighed against payment uncertainty after adjustment. Actual ARM changes depend on the note's index, margin, and caps.

Official sources

Frequently Asked Questions

What does a 5/1 ARM mean?

A 5/1 ARM has a fixed interest rate for the first 5 years, then adjusts once per year (the '1') for the remaining life of the loan. The adjustment is based on a benchmark index (typically SOFR) plus a margin. Common ARM structures are 5/1, 7/1, and 10/1 — the first number is the fixed period in years, the second is how often it adjusts after that.

How much can an ARM rate increase after the fixed period?

ARM rate increases are limited by caps. A common structure is 5/2/5: the first adjustment can't exceed 5% above the initial rate, subsequent annual adjustments can't exceed 2%, and the rate can never rise more than 5% above the start rate over the loan's life. So a 6% ARM could eventually reach as high as 11% — but never higher.

When is an ARM a better choice than a fixed-rate mortgage?

An ARM typically makes sense when: you plan to sell or refinance before the fixed period ends (capturing the lower initial rate without facing adjustments), current rates are high and likely to fall (so you'd refinance anyway), or the ARM rate is significantly lower than fixed (1%+ difference on a large loan amount). If you're uncertain about your timeline, a fixed rate provides more security.

What index do ARM rates adjust to?

Most modern ARMs adjust to the Secured Overnight Financing Rate (SOFR), which replaced LIBOR as the standard benchmark. Your rate is set as SOFR plus a lender margin (typically 2.5%–3%). When SOFR rises, your ARM rate rises at the next adjustment date; when SOFR falls, your rate can decrease as well — subject to any floor in your loan documents.

How to use this estimate

This calculator is an educational planning tool. Results are based on the figures and assumptions entered by the reader, use standard mortgage mathematics, and are not a loan offer, approval, or substitute for a lender's Loan Estimate. Taxes, insurance, fees, mortgage insurance, and available rates vary by borrower, property, lender, and location.

Review the assumptions, compare more than one scenario, and confirm the final numbers with a licensed mortgage professional before making a financial commitment.