ARM vs Fixed Rate Break-Even: The Exact Calculation Most Borrowers Skip

Why the Monthly Payment Comparison Fails You

The instinct is understandable. An ARM shows a lower rate on the term sheet, the monthly payment is smaller, and the savings seem obvious. But that framing answers the wrong question.

The right question is not "which loan has the lower payment today?" It is "at what point does the fixed rate borrower break even against the ARM borrower, accounting for the full cost of each reset?"

Those are different calculations. And the gap between them, in dollar terms, is where bad mortgage decisions live.

The break-even method forces you to define one variable most borrowers leave undefined: your actual hold period. The median tenure in a US owner-occupied home is 13.2 years, according to the National Association of Realtors. A 5/1 ARM has a fixed period of exactly 5 years. That gap matters enormously.

The Core Break-Even Formula

The break-even point is the month at which the ARM borrower's cumulative interest paid equals the fixed-rate borrower's cumulative interest paid.

Before that month, the ARM wins. After it, the fixed rate wins.

The formula has three inputs:

Monthly savings = Fixed rate payment minus ARM initial payment

Cumulative ARM savings = Monthly savings multiplied by months in the fixed window (typically 60 months for a 5/1 ARM)

Post-reset cost = The additional monthly interest burden after each reset, multiplied by the number of months you hold the loan past the initial fixed window

Break-even month = Cumulative ARM savings divided by the monthly post-reset cost increase

If your expected hold period falls before the break-even month, the ARM wins. If it falls after, the fixed rate wins.

Worked Example 1: The 5/1 ARM on a $500,000 Purchase

Assumptions:

• Loan amount: $500,000
• 5/1 ARM initial rate: 5.75%
• 30-year fixed rate: 6.75%
• Expected hold period: 8 years

Step 1: Calculate the monthly payments.

At 5.75%, the monthly principal and interest payment on $500,000 is $2,918. At 6.75%, the monthly payment is $3,243. Monthly savings with the ARM: $325.

Step 2: Calculate cumulative savings during the fixed window.

The ARM holds at 5.75% for 60 months. Total savings over those 60 months: $325 x 60 = $19,500.

Step 3: Model the reset.

After month 60, the ARM adjusts. Assume a first-year cap of 2% over the initial rate, bringing the new rate to 7.75%. The new monthly payment at that rate, on the remaining balance of approximately $463,000, is $3,509. That is $266 per month more than the fixed-rate borrower pays.

Step 4: Calculate the break-even.

$19,500 in savings divided by $266 monthly post-reset cost = 73.3 months of reset needed to burn through the savings.

Break-even point = 60 months (fixed window) + 73.3 months = 133.3 months, or roughly 11.1 years from origination.

Decision: You plan to hold for 8 years. Your hold period falls at month 96, well before the break-even at month 133. The ARM saves you money. Over your 8-year hold, the ARM borrower pays approximately $9,300 less in total interest and principal than the fixed-rate borrower.

Worked Example 2: The 7/1 ARM on a $750,000 Refinance

This scenario is more common among high-income borrowers who refinance into a jumbo product.

Assumptions:

• Loan amount: $750,000
• 7/1 ARM initial rate: 6.10%
• 30-year fixed rate: 6.90%
• Expected hold period: 10 years

Step 1: Monthly payments.

At 6.10%, monthly payment: $4,549. At 6.90%, monthly payment: $4,969. Monthly savings: $420.

Step 2: Cumulative savings over the fixed window.

84 months at $420: $35,280.

Step 3: Model the reset.

At month 85, the ARM adjusts. Assume a 2% first-cap, bringing the rate to 8.10%. Remaining balance at that point: approximately $703,000. New payment: $5,233. Additional monthly cost versus the fixed-rate borrower: $264.

Step 4: Break-even.

$35,280 / $264 = 133.6 months of reset exposure needed to exhaust the savings. Break-even = 84 + 133.6 = 217.6 months, or 18.1 years.

Decision: Your 10-year hold period ends at month 120, a full 97 months before the break-even. The ARM wins decisively. Total interest savings over the 10-year hold: approximately $31,400 compared to the fixed-rate borrower.

This is why the 7/1 ARM frequently makes sense for borrowers with a defined exit timeline, such as those approaching retirement, expecting a geographic relocation, or managing a property that will transfer to an estate.

The Variables That Change Everything

The SOFR Index and Margin

Most ARMs today adjust based on the Secured Overnight Financing Rate (SOFR) plus a lender margin, typically between 2.50% and 3.00%. Your adjusted rate at reset equals the index at that time plus the margin, subject to caps.

If SOFR sits at 4.30% at your reset date and your margin is 2.75%, your new rate is 7.05%, regardless of what prevailing market rates look like. Build this into your model. Do not assume the index stays flat.

Periodic and Lifetime Caps

A 2/1/5 cap structure means the rate can rise 2% at first adjustment, 1% per year thereafter, and 5% over the life of the loan. On a 5.75% ARM, the ceiling is 10.75%. That ceiling matters if you hold past the break-even. Model the worst-case cap scenario and confirm the payment remains serviceable.

Opportunity Cost of the Payment Difference

The monthly savings from the ARM has an opportunity cost. A borrower who invests the $325 monthly savings from Example 1 at a 7% annualized return for 60 months accumulates approximately $23,100. That additional capital changes the break-even calculation. The fixed-rate borrower now needs even longer to catch up.

Prepayment and Extra Principal

If you make extra principal payments, the loan balance at reset drops. A lower balance at reset means a smaller monthly payment even if the rate rises. This shrinks the post-reset cost burden and can extend the ARM's advantage.

When the Fixed Rate Is the Correct Answer

The ARM wins on a shorter hold period. The fixed rate wins in three specific scenarios.

First, when the rate spread is narrow. If the ARM offers only 0.375% below the fixed rate, the monthly savings are minimal. On a $500,000 loan, that spread produces only $118 per month in savings. The risk of reset exposure outweighs that margin for most hold periods.

Second, when you cannot absorb payment volatility. If a 2% rate reset would strain cash flow, the certainty of a fixed rate has value that does not appear in a pure interest calculation.

Third, when your hold period is genuinely uncertain. Divorce, job loss, health changes, and family needs disrupt planned timelines. If you cannot confidently project your hold period within a two-year window, the fixed rate removes one major variable from an already uncertain future.

Run the Numbers for Your Loan

The examples above use clean assumptions. Your loan will not. Your rate spread, your expected hold, your cap structure, your SOFR margin, and your prepayment behavior are all specific to your situation.

The CalcMoney mortgage calculator models ARM resets across multiple rate scenarios, calculates the break-even month, and shows total interest paid side by side across the full expected hold period.

Plug in your actual loan terms. The break-even number will tell you precisely which product wins, and by how much, before you sign anything.

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