ARM vs Fixed Mortgage Calculator — compare a 5/1 ARM to a fixed-rate loan

The ARM vs fixed mortgage calculator above compares a 5/1 adjustable-rate mortgage against a traditional fixed-rate loan over the actual number of years you expect to keep the loan, showing you the initial payment, the payment after the rate resets, and — most importantly — the net dollar difference between the two options over your specific time horizon.

ARMs are often pitched purely on their lower introductory rate, but that rate is only part of the story. What happens after year five matters just as much, and this calculator is built to show you both halves of that story clearly. Arb Digital built this tool so you can compare real numbers instead of relying on a lender's rate-sheet pitch alone.

What This ARM vs Fixed Calculator Does

A 5/1 ARM carries a fixed introductory rate for the first five years, then adjusts annually based on market conditions for the remainder of the term. This calculator takes your loan amount, your fixed-rate quote, your ARM's initial rate, your estimate of what the ARM rate will be after it resets, your loan term, and the number of years you expect to stay in the loan before selling or refinancing. It then computes your fixed-rate payment, your ARM payment during the first five years, your ARM payment after the reset (based on the remaining balance re-amortized at the new rate), and the total amount paid under each option over your chosen horizon. The bottom-line number shows whether the ARM actually saves you money by the time you sell or refinance, or whether it ends up costing more once the adjusted rate kicks in.

Because the adjusted rate is an estimate you control, you can run the calculator multiple times with different assumptions — a mild rate increase, a steep one, or even a rate decrease — to see how sensitive your outcome is to what happens in the market after year five.

How to Use the ARM vs Fixed Calculator

1. Enter your loan amount. Use the same amount for both the fixed and ARM comparison.
2. Enter your fixed rate and loan term. This is the rate and term for a standard fixed-rate mortgage quote.
3. Enter your ARM's initial rate. This is the rate that applies for the first 5 years of a 5/1 ARM.
4. Estimate the ARM rate after reset. Ask your lender for the ARM's rate caps and index, and enter a realistic estimate — err on the higher side to see a worst-case outcome.
5. Enter your expected time horizon. How many years do you realistically expect to keep this loan before selling or refinancing?
6. Review the net difference. This tells you, in dollars, whether the ARM's lower start rate was worth it given how things played out over your horizon.

The Formula Behind the ARM vs Fixed Comparison

The fixed-rate payment uses the standard amortization formula applied across the full loan term at the fixed rate. The ARM's initial payment uses the same formula at the ARM's initial rate over the full term, which is the payment charged during years one through five. After year five, the calculator determines the remaining loan balance using the standard remaining-balance formula, then re-amortizes that balance over whatever years are left in the term, using your estimated post-reset rate, to produce the adjusted ARM payment. Total cost under each option is simply each monthly payment multiplied by the number of months it applies within your horizon, summed together. The difference between the fixed loan's total cost and the ARM's total cost over that same horizon is the net result shown at the top of the calculator. For an explanation of how adjustable-rate mortgages are structured and regulated, see the Consumer Financial Protection Bureau.

Why 5/1 ARMs Can Save Money — Under the Right Conditions

The core appeal of a 5/1 ARM is a lower introductory rate than a comparable fixed-rate loan, which translates into a lower payment and less total interest paid during the first five years. If you sell the home, refinance, or pay off the loan before or shortly after the reset, you may never actually experience the higher adjusted-rate payment, meaning you captured the ARM's discount without ever paying its downside. This is precisely why ARMs are popular with buyers who know they're likely to move within five to seven years — for a job relocation, a planned upsize, or a starter home they don't intend to keep long-term.

The math only works in your favor, though, if your actual timeline matches your plan. Life circumstances change, housing markets soften, and refinancing isn't always available on favorable terms when you need it — all of which can leave a borrower holding an ARM well past the point where its introductory discount has been fully offset by a higher adjusted rate.

A Real-World Example

Consider a $350,000 loan with a fixed rate of 6.5% versus a 5/1 ARM starting at 5.5%, with an estimated adjusted rate of 7.5% after the reset, and a 30-year term. The fixed-rate payment comes out to roughly $2,212 a month for the entire loan. The ARM's initial payment, calculated at 5.5% over the full term, comes out to roughly $1,987 a month for the first five years — a savings of about $225 a month, or $13,500 over that five-year stretch, compared to the fixed loan. After year five, the remaining balance is re-amortized at the estimated 7.5% adjusted rate over the remaining 25 years, pushing the ARM payment up to roughly $2,371 a month, which is now higher than the fixed-rate payment.

If this borrower sells or refinances after 7 years, as in the default scenario above, they'd have paid the lower ARM payment for 5 years and the higher adjusted payment for 2 years. Comparing total cost over that 7-year horizon against what they'd have paid on the fixed loan over the same period shows whether the ARM's early discount was enough to offset the higher payments after the reset. In many cases with a horizon close to the reset point, the ARM still comes out ahead — but the margin narrows quickly the longer the borrower holds the loan past year five, which is exactly the dynamic this calculator is built to reveal.

The Risk of Holding an ARM Past the Reset

Once a 5/1 ARM resets, the new rate is determined by an index plus a margin, subject to rate caps set in your loan agreement, and it can move meaningfully higher than your fixed-rate alternative would have been. If your income, budget, or the housing market doesn't allow you to sell or refinance around that time, you could be locked into a materially higher payment for the remaining years of the loan. This calculator's "years before you sell or refinance" input exists specifically so you can see the financial consequence of that risk — try entering a longer horizon than you currently plan for, to see how the numbers change if your timeline slips.

It's also worth checking your ARM's specific rate caps — the maximum the rate can increase at the first adjustment, at each subsequent adjustment, and over the life of the loan — since these caps put a ceiling on the worst-case scenario even if you end up holding the loan longer than planned.

Choosing Between an ARM and a Fixed-Rate Loan

The decision largely comes down to certainty about your timeline. If you're confident you'll move, sell, or refinance within the ARM's fixed period — or shortly after — the lower initial rate can be a genuine, low-risk savings opportunity. If your timeline is uncertain, or you plan to stay in the home for the long haul, a fixed-rate mortgage removes the guesswork entirely: your payment never changes, regardless of what happens to interest rates in the broader market.

• Run the calculator with a conservative (higher) estimate for the post-reset rate to see your worst-case scenario, not just the best case.
• Confirm the ARM's specific index, margin, and rate caps directly from your loan estimate — these details vary by lender and loan product.
• Consider your job stability and family plans honestly when estimating your true time horizon.
• Remember that refinancing an ARM before it resets isn't guaranteed — it depends on future rates, your credit, and your home's value at that time.

Common Mistakes to Avoid

• Assuming the post-reset rate will stay low. Always test a realistic or conservative estimate, not just an optimistic one.
• Underestimating how long you'll actually keep the loan. Plans to sell or refinance within five years often slip.
• Ignoring rate caps. Know the maximum your ARM can adjust at each reset and over the life of the loan before signing.
• Comparing only the initial payment. The real comparison requires looking at total cost across your entire expected horizon, not just year one.
• Not planning a refinance backup. If your strategy depends on refinancing before the reset, have a realistic fallback if rates or your finances don't cooperate.

Related Free Tools From Arb Digital

Compare this decision against other financing paths with our Mortgage Calculator, the 15 vs 30 Year Mortgage Calculator, the Mortgage Points Calculator, the Interest-Only Mortgage Calculator, or the Mortgage Amortization Calculator for a full payoff schedule. Browse every calculator we offer on our free online tools hub.

Frequently Asked Questions

This tool provides general estimates for educational purposes only and is not financial, tax, legal, or medical advice. Figures are illustrative; consult a licensed professional for decisions.