ARM vs Fixed-Rate Mortgage: Which One Is Right for You?

Quick comparison

The three questions that decide it

Before comparing rates, answer these. They eliminate most loan-shopping ambiguity.

1. How long will you actually keep the loan? ARM savings accrue only during the fixed period. If you sell or refinance at year 4, you captured the full ARM benefit. If you hold 30 years, the math usually reverses because the fully-indexed rate exceeds the fixed rate you passed up.
2. Can you afford the worst-case payment? Teaser rates are not your real payment. With 2/1/5 caps on a $400,000 loan starting at 5.50%, the payment rises from $2,271.16 to $2,733.10 at the first adjustment and can reach $3,491.98 at the 10.50% lifetime ceiling, a jump of $1,220.82 or 53.8%. Budget on the post-adjustment number, not the origination number.
3. Do you have a credible exit? ARMs work when paired with a sale or refinance plan you actually control. If your exit depends on a rate forecast, you are speculating, not borrowing.

How an ARM works: the parts that change the comparison

An ARM and a fixed loan differ in one dimension: what happens to your rate after a set number of years. The fixed loan answers "nothing." The ARM answers with a formula, and that formula has three parts.

The index is a published market rate your loan tracks. Most conforming ARMs originated since 2024 use 30-day average SOFR, published by the New York Fed, while portfolio lenders still use the 1-year Treasury near 4.20%. The margin is the lender's fixed markup, 2.25% to 3.00% on conforming loans, and it never changes. The caps are three separate limits: how far the rate may move at the first adjustment, how far at each later adjustment, and how far above your starting rate it may ever go, with a floor running the other way.

At every adjustment the servicer adds index to margin, rounds to the nearest 0.125%, then applies the floor, the initial or periodic cap, and the lifetime cap in that order. On the loan used throughout this page, $400,000 at 5.50% with a 2.75% margin, the arithmetic lands between a 2.75% floor and a 10.50% ceiling, and today's index of 4.30% puts the fully indexed rate at 7.00% before any cap is even tested.

That last sentence is the whole comparison. An ARM starts cheaper than the fixed rate you did not take, and its own formula points at a rate higher than that fixed rate the day the fixed period ends. Whether the arithmetic favors you depends on when you leave, which is what the tables below measure.

How ARM payments are calculated

Two calculations run in sequence at every adjustment: the new rate, then the new payment.

Step 1, the new rate: index + margin, rounded, capped. SOFR 4.30% plus a 2.75% margin equals 7.05%, which rounds to 7.00% at the nearest 0.125%. On a loan that started at 5.50% with 2/1/5 caps, the first adjustment may not exceed 7.50% and the lifetime ceiling is 10.50%, so the computed 7.00% stands as written.

Step 2, the new payment: recast the balance over the remaining term. Sixty payments of $2,271.16 leave $369,842.41 outstanding with 300 payments to go. Recast that balance at 7.00% and the payment is $2,613.97, up $342.81 or 15.1%. At the 7.50% first-reset ceiling it is $2,733.10, up $461.94. At the 10.50% lifetime ceiling it is $3,491.98, up $1,220.82 or 53.8%.

A cap only matters when the index runs past it: at SOFR 4.85% the sum is 7.60%, which rounds to 7.625% and breaches the 7.50% ceiling, so you receive 7.50%. And because the balance is smaller than it was at closing, each of these payments rises by less percentage-wise than the rate does. The full sequence, including rounding and lookback windows, is worked out in how ARM payments are calculated.

Break-even table: $400k loan, 5/1 ARM at 5.50% vs 30-year fixed at 6.50%

Scenario A assumes rates rise after year 5, so the 2/1/5 caps walk the rate from 7.50% at the first adjustment to 8.50%, 9.50%, and finally the 10.50% lifetime ceiling in year 9. Scenario B assumes rates fall, so the ARM resets to 5.75% at year 6 and holds there. Net Savings is fixed-rate total payments minus ARM total payments at each horizon, so a positive number means the ARM costs less. Both columns count principal and interest only.

Read this carefully. At year 5 the ARM is ahead by $15,427 and neither path has adjusted yet, which is exactly why the decision gets made at closing instead of at year 8. In Scenario A the lead shrinks to $7,646 by year 7, turns negative between year 7 and year 10 (year 8 is a $564 loss), and compounds into a $22,785 deficit by year 10 and $244,996 by year 30. Scenario B never crosses over: a lower index is worth more the longer you hold it, ending $75,898 ahead. The fixed rate wins every path in which rates do not fall and stay down.

Worked example: monthly P+I comparison

$400,000 loan, 30-year amortization, two structures compared on the same calendar.

Payments at each stage

The fixed payment of $2,528.27 holds for all 360 payments: total interest of $510,177.95 and $910,177.95 paid overall. The ARM pays $136,269.36 through year 5 against the fixed's $151,696.33, so the fixed-period saving is $15,427 no matter what the index does later.

After that the paths diverge. With a rising index, the 2/1/5 caps move the rate to 7.50% in year 6 ($2,733.10), then 8.50% in year 7 ($2,971.84) and 9.50% in year 8 ($3,212.47), and the 10.50% lifetime ceiling binds in year 9 at $3,454.15. Cumulative payments through year 7 are $204,728.64 against the fixed's $212,374.86, so the ARM is still $7,646 ahead. Through year 10 they are $326,177.82 against $303,392.65, and the ARM is $22,785 behind.

With a falling index the same loan resets to 5.75% in year 6 ($2,326.70), paying $192,110.22 through year 7 ($20,265 ahead) and $275,871.50 through year 10 ($27,521 ahead). If the index simply stays at today's 4.30% for the life of the loan, the ARM resets to 7.00% and holds there: $920,460 paid over 30 years against the fixed's $910,178, a difference of $10,282 in the fixed rate's favor. A 7.00% fully indexed rate above a 6.50% fixed rate is the same relationship this loan has with its own start rate, only running the other way.

Rate environment matters more than the loan type

In a falling-rate environment, ARMs capture the decline on every adjustment — you keep the lower rate. In a rising-rate environment, ARMs adjust upward and the fixed rate you locked becomes retroactively cheaper. In a flat-rate environment, ARMs cost slightly more than the equivalent fixed because of the embedded optionality.

Current rates are elevated relative to the 2020-2021 lows. That tilts the calculation toward fixed for borrowers with longer horizons, but does not eliminate the ARM case for short-horizon buyers.

ARM eligibility and qualification requirements

Overlays vary by lender and program, but ARMs carry a few checks a plain 30-year fixed does not. The table shows the typical pattern at today's example pricing.

Two rows do most of the work. The qualifying-rate row means you buy the house on $2,661.21 rather than $2,271.16, and the reserves row means the emergency fund is sized to the payment you might face, not the one you start with. Credit matters twice on an ARM: it sets the pricing you get at closing and, through margin, the rate you reset to later.

Tips for choosing an ARM in 2026

• Price both structures on the same day. A fixed quote captured Monday and an ARM quote captured Thursday are not comparable. Our mortgage rates page keeps both sides of the spread in one place.
• Ask for the fully indexed rate. The same 5.50% start resets to 6.55% with a 2.25% margin and 7.30% with a 3.00% margin at SOFR 4.30%, which is $2,508.77 versus $2,685.17 a month on the year-5 balance. That $176.40 difference lasts as long as the loan does.
• Match the fixed period to a dated plan. Under five years of expected holding, the ARM usually wins; over seven, the fixed usually wins; five to seven is the gray zone where structure details decide it. Pick the product for the timeline you can document, not the one you forecast.
• Prefer 2/1/5 over 5/2/5 when both price the same. Identical lifetime ceiling, half the exposure at the first reset: $2,733.10 instead of $3,491.98 if rates spike in year 6.
• Budget from the ceiling, not the start. $3,491.98 on this loan, 53.8% above $2,271.16. If that number breaks the household budget, the savings table is not worth the exposure.
• Count the cost of changing your mind. A refinance runs 2% to 5% of the balance, or $7,397 to $18,492 on the $369,842 balance at year 5, and it needs credit, equity, and income that still qualify when you need it.
• Run both paths before committing. The Fixed vs ARM Calculator holds the loan constant and varies only the structure, which is the only way to see the crossover year for your own numbers.

Common ARM mistakes

• Qualifying on the teaser payment. Your lender tests $2,661.21 at the fully indexed rate. Your budget should work at $2,733.10 to $3,491.98, not at $2,271.16.
• Treating caps as a safety net. Caps limit the rate, not the payment. 10.50% on this balance is $3,491.98 a month, 53.8% above where you started.
• Ignoring the margin. Two loans both starting at 5.50% reset to 6.55% and 7.30% when the margins are 2.25% and 3.00%, which is $176.40 a month apart on the same balance after every adjustment.
• Using a rate forecast as an exit plan. The break-even table does not need a forecast to be correct; it only needs your sale or refinance date. If the plan requires calling the top of the rate cycle, you are speculating with your housing payment.
• Choosing 5/2/5 because the start rate looks identical. The first-reset ceiling is 10.50% instead of 7.50%, so the same index move costs $758.88 more in year 6.
• Forgetting the floor. With a 2.75% floor the rate stops falling even if the index collapses, so the downside protection you imagined is smaller than the upside protection the lender bought.

How to use the calculators

The ARM Calculator shows how your monthly P+I changes after each adjustment, given your loan amount, initial rate, margin, and caps. The Fixed vs ARM Calculator runs both structures side-by-side for the same loan amount and lets you set the assumed rate path. Use both before committing to a structure.