A 30-year fixed mortgage at 7% on a $400,000 loan runs about $2,661 a month in principal and interest. The same loan on a 5/1 ARM starting at 6% comes in near $2,398. That $263 gap is the whole debate in one line of arithmetic, and it’s also why the answer depends less on which loan is better and more on how long you plan to keep it.
Both products are ordinary tools. One trades a higher starting payment for a promise that never changes. The other trades that promise for a discount with an expiry date. Which trade makes sense comes down to numbers you can work out before you sign anything.
How Each Loan Is Built
Fixed-rate mortgages
Your rate is set at closing and stays there for the life of the loan. On a 30-year fixed, the payment you make in month one is the same payment you make in month 359. Property taxes and insurance can drift, and those usually sit in escrow, but the principal-and-interest piece is locked.
The trade-off is price. You pay for that certainty with a higher rate than an ARM’s introductory offer, and lenders know buyers will do it. Most people default to fixed simply because it’s easy to understand.
Adjustable-rate mortgages
An ARM starts with a fixed period, then adjusts on a schedule. The shorthand tells you both: a 5/1 ARM is fixed for five years and adjusts once a year after that. A 7/6 ARM is fixed for seven years, then adjusts every six months. Once the fixed window closes, the new rate is an index (commonly SOFR) plus a margin the lender sets, usually 2.25% to 2.75%. That margin never changes. The index does.
A $400,000 Example, Side by Side
Say you’re putting 20% down on a $500,000 house, so you’re financing $400,000. A lender quotes 7.0% on a 30-year fixed and 6.0% on a 5/1 ARM.
- Fixed at 7.0%: about $2,661 per month in principal and interest
- 5/1 ARM at 6.0%: about $2,398 per month
- Five-year savings on the ARM: roughly $15,800 before any difference in closing costs
Fifteen thousand dollars is real money. It’s also roughly the amount you’re risking if rates climb sharply in year six and you’re still living there.
The Intro Rate Is a Marketing Number
Lenders advertise the start rate because it’s the smallest number on the page. What actually determines your cost over time is the fully indexed rate, which is the index plus the margin, along with the caps. Ask for both in writing before you compare offers.
If SOFR sits at 4.3% and the margin is 2.75%, your fully indexed rate is 7.05% no matter what you paid in year one. That isn’t a worst case. That’s the middle of the road.
Caps Decide How Bad the Worst Case Gets
Every ARM has three caps, usually written as three numbers, like 2/2/6:
- Initial adjustment cap: the most your rate can move at the first adjustment (2%)
- Periodic cap: the most it can move at any later adjustment (2%)
- Lifetime cap: the ceiling above your start rate, ever (6%)
Run that against our example. The 6.0% start rate could reach 8.0% in year six, 10% in year seven, and stop at 12% no matter what the index does. On the remaining balance, a 12% rate means something closer to $3,900 a month. Your payment would rise by roughly $1,500 with no change to your income or your house.
Some ARMs also carry interest-only periods or payment caps that create negative amortization, where your balance grows even though you pay on time. Those structures are far rarer than they were before 2008, but they still show up in some portfolio and investor loans. Read the note, not the marketing sheet.
Where the Break-Even Point Lands
ARMs often carry lower closing costs, and sometimes the savings are large enough that the loan pays for itself in a few months. Suppose the ARM saves you $263 a month and costs $1,200 less at closing. You’re ahead by month five.
The comparison flips when you’re paying points to buy down a fixed rate. Spending $4,000 to drop from 7.0% to 6.75% saves about $67 a month, so you need roughly 60 months just to get your money back. Sell in year three and that $4,000 is gone.
Run the same math on any ARM you’re considering. Take the monthly difference, divide the extra closing costs by it, and you have your break-even in months. Compare that figure to how long you expect to own the home.
When Fixed Wins
Fixed-rate loans are the safer default more often than the ARM pitch implies. They make sense when:
- You plan to stay past the ARM’s fixed period, or you genuinely aren’t sure how long you’ll stay
- Your income is stable but your budget has no room for a jump of $500 or more
- Rates are near multi-year lows and the ARM discount is thin, since a 0.25% gap rarely justifies the risk
- You would lose sleep over a payment you can’t predict
- You’re close to retirement and want a payment that stays put
When an ARM Is the Smarter Loan
ARMs get a bad reputation they don’t always deserve. They work well when:
- You’re confident you’ll sell or refinance inside the fixed window, whether that’s a relocation, a starter home, or a medical residency
- The discount is large, typically 0.75% or more, and the caps are tight
- You expect a significant income jump: a bonus, a partnership buy-in, a business sale
- You’re buying in a competitive market where the lower payment is what gets your offer accepted
- You plan to pay the loan down aggressively, so the balance won’t matter much by adjustment time
Qualifying Rules You Should Know Before You Shop
Here’s a wrinkle most buyers only discover during underwriting. Lenders don’t always qualify you at the teaser rate. For ARMs where the intro rate lasts less than five years, they typically test you at the greater of the fully indexed rate or the intro rate plus two percentage points. A 6.0% teaser can mean being qualified as if you’re paying 8.0%.
The practical result is that an ARM may not stretch your budget as far as the advertised payment suggests, and in some cases a fixed loan qualifies you for a similar amount. Ask your loan officer which rate they’ll use for qualification before you fall in love with a house.
Five Questions for Your Loan Officer
- What’s the fully indexed rate today, and which index and margin does this loan use?
- What are all three caps, and what would my payment be at the lifetime ceiling?
- What rate will you use to qualify me?
- Is there a prepayment penalty, and how long does it last?
- What would my payment be in year six if rates move up by one point?
The Real Question Is How Long You’ll Stay
Strip away the jargon and the decision usually comes down to one honest number: how many years you’ll hold this loan. Under four or five years, an ARM with tight caps and a decent discount is often the better financial choice, and it isn’t close. Past seven years, the fixed-rate premium starts looking cheap, because you’re buying insurance against a scenario that would genuinely hurt.
Between those two windows, it’s a judgment call about your own life. Count the moves, the kids, the job, the retirement date. If the honest answer is that you don’t know, that’s information too, and it points toward the loan that can’t surprise you.