Fixed vs Adjustable-Rate Mortgages: How ARMs Work and When They Make Sense

Reading an ARM’s name

ARMs are named by their fixed period and adjustment frequency. A 5/6 ARM has a fixed rate for 5 years, then adjusts every 6 months. A 7/1 ARM is fixed for 7 years, then adjusts every year. Most new US ARMs adjust every six months and are tied to SOFR (the Secured Overnight Financing Rate), which replaced LIBOR.

• Index: the market rate the loan follows, such as 30-day average SOFR.
• Margin: a fixed number of percentage points the lender adds to the index — for example 2.75%. After the fixed period your rate is index + margin, subject to the caps.
• Caps: limits on how far the rate can move. A common structure is 2/1/5: up to 2 points at the first adjustment, 1 point at each later adjustment, and 5 points over the life of the loan.
• Floor: many ARMs cannot fall below the start rate or the margin, so falling rates help less than rising rates hurt.

A side-by-side example

Borrow $320,000 over 30 years. A fixed loan at 6.5% costs $2,022.62 a month for the whole term. A 5/6 ARM starting at 5.75% costs $1,867.43 a month for the first five years — $155.19 less each month, or about $9,300 over 60 payments.

After five years the ARM balance is $296,839. Its payment is then recalculated over the remaining 25 years at the new rate:

• If the rate rises by the full first-adjustment cap of 2 points to 7.75%, the payment becomes $2,242.11 — $374.68 more than before and $219.49 more than the fixed loan.
• If rates keep rising and the loan eventually reaches its lifetime cap of 10.75% (5.75% + 5), the payment would be $2,855.87.
• If the index falls, the payment can fall too — but only down to the floor.

When an ARM can make sense

• You are confident you will sell or move before the fixed period ends — for example a planned relocation in four years with a 5/6 ARM.
• You expect your income to rise substantially and can absorb a worst-case payment comfortably.
• The rate gap is large enough to matter. A difference of a quarter of a point rarely justifies the risk; a point or more might.
• You have savings to cover several months of the higher payment if a reset comes at a bad time.

Risks people underestimate

• Refinancing is not guaranteed. Plans to refinance before the reset can fail if rates are higher, your income has dropped, or the home has lost value.
• Payment shock compounds with escrow increases: tax and insurance can rise in the same year the rate resets.
• Selling takes time and costs money — agent fees and closing costs can be 6–10% of the price — so “we’ll just sell” is not a free exit.

Questions to ask a lender

1. What is the index, the margin and the fully indexed rate today?
2. What are the initial, periodic and lifetime caps, and is there a floor?
3. What is the worst-case payment at the lifetime cap?
4. Is there a prepayment penalty if I refinance or sell early?
5. How does the APR compare with a fixed loan from the same lender?

Run both scenarios through the mortgage calculator using the worst-case rate, not only the starting rate. If the worst case would strain your budget, a fixed rate is the safer choice. This guide is general information, not financial advice.

Seven years later: who came out ahead?

Suppose the ARM in our example resets to 7.75% after year five and stays there, and you sell after seven years. Over those 84 months the fixed loan costs $169,900 in payments and the ARM $165,857 — about $4,000 less. The remaining balances are almost identical ($289,332 for the fixed loan and $288,431 for the ARM), so in this scenario the ARM saved around $5,000.

Stretch the same comparison to ten or fifteen years and the higher post-reset payment wipes out the early savings. If rates rise further, it does so faster. That is the core trade-off of an ARM: a known saving now in exchange for an unknown cost later. It works when you have a clear reason to expect to leave the loan before the unknown part matters.

Other loan features to understand

• Rate locks: a quoted rate is usually guaranteed for 30–60 days while the loan closes. Extending a lock can cost a fee.
• Interest-only periods: some loans let you pay only interest for the first years. Payments jump sharply when principal repayment begins, and the balance does not fall in the meantime.
• Convertible ARMs: a few ARMs allow conversion to a fixed rate during a window, for a fee. Check the formula used to set the fixed rate.
• Assumable loans: FHA, VA and USDA loans can sometimes be taken over by a buyer, which can be valuable if your rate is below current market rates.

How lenders qualify you for an ARM

Because an ARM’s payment can rise, lenders usually check that you could afford more than the starting payment. For ARMs with short fixed periods, many lenders qualify borrowers at a higher rate than the introductory one — often the greater of the fully indexed rate or the start rate plus two points. If you only just qualify for an ARM at its teaser payment, that is a signal the loan is too large for your budget once it resets.

Refinancing as an exit

Many ARM borrowers plan to refinance into a fixed rate before the first reset. That can work, but it costs money — refinancing typically carries closing costs of 2–5% of the loan — and it depends on qualifying again at a time you cannot choose. Treat refinancing as a possible option, not as the plan your budget relies on.