The Trade You Are Making
An adjustable-rate mortgage trades you a lower rate today for an unknown rate later. The lender discounts the first five, seven, or ten years. After that, your rate floats with the market.
Banks do not offer that discount out of generosity. A fixed-rate loan forces the lender to eat the risk that rates rise. An ARM hands that risk back to you, and the lower intro rate is what they pay you to take it.
Sometimes that trade is smart. Usually it is not. The difference comes down to one question: will you still have this loan when the fixed period ends?
How the Adjustment Actually Works
When the intro period expires, your rate resets on a formula: index plus margin. The Consumer Financial Protection Bureau lays it out plainly. The index is a published market rate, and most new ARMs use averages of SOFR, the Secured Overnight Financing Rate. The margin is a fixed add-on written into your contract that never changes.
Index at 4%, margin of 2.75%, new rate 6.75%. That is the whole formula.
Translation: nobody at the bank picks your new rate. The market does, plus a markup you agreed to on day one. The margin is negotiable before closing and untouchable after, so compare margins across lenders, not just intro rates.
Caps: The Fine Print That Saves You
Every ARM has three caps, and they matter more than the teaser rate.
- Initial adjustment cap. Caps the first reset. The CFPB notes this cap is commonly 2 or 5 percentage points. That gap is enormous. A 5% loan with a 2-point initial cap tops out at 7% on the first reset. With a 5-point cap, it can hit 10%.
- Subsequent adjustment cap. Caps each later reset, commonly 1 to 2 points per adjustment.
- Lifetime cap. Caps total movement, generally 5 points above your start rate on hybrid ARMs.
Before you sign, price the ceiling. Take your loan amount, apply the lifetime-capped rate, and look at the payment. Use our mortgage calculator to run it. If the worst-case payment would break your budget, the ARM is the wrong loan no matter how pretty the intro rate looks. The caps are listed on your Loan Estimate. The CFPB’s CHARM handbook shows exactly where to find them.
When an ARM Is the Right Call
One situation, stated plainly: you are confident you will sell or refinance before the first adjustment.
Buying a starter home you will outgrow in four years? A 5/6 or 7/6 ARM lets you pocket the rate discount and exit before the risk window opens. Military family that relocates on schedule? Same logic. The intro period is the loan, and the adjustable years are someone else’s problem.
Here’s the catch. “I’ll just refinance before it adjusts” is a plan that leans on future rates, future home values, and your future income all cooperating. Plenty of borrowers planned to refinance in 2022 and met 7% instead. If your exit plan is refinancing rather than selling, you are not avoiding the rate risk. You are rescheduling it.
If you plan to stay put, take the fixed rate and be done with it.
Rates Can Fall Too
Fairness requires saying it. Adjustments cut both ways. If the index drops, your rate and payment drop at the next adjustment, no refinance required. Borrowers holding ARMs through a falling-rate stretch get the benefit automatically.
Treat that as a bonus, not a strategy. Budget for the cap. Hope for the floor.
Do This Before You Sign
Get the numbers in writing and compare at least three offers. For each one, write down five things: intro rate, intro period, margin, all three caps, and the worst-case payment at the lifetime cap.
Then stack the ARM against a fixed quote from the same lender, using real dollars over your realistic holding period. Suppose the ARM saves you $200 a month for five years: that is $12,000. Now weigh it against what happens in year six if you are still holding the loan and the rate has reset to its cap. If the savings clearly beat the realistic downside, the ARM earns its place. If the comparison is close, or your timeline is fuzzy, take the fixed loan. Start with our best mortgage lenders comparison, and see the full menu of loan structures at the mortgages hub.