Fixed-rate and adjustable-rate mortgages, and when each one makes sense

A fixed-rate mortgage keeps the same interest rate, and therefore the same principal-and-interest payment, for the life of the loan. An adjustable-rate mortgage (ARM) has a fixed initial rate for a set period (typically 5, 7, or 10 years) and then adjusts periodically against a published index for the rest of the term.1 The choice between them is mostly a question of who bears the interest-rate risk: the lender, in the case of a fixed, or the borrower, in the case of an ARM.

How a fixed-rate loan actually works

The 30-year fixed-rate mortgage is the default product in the US, and it's a slightly weird one by international standards. Most countries don't offer 30-year fixed-rate consumer mortgages, because the lender is locked into a long-dated rate exposure that's difficult to hedge. The reason it works in the US is the secondary mortgage market: Fannie Mae and Freddie Mac buy conforming fixed-rate loans from originating lenders and bundle them into mortgage-backed securities sold to investors.3 The originator collects fees for making the loan and gets out of the rate-risk business.

For the borrower, the upside is simple: the rate that's locked at closing is the rate that applies for 30 years. Property tax and insurance can rise (and almost always do), but the principal-and-interest portion of the monthly payment is fixed. Long-range financial planning is genuinely possible.

How an ARM actually works

An ARM has three distinct phases written into the loan documents.2 The initial fixed period is usually 5, 7, or 10 years (that's the first number in "5/1 ARM" or "7/6 ARM") and the rate is fixed at the initial rate during this period, typically lower than a comparable 30-year fixed. The adjustment frequency is the second number: 1 means the rate adjusts annually after the initial period, 6 means every six months. The cap structure is written as three numbers like 2/2/5, defining the maximum first adjustment, the maximum subsequent adjustment, and the maximum total increase over the life of the loan respectively.

The rate after the initial period is typically the index (often SOFR-based now, replacing LIBOR) plus a fixed margin specified at closing, subject to the caps. So a 5/1 ARM at 6.0% with a 2/2/5 cap structure could go as high as 8.0% in year 6, no higher than 10.0% in year 7, and no higher than 11.0% over the life of the loan, depending on what the index does.

The PITI calculator at /tools/mortgage-payment models the three standard ARM scenarios (rates flat, rates up by 2%, and the lifetime cap) so the post-adjustment payment range is visible up front.

When an ARM tends to make sense

The case for an ARM is strongest in two situations. The first is when the borrower expects to sell or refinance before the initial fixed period ends, a 7/1 ARM at a rate 0.5–1.0% below a 30-year fixed saves real money over the first seven years, and if the loan is gone before year eight the post-adjustment risk never matters. The second is when the yield curve makes the rate gap large enough to justify the option cost. When 30-year fixed rates and 5/1 ARM rates are within 0.25%, the ARM rarely pays for itself; when the gap is 1.0%+, the math shifts meaningfully.

The case against an ARM is mostly about the borrower's tolerance for payment uncertainty after the initial period, and the realistic likelihood of being able to refinance if rates go up. If rates rise meaningfully and the borrower's financial picture also weakens (job change, illness, value drop), refinancing out of the ARM may not be available on terms that solve the problem.

What "interest-only" actually adds

An interest-only (IO) mortgage is a separate product, sometimes layered on top of an ARM and sometimes a standalone fixed product. During the IO period, the borrower pays only interest each month, and the loan balance doesn't change. When the IO period ends, the loan amortizes over the remaining term, which produces a step up in monthly payment that is often substantial, 30 to 50 percent in a typical 10-year IO followed by 20-year amortizing.

IO loans are not the historical 2007-vintage product. The current CFPB Qualified Mortgage rules require the lender to qualify the borrower against the post-IO amortizing payment, not the IO-period payment. This is a meaningful guardrail. IO loans are now mostly used by borrowers with variable income (commission-based, deal-based) who want flexibility on the monthly during low-income periods, rather than by borrowers stretching to qualify.

A useful frame, in one sentence

A 30-year fixed is paying for certainty. An ARM is selling some of that certainty back in exchange for a lower initial rate. Whether the trade is good depends on how long the loan is going to be on the books, what the rate gap is at the moment of decision, and how the borrower's financial picture holds up if rates move against them. The PITI calculator and the rate-sensitivity sliders are useful for putting concrete numbers on each of those.