Fixed-Rate vs Adjustable-Rate Mortgages

A fixed-rate mortgage keeps the same interest rate for the life of the loan. An adjustable-rate mortgage, or ARM, keeps a stated rate for an initial period, then can move on a schedule tied to an index plus a margin, within caps. Both are common on U.S. home purchases and refinances. Both can be fully amortizing. They are not the same product with a different advertisement.

The useful comparison is not which loan is better in general. It is which payment path you can live with if rates rise, how long you expect to keep the mortgage, and whether a lower initial payment is worth that uncertainty. This guide explains how each structure works, walks through a labeled example, and lists what this comparison cannot decide for you.

How a fixed-rate mortgage behaves

On a 30-year or 15-year fixed loan, the note rate does not change. Principal and interest stay the same. Taxes and insurance can still change, so the full escrowed payment is not frozen. You usually pay for that certainty with a higher initial rate than a comparable ARM. Two fixed quotes are mostly a comparison of rate, points, and fees. Prepaying principal shortens the schedule without changing the required payment. Refinancing later is optional if rates fall. In a rising-rate world, your rate does not climb with the index. That protection does not show up on a year-one payment table.

How an adjustable-rate mortgage is built

An ARM has several moving pieces that a single teaser rate does not show.

The initial period is the time the start rate is locked. Five, seven, and ten years are common on modern hybrid ARMs. During that period, the loan behaves much like a fixed-rate loan. After that, the rate resets on a stated cycle.

The index is a published market rate named in the note, such as a Treasury-based index or SOFR, depending on the product and origination year. You do not control the index. The margin is a fixed number of percentage points the lender adds to the index. The fully indexed rate is index plus margin, before caps.

If the index is 4.00% and the margin is 2.75%, the fully indexed rate is 6.75% before applying caps. If the start rate was 5.50%, the first adjustment could move toward 6.75% unless a cap stops it short. If the index later falls, the rate can fall too, subject to a floor that is often the margin.

Caps, floors, and adjustment frequency

Caps limit how much the rate can change. A common educational pattern is an initial adjustment cap, a periodic cap, and a lifetime cap, written as something like 5/1/5 or 2/1/5 depending on the product. Those numbers are examples of a format, not a promise of your loan’s caps. The first number often limits the first adjustment. The second limits later adjustments. The third limits the total increase from the start rate over the life of the loan.

Adjustment frequency after the initial period may be every six months or every year. A 5/6 ARM and a 5/1 ARM with the same start rate are not interchangeable. More frequent adjustments mean the payment can change more often. The Loan Estimate must show an example of how the payment could change. Read that example. It is still an illustration, not a forecast of the index.

A worked example comparing a 5/6 ARM and a 30-year fixed

The figures below are an illustrative example with labeled assumptions. They are not a forecast of SOFR, Treasuries, or future mortgage spreads.

Assume a $340,000 loan amount, 30-year amortization, and no extra fees in the payment comparison. Taxes and insurance are omitted so the example isolates principal and interest.

Quote A: 30-year fixed at 6.50%. Principal and interest are about $2,149.

Quote B: 5/6 ARM at 5.75% start rate, 2.75% margin, lifetime cap 5 percentage points above the start rate. Principal and interest during the first five years are about $1,985. The monthly difference is about $164 while the start rate lasts.

If the household keeps the ARM through year five and then the fully indexed rate is 7.75% at the first reset, subject to the initial cap, the payment can rise even if they never missed a payment and never spent more on the house. Using the same $340,000 remaining-balance idea for teaching (the actual remaining principal would be lower after five years of amortization), a rate near 7.75% on a remaining term of 25 years produces a higher payment than $1,985. The exact new payment depends on remaining principal, remaining term, and the capped rate. The point of the example is the direction: the ARM’s advantage can shrink or reverse after the fixed period.

If instead they sell or refinance in year four, they may never see an adjustment. Savings of $164 a month for 48 months is about $7,872 before principal differences, closing-cost differences, and refinance risk. If they stay 12 years and the index stays high, the ARM can cost more than the fixed loan. If the index stays low, the ARM can cost less. Neither path is promised. Use a mortgage calculator for the fixed payment. For the ARM, model at least three cases: the start rate continues, a mid-range fully indexed rate, and the lifetime cap. Those are scenarios, not probabilities.

When an ARM is sometimes considered

People with a documented reason to sell or pay off the loan during the initial period sometimes accept ARM reset risk for a lower start rate. Plans change. A delayed listing or a weak sale can leave you holding the ARM into the adjustment years. Qualifying at the start rate without stress-testing the capped payment is how people end up house-poor in year six. Ask the lender what rate they used for qualification and what the payment would be at the cap. Temporary buydowns are not ARMs, but they also make the first years cheaper than later years. Read which structure you are signing.

Risks that calculators can understate

Payment shock is the main ARM risk. Taxes and insurance rising at the same time as a rate reset can stack. Confirm the ARM is fully amortizing. Refinancing is not a hedge you can count on. If credit falls, value dips, or income becomes irregular, the door can close.

• Compare start rate, margin, index name, and all three caps, not just the advertised start rate.
• Compare APR, points, and origination fees on the same loan amount and term.
• Ask whether the payment after the initial period is still fully amortizing over the remaining term.
• Stress-test the lifetime cap payment against today’s budget, not against an expected raise.

An ARM’s lower start rate is compensation for taking interest-rate risk the fixed-rate borrower is paying the lender to hold. If you cannot explain the cap structure in one paragraph, you are not ready to use the teaser payment as your housing budget.

What this comparison does not decide

This guide does not predict where mortgage rates or the ARM index will be in five years. It does not tell you whether to buy now or wait, and it does not choose 15 years versus 30 years. Affordability still depends on taxes, insurance, maintenance, and cash reserves. An ARM that looks comfortable in year one can consume an emergency fund in year six if the payment jumps.

A mortgage calculator can show a payment at whatever rate you type. It cannot apply your ARM’s periodic cap or know remaining principal at the first reset unless you amortize first. Ask the lender to print the maximum payment under the lifetime cap. If you already have an ARM and an adjustment is approaching, gather remaining principal, the current index, the margin, and the applicable cap, then decide whether to keep the loan, pay extra principal, or shop a refinance.