Fixed-Rate vs. Adjustable-Rate Mortgages: How Each One Behaves Over Time

How Each Mortgage Is Structured

A fixed-rate mortgage does exactly what its name implies: the interest rate is set at closing and never changes for the life of the loan — whether that's 15 or 30 years. Your principal and interest payment stays the same every month, regardless of what happens in the broader economy or financial markets.

An adjustable-rate mortgage (ARM) is more complex. It begins with a fixed introductory period — commonly three, five, seven, or ten years — during which the rate is locked. After that period ends, the rate resets periodically (often annually) based on a benchmark market index, such as the Secured Overnight Financing Rate (SOFR), plus a lender margin. The result is a monthly payment that can go up or down. To understand what drives those benchmark rates, see our article on how mortgage interest rates are actually set.

ARMs are typically named by their structure. A 5/1 ARM has a five-year fixed period and then adjusts once per year. A 7/6 ARM has a seven-year fixed period and adjusts every six months thereafter.

Rate Caps: The Safety Rails on an ARM

One of the most misunderstood features of an ARM is its cap structure. Caps are contractual limits on how much your interest rate can move. There are typically three types:

• Initial cap: The maximum the rate can increase at the first adjustment — often 2 percentage points above the introductory rate.
• Periodic cap: The maximum increase allowed at each subsequent adjustment, commonly 1–2 percentage points.
• Lifetime cap: The maximum total increase over the full life of the loan, often 5–6 percentage points above the initial rate.

These caps prevent worst-case scenarios, but a loan starting at 5% with a 5-point lifetime cap could still reach 10% — a significant payment increase. Before signing an ARM, ask your lender to show you payment scenarios at various rate levels so you understand your exposure.

Cost Comparison Over Time

In the early years, ARMs almost always carry a lower interest rate than comparable fixed-rate loans. Lenders offer this discount — sometimes called the teaser rate — because the borrower accepts the risk that the rate may rise later. If you sell or refinance before the adjustment period begins, you may never experience a rate increase at all.

For borrowers who stay in the home long-term, however, the math can shift. If market rates rise and your ARM adjusts upward, you could eventually pay more in interest than you would have with a fixed loan from the start. The break-even point depends on how much your rate rises, how quickly, and how much lower your introductory rate was.

It's also worth considering loan type alongside rate structure. Our comparison of conventional, FHA, VA, and USDA loans explains how program eligibility and costs interact with your rate choice.

If your situation changes after closing — rates drop, your credit improves, or your needs shift — refinancing from an ARM to a fixed loan is a common strategy. The full picture of refinancing is worth reviewing before you rely on that as a backup plan, since it carries its own costs.

This article provides general educational information about mortgage structures and is not personalized financial or lending advice. Consult a licensed mortgage professional or financial adviser to evaluate options based on your specific circumstances.