Fixed vs Adjustable-Rate Mortgage: How ARMs Adjust, Caps, and When Each Fits
Published 2026-11-08 · eLoanTree Blog
A mortgage is, at its simplest, a loan secured by a home. The biggest choice inside that loan — after how much you borrow and for how long — is how the interest rate behaves over time. With a fixed-rate mortgage, the rate is set when you take out the loan and does not change.
With an adjustable-rate mortgage, or ARM, the rate stays fixed for an initial period and then can move up or down. Neither choice is right for everyone. The right one depends on how long you expect to keep the loan, how much payment uncertainty you can handle, and what happens if rates move against you.
Key Takeaways
- A fixed-rate mortgage locks the rate at closing, so the principal-and-interest payment stays the same for the life of the loan.
- An ARM is fixed for an initial period — for example, a 5/6 ARM is fixed for 5 years, then adjusts every 6 months — and the new rate is the index plus the margin; for most conforming ARMs today the index is the 30-day Average SOFR.
- Three caps limit ARM movement: the initial adjustment cap, the periodic cap, and the lifetime cap, commonly written as 2/1/5 or 5/1/5; on a 2/1/5 ARM starting at 6%, the rate can never go above 11%.
- ARMs usually start with a lower rate because the borrower accepts the risk that the rate will rise later — the lower starting rate is a trade, not a gift.
- Choose a fixed rate when you plan to stay for a long time, need payment stability, or have little room for a higher payment; consider an ARM only if the loan will likely end before the fixed period does, you can afford the payment at the lifetime-cap rate, and you understand the index, margin, and all three caps.
- Before choosing an ARM, ask the lender to show you the payment at the lifetime-cap rate — if that payment would break your budget, the ARM is too risky.
1. How a fixed-rate mortgage works
A fixed-rate mortgage is the simpler of the two. Your interest rate is locked in at closing, and your principal-and-interest payment stays the same for the life of the loan. If you take out a 30-year fixed loan, the rate you close with is the rate you pay in year 1 and in year 29, assuming you do not refinance.
Fixed-rate loans are the default choice for most first-time buyers, and for good reason. If you plan to stay in the home for many years, if you want a payment you can plan around, and if you would lose sleep over a payment that might rise, a fixed rate is usually the safer starting point.
2. How an ARM works
An ARM has two phases. First comes a fixed period, during which the rate does not change. Then comes an adjustment period, during which the rate can move at scheduled intervals.
ARMs are named for those two phases. A 5/6 ARM, for example, has a fixed rate for the first 5 years, then adjusts every 6 months. A 7/6 ARM is fixed for 7 years and then adjusts every 6 months. A 10/6 ARM is fixed for 10 years and then adjusts every 6 months. Older naming, such as 5/1, meant five years fixed and then an adjustment once a year; the newer 6-month adjustment pattern is now common on conforming ARMs.
Once the fixed period ends, your new rate is not chosen at random. It is calculated from two parts: an index plus a margin. The index is a published market rate that moves with the economy.
For most conforming ARMs today, the index is the 30-day Average SOFR (Secured Overnight Financing Rate), which replaced LIBOR for US dollar ARMs when LIBOR was retired on June 30, 2023. The margin is a fixed number of percentage points the lender adds on top of the index, and it is set when you take out the loan. It does not change over the life of the loan.
So the formula is simple: new rate = current index + margin. If the index rises, your rate rises. If the index falls, your rate falls. The margin is the lender's fixed markup; the index is the moving part.
3. Caps: the guardrails on an ARM
Caps are what keep an ARM from moving without limit. The Consumer Financial Protection Bureau (CFPB) describes three kinds of caps, and every ARM has them:
The initial adjustment cap limits how much the rate can change the first time it adjusts, after the fixed period ends. The periodic (or subsequent) adjustment cap limits how much the rate can change at each adjustment after the first. The lifetime adjustment cap limits how much the rate can change in total over the life of the loan, measured from the starting rate.
Cap structures are usually written as three numbers: initial / periodic / lifetime. Two common structures on conforming ARMs are 2/1/5 and 5/1/5. Take a 2/1/5 example. Suppose your ARM starts at 6%. At the first adjustment, the rate can rise at most 2 percentage points, to 8%, or fall at most 2 points, to 4%.
At each later adjustment, it can move at most 1 point from the previous rate. And over the whole life of the loan, it can never go above 11% — the starting rate plus the 5-point lifetime cap — no matter what the index does.
That worst case matters more than the starting rate. Before choosing an ARM, ask the lender to show you the payment at the lifetime-cap rate. If that payment would break your budget, the ARM is too risky for you, even if the starting rate looks attractive.
The caps are disclosed in your loan documents, and the Loan Estimate and the CFPB's Consumer Handbook on Adjustable-Rate Mortgages (the CHARM booklet) explain them in plain language.
Source: CFPB Consumer Handbook on Adjustable-Rate Mortgages (CHARM), as cited in the article
4. Why ARMs start lower — and what that costs
ARMs usually start with a lower rate than a comparable fixed-rate loan. That is not a gift; it is a trade. The lender is offering a lower starting rate because you are accepting the risk that the rate will rise later. The lender's risk is lower, so the starting price is lower.
5. When each fits
Choose a fixed rate when you plan to stay put for a long time, when payment stability matters more than the lowest possible starting rate, when your budget has little room for a higher payment, or when you simply do not want to track the index and the adjustment schedule. That covers most buyers, and there is nothing unsophisticated about choosing certainty.
Consider an ARM when you have a clear, realistic reason to expect the loan to end before the fixed period does, when you can comfortably afford the payment at the lifetime-cap rate, and when you understand the index, the margin, and all three caps on the specific loan you are being offered — not on ARMs in general. If any of those three conditions is missing, the lower starting rate is not worth the risk.
6. The bottom line
A fixed-rate mortgage gives you a payment that never changes because the rate changed. An ARM gives you a lower starting rate in exchange for a rate that follows an index plus a margin, held in check by initial, periodic, and lifetime caps.
The question is not which loan is better in the abstract. It is which risk you would rather carry: the risk of missing out if rates fall, or the risk of a higher payment if rates rise. Answer that honestly, check the worst-case payment, and the choice usually becomes clear.
Sources: Consumer Financial Protection Bureau, Consumer Handbook on Adjustable-Rate Mortgages (CHARM) and CFPB guidance on ARM rate caps; conforming ARM index and cap structures as described in 2026 lender and CFPB-sourced guides. Rate examples in this article are illustrations, not quotes.
Educational only — eLoanTree is not a lender or broker and does not make loan decisions, guarantee rates or approval, or collect documents or SSNs.
Sources & further reading
Every figure in this article is dated and sourced in the text above. These official sources were used — open them to check the current figures: