Fixed-Rate vs. Adjustable-Rate Mortgages: 5 Key Differences
Fixed-rate vs. adjustable-rate mortgages: how the rate changes, index plus margin, rate caps, and a $400,000 payment example.
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Anyone shopping for a home loan meets the same fork in the road: lock in a fixed rate or start with an adjustable one. A fixed-rate mortgage keeps the same interest rate for the life of the loan, so the principal-and-interest payment stays the same. An adjustable-rate mortgage (ARM) usually starts with a lower rate, then the rate can move up or down, which changes your payment.
In this guide:
- A side-by-side table of the two loan types
- Five differences: the rate, the starting rate, adjustments, caps, and a worked example
- A 3-step check to run before you choose
Fixed-rate vs. ARM at a glance
| Feature | Fixed-rate | Adjustable-rate |
|---|---|---|
| Rate changes | Never | Can go up or down |
| Starting rate | Often higher | Often lower at first |
| Payment | Same principal and interest each month | Can change at each adjustment |
| Adjustment rule | None | New index plus a set margin, subject to caps |
What the table shows: the fixed-rate loan trades a higher starting rate for certainty, while the ARM trades certainty for a lower starting rate that can move. The five differences below explain which trade may fit your situation.
5 differences that matter
Each difference below comes down to one question: how much uncertainty are you willing to carry in exchange for a lower starting payment? Your answer depends on your budget, how long you expect to keep the loan, and how comfortable you are with a payment that can change.
1. What happens to the rate
With a fixed-rate mortgage, the interest rate is set when you take out the loan and will not change, according to the Consumer Financial Protection Bureau (CFPB). With an ARM, the interest rate may go up or down.
That single difference drives the rest. Budgeting is straightforward with a fixed rate, while an ARM asks you to plan for a range of possible payments.
2. The starting rate
Many ARMs start at a lower rate than fixed-rate loans. The CFPB says this initial rate may stay the same for months, one year, or a few years. After that, the rate adjusts at regular intervals set by the loan contract.
The lower start is the ARM's main appeal. It is also the reason to read the adjustment terms before signing, because the CFPB advises asking how soon your payment could go up.
3. How ARM adjustments work
Part of an ARM's rate is tied to a broad measure of interest rates called an index. At each adjustment, your actual rate is based on the new index plus a set margin, subject to any caps. The margin is a number of percentage points the lender adds to the index.
When the index rises, your rate can rise with it. When the index falls, your rate may fall too, depending on the terms of your loan.
4. Caps limit the swing
Some ARMs set a cap on how high the interest rate can increase at any time or over the life of the loan. The CFPB describes three kinds. An initial adjustment cap limits the first change, a subsequent adjustment cap limits each later change, and a lifetime cap limits the total change from the starting rate.
The CFPB lists typical figures, not fixed rules: an initial cap of two or five percentage points, a subsequent cap of one or two, and a lifetime cap of five. Some ARMs also limit how far the rate may decrease. Caps do not prevent increases; they bound them. The lifetime cap tells you the highest rate the contract allows, so check your own loan's numbers.
5. Worked example: what the lower start is worth
Take a $400,000 loan. A 30-year fixed rate at 6.5% gives a principal-and-interest payment of about $2,528 per month, every month. An ARM starting at 5.5% gives about $2,271 per month during the initial period, which is about $257 less.
If the ARM later adjusts upward, that gap shrinks or reverses. As an illustration only, if the rate rose by 5 percentage points to 10.5% on the same balance, the payment would be roughly $3,659 on a 30-year schedule. The real figure would depend on the remaining balance and term.
Assumptions: 30-year term, rates held constant within each scenario, taxes and insurance excluded. These rates are examples, not current offers.
Before you choose: 3 steps
- Ask the lender the CFPB's questions: how high or low your rate and monthly payment can go with each adjustment, how often the rate adjusts, how soon your payment could go up, whether there is a cap on how high and a limit on how low the rate can go, and whether you could still afford the loan at the maximums.
- Model the worst case. Calculate the monthly payment at the lifetime cap rate and compare it with your budget, not just the introductory payment.
- Match the loan to your time horizon. An ARM's lower start helps most if you expect to sell or refinance before the adjustments begin. A fixed rate suits those who want the same payment for years.
Before you sign
Get the index name, the margin, and each cap in writing as part of the loan paperwork. Ask what the contract says if the index is discontinued or replaced.
It also helps to compare loan estimates from more than one lender side by side, since the margin and caps can differ even when the starting rate looks the same. A housing counselor or a licensed mortgage professional can help you check the numbers for your own situation.
This article is for general information, not financial advice.