The fixed-rate vs. adjustable-rate mortgage decision shapes your housing costs for years, sometimes decades. A fixed-rate loan keeps the same interest rate for its whole term, so your principal and interest payment never changes. An adjustable-rate mortgage (ARM) usually starts with a lower rate for a set period, then resets on a schedule based on market rates, which means your payment can rise or fall.
Neither option is automatically better. The right fit depends on how long you expect to keep the loan, how much payment movement your budget can handle, and how comfortable you are with uncertainty. This guide explains how each loan works, how ARM rates are calculated and capped, and how to compare them using a worked example.
How a Fixed-Rate Mortgage Works
With a fixed-rate mortgage, the interest rate is set at closing and stays the same until the loan is paid off. Common terms include 15 and 30 years. Because the rate never changes, the principal and interest portion of your payment is identical every month. Property taxes and homeowner’s insurance collected through escrow can still change, so your total payment may shift slightly, but the loan cost itself is locked in.
The main advantage is predictability. You can plan a long-term budget without worrying about rising rates. The trade-off is that fixed rates often start higher than the introductory rate on a comparable ARM, and if market rates fall, you would need to refinance to benefit, which comes with closing costs.
How an Adjustable-Rate Mortgage Works
The Consumer Handbook on Adjustable-Rate Mortgages, published by the Federal Reserve and updated by the CFPB, explains that most ARMs begin with an initial period at a set rate. That period can range from one month to five years or more. After it ends, the rate adjusts at regular intervals.
ARMs are often described with two numbers. In a 5/1 ARM, for example, the rate is fixed for the first five years, then adjusts every year after that. The first number is the length of the initial period; the second tells you how often the rate changes afterward. Always confirm the exact adjustment schedule in your loan documents rather than relying on the label alone.
On a Loan Estimate, you can spot an ARM in the Loan Terms table: the interest rate row will say YES under “Can this amount increase after closing?” ARMs also come with an Adjustable Interest Rate table on page 2. Our guide on how to read a Loan Estimate walks through where to find these details.
Index, Margin and Caps: The Moving Parts of an ARM
Once the initial period ends, your new rate is calculated using two pieces, according to the CFPB:
- The index: a published interest rate that moves with overall market conditions. Your lender chooses it when you apply, and it generally stays the same for the life of the loan.
- The margin: a fixed number of percentage points added to the index. It is set in your loan agreement and does not change after closing. The CFPB notes margins vary by lender, so compare them and ask whether they are negotiable.
Index plus margin is called the fully indexed rate. The ARM handbook gives a simple illustration: with a 4% index and a 3% margin, the rate would be 7%. If the index rose to 5%, the rate would become 8%.
Caps limit how far the rate can move:
- Periodic adjustment cap: limits how much the rate can change from one adjustment to the next. Some ARMs allow a larger change at the first adjustment than at later ones.
- Lifetime cap: limits how high the rate can go over the life of the loan. The handbook notes virtually all ARMs must have one.
- Payment cap: some loans limit how much the payment can rise at each adjustment. Be careful here: if the payment is capped but the rate is not, unpaid interest can be added to your balance, which is called negative amortization.
The handbook also notes that a rate increase the cap prevents in one period may carry over to a later adjustment, so ask your lender whether carryover applies.
Fixed-Rate vs. Adjustable-Rate Mortgage: Comparison Table
| Feature | Fixed-Rate Mortgage | Adjustable-Rate Mortgage |
|---|---|---|
| Interest rate | Same for the entire loan | Set for an initial period, then adjusts |
| Starting rate | Often higher than an ARM’s initial rate | Often lower at first |
| Payment predictability | High for principal and interest | Lower after the initial period |
| Effect of falling market rates | No change unless you refinance | Rate may fall at adjustment, subject to any floor |
| Effect of rising market rates | No change | Rate and payment may rise, up to the caps |
| Comparing APRs | Comparable with other fixed loans | Compare with other ARMs; not directly with fixed-rate APRs |
| Often suits | Long-term owners who value stability | Borrowers with a shorter horizon who can absorb higher payments |
A Hypothetical Payment Comparison
The figures below are hypothetical, rounded and for illustration only. They cover principal and interest only and do not reflect current market rates or any lender’s offer.
A borrower is choosing between two $300,000, 30-year loans:
- Fixed rate at 6.5%: about $1,896 per month for all 30 years.
- 5/1 ARM starting at 5.75%: about $1,751 per month for the first five years, roughly $145 less per month than the fixed loan.
After five years, the ARM balance would be about $278,300. What happens next depends on the market:
- If the rate reset to 7.75%, the payment on the remaining 25 years would rise to about $2,102, roughly $351 more than before and $206 more than the fixed-rate payment.
- If the rate reset to 4.75%, the payment would fall to about $1,587.
Over the first five years the ARM saves roughly $8,700 in payments in this example. Whether that saving is worth it depends on where rates go and whether you still have the loan when it adjusts. The handbook’s own examples show how quickly payments can jump when an introductory rate ends, something often called payment shock.
Questions to Ask Before Choosing
The ARM handbook includes a worksheet of questions. Some of the most useful:
- What is the initial rate, how long does it last, and what is the APR?
- Which index is used, what is its current value, and what is the margin?
- What are the periodic and lifetime caps, and is there a rate floor?
- Is there a payment cap, and could the loan negatively amortize?
- What would my payment be if the index stayed flat, rose 2 percentage points, or fell 2 percentage points?
- What is the highest payment possible in the first few years after adjustment?
- Is there a prepayment penalty or a balloon payment?
Then ask yourself: How long do I plan to stay in the home? Could I afford the maximum payment? Are other big expenses on the way? Our step-by-step guide to choosing the right mortgage can help frame those answers.
Notices and Protections for ARM Borrowers
According to the ARM handbook, your servicer must notify you at least seven months before the first rate adjustment, and at least 60 days before a new payment takes effect for later adjustments. The handbook also notes that lenders generally must evaluate your ability to repay using the fully indexed rate or the highest rate expected in the first five years, not just the introductory rate. Some ARMs are convertible to a fixed rate at set times, though the handbook warns this may come with a higher rate or fees.
Frequently Asked Questions
Can an ARM rate go down?
Yes. If the index falls, your rate can drop at the next adjustment, although many loans have a floor that limits how low the rate can go.
Can I refinance an ARM into a fixed-rate loan?
Often, yes, but refinancing involves qualifying again and paying closing costs. Falling home values can make refinancing harder, as the handbook notes.
Why can’t I compare an ARM’s APR with a fixed loan’s APR?
The handbook explains that ARM APRs can be compared with other ARMs, but not directly with fixed-rate APRs, because the ARM APR rests on assumptions about future rate changes.
Are interest-only ARMs the same as regular ARMs?
No. With an interest-only ARM, you pay only interest for a set time, often 3 to 10 years according to the handbook, and payments rise once principal repayment begins, even if the rate stays the same.
Bottom Line
A fixed-rate mortgage buys certainty: the same principal and interest payment for the life of the loan. An adjustable-rate mortgage typically offers a lower starting rate in exchange for the risk that payments climb later, within limits set by its caps. If you expect to keep the loan long term and value stability, a fixed rate is often the simpler fit. If your time horizon is shorter and you can absorb a higher payment, an ARM may be worth considering. Either way, compare full Loan Estimates and stress-test the worst-case payment before you commit.
Sources
- Consumer Financial Protection Bureau – Consumer Handbook on Adjustable-Rate Mortgages
- Consumer Financial Protection Bureau – For an ARM, what are the index and margin, and how do they work?
- Consumer Financial Protection Bureau – Loan Estimate Explainer
This article is for general information only and is not financial advice. See our Disclaimer.
