Key takeaways
- A fixed-rate mortgage keeps the same interest rate, and the same principal-and-interest payment, for the entire term.
- An adjustable-rate mortgage (ARM) starts with a fixed introductory period, then its rate can rise or fall with a market index.
- Caps limit how far an ARM’s rate can move at each adjustment and over the life of the loan. They are the most important numbers to read.
- A useful test before choosing an ARM: could you afford the payment at the highest rate the loan allows?
One of the first structural choices you make with a mortgage is how its interest rate behaves over time. A fixed rate buys certainty. An adjustable rate usually starts lower in exchange for accepting some uncertainty later. Neither is better in general; the right fit depends on your budget, your time horizon and how much payment risk you can live with.
How a fixed-rate mortgage works
With a fixed-rate mortgage, the interest rate is set when you close and never changes. Your principal and interest payment is the same in year one as it is in the final year. Your total monthly payment can still move if it includes escrow for property taxes and insurance, since those bills change, but the loan itself stays put.
The term shapes the trade-off. A 30-year fixed loan spreads repayment out, giving a lower monthly payment but more total interest. A 15-year fixed loan has a noticeably higher payment, typically carries a lower rate than a comparable 30-year loan, and costs far less interest overall.
The appeal is predictability. The cost is that fixed rates are often higher than an ARM’s starting rate, and if market rates later fall, the only way to capture them is to refinance, which comes with its own costs.
How an adjustable-rate mortgage works
An ARM’s name tells you its structure. In a 5/6 ARM, the rate is fixed for the first five years and then adjusts every six months. A 7/6 ARM is fixed for seven years, a 10/6 for ten. Older designs such as the 5/1 ARM adjust once a year after the fixed period.
When the rate adjusts, it is recalculated with a simple formula:
- Index: a benchmark interest rate that moves with the market. Many newer ARMs use the Secured Overnight Financing Rate, known as SOFR.
- Margin: a fixed number of percentage points added to the index. It is set in your loan documents and does not change.
Index plus margin gives the fully indexed rate, subject to the loan’s caps.
Caps: the numbers that limit your risk
ARM caps are usually written as three numbers, such as 2/1/5:
- Initial adjustment cap (2): the most the rate can change at the first adjustment.
- Subsequent adjustment cap (1): the most it can change at each later adjustment.
- Lifetime cap (5): the most it can ever rise above the starting rate.
Some ARMs also have a floor, a minimum rate that applies even if the index falls sharply. Your Loan Estimate shows the highest possible payment and the earliest date it could arrive.
- Loan amount and type
- $300,000, 30-year 5/6 ARM
- Starting rate (hypothetical) and caps
- 5.75%, 2/1/5
- Payment for the first five years
- $1,750.72
- Balance after five years
- about $278,287
- Payment at the first-adjustment maximum, 7.75%
- about $2,102
- Payment at the lifetime maximum, 10.75%
- about $2,677
Principal and interest only, recalculated on the remaining balance over the remaining 25 years. Hypothetical figures for illustration, not a quote.
Fixed and adjustable, side by side
| Feature | Fixed rate | Adjustable rate |
|---|---|---|
| Interest rate | Set at closing for the full term | Fixed at first, then follows an index within caps |
| Payment predictability | Principal and interest never change | Can rise or fall after the introductory period |
| Starting rate | Often higher | Often lower |
| If market rates fall | Refinancing needed to benefit | Rate may fall at the next adjustment, subject to any floor |
| If market rates rise | No effect on your rate | Rate may rise, up to the caps |
Questions to ask before choosing
How long do you expect to keep the loan?
If you are confident you will sell before the fixed period ends, an ARM’s lower starting rate may matter more than what happens afterward. If you plan to stay for many years, the certainty of a fixed rate carries more weight.
Could your budget absorb the maximum payment?
Run the numbers at the lifetime cap, as in the example above. If that payment would put real strain on your household, treat it as a serious warning rather than an unlikely footnote.
What exactly are the terms?
Ask which index the loan uses, what the margin is, how often the rate adjusts, what the caps and floor are, and whether there is a prepayment penalty. Avoid features you do not fully understand, especially any that allow the balance to grow, known as negative amortization.
Plans change
Many ARM decisions rest on a plan to sell or refinance before the rate adjusts. Refinancing later depends on your credit, income, home value and market rates at that future date, none of which you can know today. Treat it as a possibility, not a certainty.
A word on timing the market
The gap between fixed rates and ARM starting rates changes with market conditions. Sometimes it is wide enough to be tempting; sometimes it is barely there. Nobody can reliably predict where rates will go next. The practical approach is to compare real Loan Estimates for both loan types on the same day and decide based on the payments you can comfortably carry.
Helpful official resources
- Consumer Financial Protection BureauThe federal agency responsible for protecting consumers of financial products, including mortgages.consumerfinance.gov
- U.S. Department of Housing and Urban DevelopmentThe federal department that oversees the Federal Housing Administration and HUD-approved housing counseling.hud.gov
- Federal Deposit Insurance CorporationThe independent federal agency that insures bank deposits and supervises many banks.fdic.gov
About this guide. OwnMG publishes general educational information. It is not financial, legal or insurance advice, and OwnMG is not a lender, insurer, broker or government agency. Rules, limits and fees change, so confirm current details with your lender or the agency involved. Spotted something out of date? Tell us at info@ownmg.com.
