An adjustable-rate mortgage (ARM) is a home loan with an interest rate that changes periodically after an initial fixed period. Unlike a fixed-rate mortgage — where your rate stays the same for the life of the loan — an ARM starts with a fixed rate for 3, 5, 7, or 10 years, then adjusts annually based on a market index. ARMs can offer lower initial rates than fixed mortgages, but they carry the risk of payment increases when rates adjust.
How an ARM Works
Most ARMs are named with two numbers separated by a slash, like 5/1 or 7/1:
- The first number is the length of the initial fixed-rate period (in years)
- The second number is how often the rate adjusts after that period (1 = annually)
So a 5/1 ARM has a fixed rate for five years, then adjusts every year thereafter. A 7/6 ARM (increasingly common) is fixed for seven years and then adjusts every six months.
What Determines the Adjusted Rate?
After the fixed period, your rate is calculated by adding a margin (set by the lender at origination, typically 2.5%–3.5%) to a benchmark index. Common indexes include:
- SOFR (Secured Overnight Financing Rate): The most common index for new ARMs since replacing LIBOR
- CMT (Constant Maturity Treasury): Based on U.S. Treasury yields
If SOFR is 4.5% and your margin is 2.75%, your new rate would be 7.25%. That rate applies until the next adjustment period.
Rate Caps: The Protection Limits
ARMs include caps that limit how much the rate can change, expressed as three numbers (e.g., 2/2/5):
- Initial cap (first number): Maximum rate increase at the first adjustment. A 2% cap means your rate cannot jump more than 2% above the initial fixed rate at the first reset.
- Periodic cap (second number): Maximum rate change at each subsequent adjustment (typically 1%–2%).
- Lifetime cap (third number): Maximum total rate increase over the life of the loan. A 5% lifetime cap on a 6% initial rate means your rate can never exceed 11%.
ARM vs. Fixed-Rate Mortgage
- Fixed-rate: Rate never changes. Predictable monthly payment for the life of the loan. Higher initial rate than ARM. Best for long-term homeowners who want stability.
- ARM: Lower initial rate. Payment can change after fixed period. Best for borrowers who plan to sell or refinance before the fixed period ends.
When an ARM Makes Sense
ARMs work best in specific situations:
- You plan to sell or move within the initial fixed period (5–10 years)
- You expect interest rates to fall during your ownership, making refinancing advantageous later
- You need the lower initial payment to qualify or to free up cash for other priorities
- You have a high income with significant flexibility to absorb payment increases
When an ARM Is Risky
- You plan to stay in the home long-term beyond the fixed period
- Your budget is tight and a payment increase of $300–$600/month would cause hardship
- Rates are already low and are more likely to rise than fall
- You are counting on refinancing but cannot guarantee you will qualify at future rates
Payment Shock: The Real Risk
Payment shock is the increase in monthly payment when an ARM first adjusts. On a $400,000 loan at 5.5% (fixed ARM rate), the monthly principal and interest payment is about $2,270. If rates rise and the ARM adjusts to 8.5%, the payment jumps to around $3,070 — an increase of $800 per month. That kind of increase can strain or break a household budget that was not prepared for it.
How to Evaluate an ARM Offer
Ask your lender for the worst-case scenario: apply the lifetime cap to your initial rate and calculate the maximum possible payment. If you can afford that payment, the ARM carries less risk. If that payment would strain your finances, proceed with caution or choose a fixed-rate mortgage.
Bottom Line
An ARM is not inherently bad or good — it is a tool that fits specific circumstances. If you know you will sell within five to seven years, a 5/1 or 7/1 ARM can save meaningful money on interest. If you plan to stay put for the long term, a fixed-rate mortgage’s predictability is usually worth the slightly higher initial rate.