See your actual ARM payment across all three phases
Try the ARM Mortgage Calculator →With the 30-year fixed sitting at 7.28% — six straight weekly increases — adjustable-rate mortgages are back in buyers' search bars. An ARM trades rate certainty for a lower starting payment, and understanding exactly what you're trading away matters more than the headline rate.
Here's how ARMs actually work: what the numbers in "5/1" or "7/1" mean, what protects you from a runaway rate, and who an ARM genuinely makes sense for in this market.
An adjustable-rate mortgage starts with a lower fixed rate for an initial period, then adjusts periodically based on market conditions for the rest of the loan term. The appeal is straightforward: a lower rate than a 30-year fixed means a lower payment today, which can mean qualifying for more home or simply paying less during the years you actually plan to own it.
The naming convention tells you the whole structure:
So a 7/1 ARM means: your rate is locked for 7 years, then it can adjust once a year for the remaining life of the loan.
| Scenario (on $400,000) | Rate | Approx. Payment |
|---|---|---|
| 5/1 ARM — intro rate | 6.10% | $2,424/mo |
| 30-year fixed — today's rate | 7.28% | $2,737/mo |
That's roughly $313/month lower during the fixed period — about $18,800 over 5 years. Use the ARM mortgage calculator above to run this with your own loan amount and rates.
When the fixed period ends, your new rate isn't arbitrary. It's calculated as:
New Rate = Index Rate (published, moves with the market) + Margin (fixed at origination, never changes)
The index is typically tied to SOFR (the Secured Overnight Financing Rate) on current ARMs. The margin is set when you take out the loan and stays constant for the life of the loan — it's the lender's fixed spread above the index. You can find your loan's specific index and margin in your original loan documents.
This is the part that matters most and gets explained least. ARMs come with caps that limit how much the rate can move:
These are often written as a three-number structure like "5/2/5" — meaning up to a 5-point jump at first adjustment, up to 2 points at each adjustment after that, and never more than 5 points above the start rate over the life of the loan. Caps vary by lender and loan product, so confirm the actual numbers on your loan estimate rather than assuming a standard structure.
Run your own ARM numbers — intro, adjustment, and worst case
ARM Mortgage Calculator → Standard Mortgage Calculator →The first number is how many years the rate stays fixed. The second is how often it can adjust afterward, in years — so a 5/1 ARM is fixed for 5 years, then can adjust once a year after that.
ARMs have rate caps that limit how much the rate can rise: an initial adjustment cap, a periodic cap for each later adjustment, and a lifetime cap over the starting rate. A common structure is 5/2/5 — check your specific loan's caps before assuming a worst case.
Your new rate is a published index rate (commonly SOFR-based) plus a fixed margin set at origination. The margin never changes; the index moves with the market, within your loan's caps.
Not necessarily. It depends on how long you plan to keep the loan. If you'll sell, refinance, or pay it off before the fixed period ends, an ARM's lower intro rate can save real money. If you plan to stay long-term, the uncertainty after the fixed period is a real risk to weigh against the upfront savings.
Yes, assuming you qualify at the time. Many ARM borrowers plan to refinance into a fixed rate (if rates improve) or sell before the fixed period ends — but that plan depends on market conditions and your finances holding up, not a guarantee.