What Is Adjustable-Rate Mortgage?
ARMs start with a lower rate than fixed-rate mortgages to compensate for future uncertainty. After the initial period, the rate adjusts—usually annually—based on a benchmark (SOFR, Treasury, etc.) plus a fixed margin. Caps limit how much the rate can rise per adjustment and over the life of the loan. For investors planning to sell or refinance within 5–7 years, an ARM can reduce payments and improve cash flow. For long-term holds, a fixed rate eliminates interest-rate-cycle risk. Negative leverage can worsen if rates rise and your ARM adjusts up.
An adjustable-rate mortgage (ARM) is a loan whose interest rate changes periodically after an initial fixed period (e.g., 5, 7, or 10 years). The rate is tied to an index plus a margin: Rate = Index (e.g., SOFR) + Margin.
At a Glance
- What it is: Mortgage with rate that adjusts after initial fixed period
- Formula: Rate = Index + Margin (e.g., SOFR + 2.25%)
- Initial period: Often 5, 7, or 10 years fixed
- Caps: Per-adjustment and lifetime limits on rate increases
- Use case: Short-term holds, planned refinance, or rate bet
Rate = Index (e.g., SOFR) + Margin
How It Works
Index + margin
The rate isn't arbitrary. It's calculated as:
Rate = Index + Margin
- Index: A published benchmark—SOFR (Secured Overnight Financing Rate), 1-Year Treasury, or similar. The index moves with the market.
- Margin: A fixed add-on set at origination (e.g., 2.25%). Your margin never changes.
Example: SOFR is 4.5%, margin is 2.25%. Your rate = 6.75%. When SOFR moves to 5%, your rate becomes 7.25%.
Adjustment schedule
A "5/1 ARM" means 5 years fixed, then annual adjustments. A "7/1 ARM" means 7 years fixed, then annual. The first number is the initial period; the second is how often it adjusts after that.
Caps protect you (somewhat)
- Initial cap: Limit on the first adjustment (e.g., 2%).
- Periodic cap: Limit per adjustment (e.g., 2% per year).
- Lifetime cap: Maximum rate over the loan (e.g., 5% above start rate).
A 5/1 ARM starting at 5% with a 2/2/5 cap structure can rise at most 2% at first adjustment, 2% per year after, and never more than 10% (5% + 5% lifetime).
When ARMs make sense
- You plan to sell or refinance before the first adjustment
- You want lower payments now and accept future rate risk
- You believe rates will fall (your ARM will follow)
- Short-term interest-rate-cycle play
Real-World Example
David buys a duplex in Denver for $380,000 with a 7/1 ARM at 5.5% (vs 6.75% for a 30-year fixed). His payment is $2,158 vs $2,463—$305/month less. He plans to refinance or sell within 7 years.
Year 1–7: He pays 5.5%. Cash flow is stronger. At year 7, SOFR has risen; his rate adjusts to 7.25% (within the 2% periodic cap). His payment jumps to $2,590. He refinances into a fixed-rate mortgage at 6.5% before the next adjustment. He captured 7 years of savings and locked in before rates went higher.
If he'd held the ARM and rates kept rising, he'd face negative leverage—debt cost exceeding returns. The refinance was his exit.
Pros & Cons
- Lower initial rate and payment than fixed-rate mortgage
- Can improve cash flow for short-term holds
- If rates fall, your payment drops without refinance
- Caps limit worst-case increases
- Useful when you expect to sell or refi before adjustment
- Rate and payment uncertainty after initial period
- Negative leverage risk if rates rise
- Refinance may be costly or unavailable when you need it
- Lifetime cap can still mean a painful payment increase
- Complexity—many borrowers don't understand the mechanics
Watch Out
Payment shock: When the ARM adjusts, your payment can jump. Model the worst case (rate at lifetime cap) and ensure you can afford it.
Refinance timing: Don't assume you'll refinance when the ARM adjusts. If rates are high or your equity is down, you may be stuck with the adjusted rate.
Index changes: Lenders have moved from LIBOR to SOFR. Know which index your loan uses and how it's behaved historically.
Balloon ARMs: Some ARMs have a balloon payment at the end of the initial period. You must refinance or pay off—there's no "adjust and continue" option. Read the note.
The Takeaway
ARMs offer lower upfront costs in exchange for future rate risk. Use them when you have a clear exit—sell or refinance—before the first adjustment. For long-term holds, the certainty of a fixed-rate mortgage usually wins. Always model the capped worst case before you sign.