What is an Adjustable-Rate Mortgage?
An adjustable-rate mortgage (ARM) is a loan with an interest rate that changes periodically based on market conditions. Unlike a fixed-rate mortgage where the rate stays the same for the entire loan term, an ARM starts with a fixed rate for an initial period (typically 3, 5, 7, or 10 years), then adjusts periodically (usually annually) based on a specified index plus a margin.
For real estate investors, ARMs offer a strategic trade-off: lower initial monthly payments in exchange for future rate uncertainty. If you plan to sell or refinance the property before the adjustment period begins, an ARM can save you money compared to a fixed-rate loan. But if you end up holding longer than planned and rates have risen, your monthly payment can increase substantially.
How ARM rates are calculated
Every ARM has two components that determine the rate after the initial fixed period:
Index: A benchmark interest rate that the ARM tracks. Common indexes include the Secured Overnight Financing Rate (SOFR), the 1-Year Treasury rate, and the 11th District Cost of Funds Index (COFI). The index moves with the broader interest rate market. When the Federal Reserve raises rates, these indexes typically increase.
Margin: A fixed percentage added to the index that represents the lender's markup. Margins typically range from 1.75% to 3.50% and are set at origination and never change. If your ARM has a margin of 2.50% and the index is at 4.00% on your adjustment date, your new rate would be 6.50%.
ARM rate formula: New rate = Current index value + Fixed margin
Example: SOFR at 4.25% + 2.50% margin = 6.75% adjusted rate
Common ARM structures
ARMs are described with two numbers separated by a slash. The first number is the initial fixed-rate period in years. The second number is how often the rate adjusts after that.
| ARM type | Fixed period | Adjustment frequency | Best for |
|---|---|---|---|
| 3/1 ARM | 3 years | Annually | Short-term flips or holds |
| 5/1 ARM | 5 years | Annually | Medium-term holds with refinance plan |
| 7/1 ARM | 7 years | Annually | Value-add with stabilization timeline |
| 10/1 ARM | 10 years | Annually | Longer holds, near-fixed stability |
| 5/6 ARM | 5 years | Every 6 months | Newer structure, more frequent adjustment |
Rate caps
ARMs include caps that limit how much the rate can change. Three caps apply:
Initial adjustment cap: Limits the rate increase at the first adjustment. Typically 2% for a 5/1 ARM. If your initial rate is 5.50%, the rate can't go above 7.50% at the first adjustment.
Periodic adjustment cap: Limits the rate increase at each subsequent adjustment. Typically 2% per adjustment period. Even if the index jumps 4% in a year, your rate can only increase 2%.
Lifetime cap: Limits the total rate increase over the life of the loan. Typically 5-6% above the initial rate. An ARM starting at 5.50% with a 5% lifetime cap can never exceed 10.50%.
A common cap structure written as 2/2/5 means: 2% initial cap, 2% periodic cap, 5% lifetime cap. Understanding these caps is essential for modeling worst-case payment scenarios on investment properties.
When investors choose ARMs
The BRRRR strategy (Buy, Rehab, Rent, Refinance, Repeat) is a natural fit for ARMs. If your plan is to buy, renovate, stabilize, and refinance within 2-3 years, a 5/1 ARM gives you the lowest payments during the renovation and stabilization phase while you're not yet collecting full rent. You refinance into permanent fixed-rate financing before the ARM adjusts.
Value-add multifamily investors often use 7/1 or 10/1 ARMs because their business plan involves 2-3 years of renovation and lease-up followed by a refinance at the higher stabilized value. The ARM's lower payments during the capital-intensive improvement phase preserves cash flow when it's most needed.
The risk is that your refinance plan fails. If property values decline, or your renovation runs over budget and timeline, or lending standards tighten, you might not be able to refinance before the ARM adjusts. At that point, your holding costs increase, potentially turning a profitable investment into a cash-flow-negative situation.
ARM vs. fixed rate: the math
On a $200,000 investment property loan, the difference between a 5/1 ARM at 6.25% and a 30-year fixed at 7.00% is about $100/month in payment savings. Over a 5-year hold, that's $6,000 in savings. If you refinance before the adjustment, you captured the savings with no additional risk. If you can't refinance and the rate adjusts upward, the savings evaporate quickly.