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How Adjustable-Rate Mortgages Work
An adjustable-rate mortgage (ARM) starts with a fixed introductory rate for a set period, typically 3, 5, 7, or 10 years, then adjusts periodically based on a benchmark index plus a margin. A 5/1 ARM means the rate is fixed for 5 years, then adjusts every 1 year. A 7/6 ARM is fixed for 7 years, then adjusts every 6 months. The introductory rate is typically 0.5% to 1.5% lower than a comparable 30-year fixed rate, which translates to meaningful monthly savings during the fixed period.
After the fixed period, your rate adjusts based on the index + margin formula. Common indices include SOFR (Secured Overnight Financing Rate) and the 1-year Treasury rate. The margin is typically 2% to 3%, set at origination and fixed for the life of the loan. If SOFR is 4.5% and your margin is 2.5%, your adjusted rate would be 7.0%. ARMs include rate caps that limit how much the rate can change: initial adjustment cap (typically 2%), periodic cap (2% per adjustment), and lifetime cap (5-6% above the initial rate).
When an ARM Makes Financial Sense
ARMs are strategically advantageous in specific situations. You plan to sell or refinance within the fixed period: if you know you will move in 5-7 years, a 7/1 ARM at 5.75% versus a 30-year fixed at 6.5% saves approximately $150/month or $12,600 over 7 years on a $400,000 loan. You expect rates to decline: if you believe interest rates will fall, an ARM benefits automatically without refinancing costs. You are buying a starter home: the lower initial payment helps with affordability and you plan to sell before the rate adjusts.
ARMs carry risk if rates rise significantly after the fixed period. The worst-case scenario on a 5/1 ARM with a 5% lifetime cap: your 5.75% rate could eventually reach 10.75%. On a $400,000 loan, that would increase your payment from approximately $2,334 to $3,763, a $1,429/month increase. Before choosing an ARM, ensure you can afford the worst-case payment. Compare scenarios with our Mortgage Calculator and Refinance Calculator.
ARM Rate Caps Explained
Every ARM has three caps that protect borrowers from extreme rate increases. The initial adjustment cap limits the first rate change, typically 2% above the introductory rate. The periodic adjustment cap limits each subsequent adjustment, usually 2% per period. The lifetime cap sets the maximum rate over the life of the loan, typically 5-6% above the initial rate. A 5/1 ARM starting at 5.5% with a 2/2/5 cap structure means: first adjustment cannot exceed 7.5%, each subsequent adjustment cannot increase more than 2%, and the rate can never exceed 10.5%. Understanding these caps helps you calculate your worst-case scenario before committing to an ARM.