How Does a 5/1 ARM Work? Adjustable Rate Mortgages Explained

An adjustable-rate mortgage sounds complicated, but the mechanics are straightforward once you understand the structure. The "5/1" means 5 years fixed, then adjusts every 1 year. The initial rate is set at origination. After the fixed period, the rate adjusts based on a market index plus a lender margin. Caps limit how much the rate can move at each adjustment and over the life of the loan.

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Key Facts at a Glance
Fixed period
5 years
Adjustment frequency
Every 12 months after year 5
Index (modern ARMs)
SOFR (replaced LIBOR)
Typical margin
2.5–3.0% above index
Common cap structure
2% initial / 2% periodic / 5% lifetime

Frequently Asked Questions

What does the 2/2/5 cap structure mean?
The three numbers represent limits on rate increases: the first cap (2%) is the maximum increase at the first adjustment after the fixed period; the periodic cap (2%) is the maximum at any single subsequent adjustment; the lifetime cap (5%) is the total maximum increase over the life of the loan. So a 6.5% ARM with 2/2/5 caps can never exceed 11.5%.
Most ARMs originated since 2023 use SOFR (Secured Overnight Financing Rate) as the index. Older ARMs used LIBOR, which was phased out in 2023 and transitioned to SOFR. Your new rate at each adjustment = current index + your margin. The margin is fixed for the life of the loan; only the index changes.
Your lender will notify you 60–120 days before any rate change. The new rate (index + margin, subject to caps) is calculated, and your payment is recalculated based on the remaining balance and remaining loan term. If your rate goes from 6.5% to 8.5% after year 5, your payment increases meaningfully — model this scenario before taking an ARM.
Common ARM types include 3/1 (fixed 3 years, adjusts annually), 7/1 (fixed 7 years), 10/1 (fixed 10 years), and 5/6 (fixed 5 years, adjusts every 6 months — more common after LIBOR transition). Longer initial fixed periods have slightly higher starting rates but more protection.
Some ARMs have a built-in conversion option allowing you to switch to a fixed rate at specified points. Most borrowers refinance instead. The risk: refinancing requires qualifying again at that point in time, and rates may be higher. Going into an ARM with a clear refinance plan and the financial flexibility to execute it is key.
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