Adjustable Rate Mortgages (ARMs) Explained

How an ARM Works

An ARM has two phases: a fixed-rate period at the start (typically 5, 7, or 10 years), followed by a variable-rate period where the rate adjusts on a schedule (usually every 6 or 12 months) based on a published index plus a margin.

Naming convention: a 7/6 ARM is fixed for 7 years, then adjusts every 6 months. A 5/1 ARM (older convention) was fixed for 5 years, then adjusted yearly.

Index and Margin

After the fixed period, your rate becomes index + margin. Most modern ARMs use the SOFR index (Secured Overnight Financing Rate) plus a margin of 2.25–2.75% set at origination. If SOFR is 4.50% and your margin is 2.50%, your new rate would be 7.00% — capped by the rate caps below.

Rate Caps Protect You

Every ARM has three caps, often expressed as 2/1/5 or 5/2/5:

• Initial cap — the maximum the rate can jump at the first adjustment (e.g. 2% above your start rate).
• Periodic cap — the max change at any subsequent adjustment (e.g. 1%).
• Lifetime cap — the max the rate can ever reach above the start rate (e.g. 5%).

A 5/2/5 ARM that starts at 6% could rise to a maximum of 11% over the loan's life — but only 2% per adjustment, and 5% above the start rate even after 30 years.

When ARMs Make Financial Sense

• You'll move in 5–8 years. Most American homeowners move or refinance within 7 years. A 7/6 ARM gives 7 years of lower rate without ever entering the variable phase.
• You'll pay it off early. If you plan aggressive principal reduction or an inheritance/bonus paying off the loan, the lower starting rate compounds in your favor.
• Rates are elevated. ARMs typically run 0.50–1.0% below 30-year fixed. When fixed is high, the gap widens.
• Income trajectory is strong. If your income will be significantly higher in 5–7 years, you have more room to absorb future adjustments.

When ARMs Are a Bad Idea

• You plan to stay 10+ years and want payment certainty
• You'd be stretched at the maximum adjusted rate
• Fixed rates are already at historical lows — there's less room for ARMs to beat them
• Your income is volatile or seasonal

Sample ARM vs. Fixed Comparison

$400,000 loan, 30-year amortization:

• 30-year fixed at 7.0%: $2,661/mo. Total interest over 7 years if you sell: ~$135,200.
• 7/6 ARM at 6.0% start: $2,398/mo for 7 years. Total interest over 7 years if you sell at year 7: ~$115,400.
• Savings if you sell at year 7: ~$22,000 (plus ~$2,200/yr in lower payments along the way).

If you hold past year 7 and rates have risen, that ~$22k cushion gets consumed in the variable years. The math is heavily dependent on what rates do — but the cushion buys you flexibility.