A mortgage with an interest rate that changes periodically based on a market index.
An adjustable-rate mortgage (ARM) starts with a low introductory interest rate (called the "teaser rate") that remains fixed for a set period, typically 3, 5, 7, or 10 years. After this period, the rate adjusts periodically—usually annually—based on a specific index plus the lender's margin. ARMs often have annual and lifetime interest rate caps that limit how much the rate can increase per adjustment and overall. While ARMs can offer lower initial payments, they carry the risk of significantly higher payments when rates reset, making them suitable mainly for borrowers planning to sell or refinance before rates adjust.
An ARM has two phases: a FIXED introductory period (the "teaser" — typically 3, 5, 7, or 10 years) and an ADJUSTMENT period (annually, or in some cases every 6 months) after the teaser ends. The post-teaser rate is set as: new_rate = index_rate + margin, subject to caps. The INDEX is a published benchmark — since the LIBOR phase-out completed in 2023, new ARMs typically use the 30-day Average SOFR or CMT (Constant Maturity Treasury). The MARGIN is a fixed spread set at origination (typically 2.0%-3.0% for prime borrowers). CAPS limit how much the rate can move: initial-adjustment cap (e.g., 2% above start rate), periodic cap (e.g., 2% per adjustment thereafter), and lifetime cap (e.g., 5% above start rate). The "5/1 ARM" naming convention: 5 = fixed-rate years, 1 = adjustments per year after. A "5/6 ARM" adjusts every 6 months after the 5-year fixed period. Common mistakes: (1) misreading the cap structure — "2/2/5" means 2% initial cap, 2% periodic cap, 5% lifetime cap, but some lenders write the caps in different orders, (2) confusing teaser rate with note rate — the teaser is in effect ONLY during the fixed period and may bear no relation to future payments, (3) ignoring the index trajectory — SOFR moves with Fed policy, so estimating post-teaser payments requires modeling rate scenarios, (4) assuming you'll refinance "when rates drop" — refinance window depends on market conditions you can't control, and a refinance costs 2-5% of loan amount, (5) failing to compute payment shock at maximum allowed adjustment — under a 2/2/5 cap, a 5% start rate could climb to 7% in year 6 and 10% over the life of the loan. The CFPB CHARM Booklet (Consumer Handbook on Adjustable-Rate Mortgages) is the authoritative consumer guide; lenders are required to provide it to ARM applicants.
A fixed-rate mortgage has the same interest rate (and same monthly principal-plus-interest payment) for the entire loan term — typically 15 or 30 years. An ARM starts with a fixed rate for a defined introductory period (3, 5, 7, or 10 years), then adjusts periodically based on a market index plus a fixed margin. ARMs typically offer a LOWER initial rate than fixed-rate loans for the same term, but they carry the risk of higher payments after the fixed period ends. The CFPB CHARM Booklet explains the full mechanics.
A 5/1 ARM has a fixed interest rate for the first 5 YEARS, then adjusts ONCE PER YEAR thereafter based on the underlying index (typically 30-day Average SOFR or CMT) plus a fixed margin, subject to caps. A 5/6 ARM is similar but adjusts every 6 months after the fixed period. A 7/1 ARM has a 7-year fixed period; a 10/1 ARM has 10 years. Longer fixed periods typically carry slightly higher initial rates than shorter fixed periods because the lender takes more rate risk.
Caps limit how much the interest rate can move during an adjustment. A common structure is "2/2/5": 2% maximum initial adjustment (when the fixed period ends), 2% maximum per subsequent annual adjustment, and 5% lifetime cap above the START rate. Under that structure, an ARM that starts at 5.0% could climb at most to 7.0% at the first adjustment, and at most to 10.0% over the life of the loan. Some loans use "5/2/5" (5% initial cap) or "2/1/5" structures. The CFPB requires caps to be disclosed in the Loan Estimate.
ARMs are best suited for borrowers who EXPECT TO SELL or REFINANCE before the fixed period ends. If you're reasonably confident you'll move within 5 years, a 5/1 ARM at a meaningfully lower initial rate than a comparable 30-year fixed could save substantial interest over the holding period. ARMs are riskier when rates are at cycle lows (limited room to fall further) and safer when rates are at cycle highs (more likely to adjust DOWN). The CFPB guidance emphasizes stress-testing your budget against the maximum allowed payment under the cap structure before signing.
Most ARMs originated after 2022 reference the 30-day Average Secured Overnight Financing Rate (SOFR) or the Constant Maturity Treasury (CMT) index. SOFR replaced LIBOR after the 2023 LIBOR phase-out — LIBOR was discontinued because of fraud-related concerns and the lack of underlying transaction volume in the unsecured interbank lending market. SOFR is based on actual overnight Treasury repo transactions and is published daily by the Federal Reserve Bank of New York (also available on FRED). The margin on top of SOFR is typically 2.0%-3.0% for prime borrowers.
Yes. The post-teaser rate is index + margin, capped on the upside by the rate caps. If the index falls between adjustments, the new rate is lower (subject to the margin floor — some ARMs have a floor at or near the margin so the rate cannot drop arbitrarily low). The 2020-2021 low-rate environment is an example where many ARMs adjusted DOWN. Historical SOFR (or LIBOR before phase-out) data on FRED shows the full cycle of rate moves. The risk asymmetry — caps limit upside but few floors limit downside — means ARMs can occasionally produce favorable resets, though most homeowners refinance into fixed-rate loans during rate troughs rather than ride ARMs through them.
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