When you take out a mortgage, the biggest choice you make after the loan amount and term is whether the interest rate is fixed or adjustable. A fixed-rate mortgage locks your interest rate for the entire loan term — usually 15 or 30 years. An adjustable-rate mortgage (ARM) starts with a lower introductory rate that resets periodically based on market conditions. ARMs were widely blamed for the 2008 housing crisis, but the product itself isn’t the problem — it’s whether borrowers understand what they’re signing. This guide explains how each works, what they cost, and when each makes sense.
Fixed-Rate Mortgages
The interest rate is set at closing and never changes. Your principal and interest payment is identical for the life of the loan (taxes and insurance can still fluctuate). The most common terms are:
- 30-year fixed — the most popular by a wide margin. Lower monthly payment, but more total interest over the life of the loan
- 15-year fixed — higher monthly payment, much less total interest, faster equity build-up. Common for refis and for buyers who can comfortably afford the higher payment
- 20-year, 25-year, 10-year — less common; available from many lenders
The trade-off is clear: a fixed-rate mortgage gives certainty (you know exactly what you’ll pay for 30 years) in exchange for typically paying 0.25-0.75 percentage points more than an ARM’s initial rate.
Adjustable-Rate Mortgages (ARMs)
An ARM has two phases:
- Initial fixed period — the rate is locked at a typically lower rate than a comparable fixed-rate mortgage. Common lengths: 5, 7, or 10 years
- Adjustment period — after the initial period, the rate adjusts periodically (usually every 6 or 12 months) based on a market index plus a margin
Naming convention: a “7/6 ARM” has a fixed rate for the first 7 years, then adjusts every 6 months. A “5/1 ARM” is fixed for 5 years then adjusts annually. (Older “5/1” naming has largely been replaced by “5/6” as ARMs moved from 1-year to 6-month adjustments after the LIBOR-to-SOFR transition.)
How the Rate Resets
At each adjustment, the new rate is calculated as:
New rate = Index + Margin (subject to caps)
- Index — a market reference rate that changes daily. Most current ARMs use SOFR (Secured Overnight Financing Rate). Older ARMs used LIBOR or Treasury yields. The lender does not control this.
- Margin — a fixed markup set at loan origination, typically 2.0-3.5 percentage points. The lender’s profit and the riskier-loan premium. This number does not change for the life of the loan.
Caps limit how much the rate can change:
- Initial cap — maximum rate change at the first adjustment (often 2%)
- Periodic cap — maximum change at each subsequent adjustment (often 1-2%)
- Lifetime cap — maximum increase above the starting rate over the loan’s life (often 5%)
So a 5/6 ARM starting at 5.5% with caps of 2/1/5 would have:
- Initial rate (years 1-5): 5.5%
- Maximum rate at first reset (year 5.5): 7.5%
- Maximum subsequent change every 6 months: +/-1%
- Lifetime maximum rate: 10.5%
Caps are your protection but they also reveal the real worst case: in this example, your rate could nearly double over the life of the loan if market rates spike and stay high.
When ARMs Make Sense
- You know you’ll sell or refinance within the fixed period — if you’ll be out of the house in 5-7 years (job change, growing family, retirement relocation), a 5/6 or 7/6 ARM may save real money during the years you actually have the loan
- You expect rising income — if your salary will be significantly higher by the time the rate adjusts, you can absorb increases
- You’re paying down principal aggressively — if you’ll have substantial equity (or even pay off the loan) before the fixed period ends
- Rate environment favors ARMs — when the spread between ARM and fixed rates is wide (>0.75%), the trade-off improves. When the spread is narrow, fixed is usually the better deal
- You can comfortably afford the maximum payment — not the starting payment. If you can’t make ends meet at the lifetime-cap rate, an ARM is too risky
When Fixed Is the Safer Choice
- You plan to stay 10+ years — the longer the time horizon, the more you benefit from fixed-rate certainty
- Tight budget — if a 2-3 percentage point rate jump would strain your budget, you can’t safely take an ARM
- Rates are at historic lows — locking in a low rate for 30 years is valuable; ARMs make less sense when there’s little room for rates to fall further
- You value predictability over savings — for many people, the certainty of a fixed payment is worth a slightly higher rate
- You’re stretching to afford the home — if you’re using ARM rates to qualify for a more expensive house, you’re taking on risk you can’t absorb if rates rise
Questions to Ask About Any ARM
- What is the initial fixed period? After that, how often does the rate adjust?
- What index is used? What’s the current value of that index?
- What is the margin?
- What are the rate caps (initial, periodic, lifetime)?
- What is the maximum monthly payment at the lifetime cap, on the original loan balance?
- Is there a prepayment penalty? (Most modern ARMs do not have them, but verify)
- How does the ARM reset compare to fully indexed rate today — would your rate rise at first adjustment even if the index doesn’t change?
Have your loan officer run an amortization schedule showing both the best case (rate stays low) and worst case (rate climbs to caps) for each year. If the worst case isn’t affordable, don’t take the ARM.
Educational only. ARM structures, indexes, margins, and caps vary by lender and loan program. Rate environments and the spread between ARM and fixed-rate mortgages change continuously. Consult a loan officer and consider running scenarios with a mortgage calculator before deciding.