A fixed-rate mortgage locks your interest rate for the life of the loan, so your principal and interest payment never changes. An adjustable-rate mortgage (ARM) starts with a lower rate for an introductory period — usually 5 or 7 years — then adjusts periodically based on a market index plus a margin, within set caps. There's no universal winner: fixed rates suit borrowers who want certainty or plan to stay put, while ARMs suit borrowers who want a lower initial payment and expect to sell or refinance before the rate resets.
The comparison sounds abstract until you see it in dollars. Below, we run both loan types through the same $400,000 scenario used elsewhere on this site, so the numbers are directly comparable to the mortgage payoff and discount points articles.
What Is a Fixed-Rate Mortgage?
A fixed-rate mortgage charges the same interest rate for the entire term, whether that's 15, 20, or 30 years. Your principal and interest payment is set on day one and never moves, regardless of what happens to market rates afterward. Property taxes and insurance, if escrowed, can still change your total monthly bill, but the loan portion stays fixed.
This predictability is the entire appeal. You can budget years in advance knowing exactly what you owe each month, and you're fully insulated if rates rise later. The tradeoff is that fixed rates are typically priced higher than an ARM's introductory rate, since the lender is taking on the long-term interest rate risk instead of you.
What Is an Adjustable-Rate Mortgage (ARM)?
An ARM charges a fixed rate for an initial period — commonly 5 or 7 years, written as "5/1" or "7/1" — and then adjusts once a year for the remainder of the term. The new rate is calculated as a market index (such as SOFR) plus a fixed margin set by the lender. Because the borrower shares in the interest rate risk after the intro period, ARMs typically start at a lower rate than a comparable fixed loan.
Rate caps limit how much an ARM can move at each adjustment and over its lifetime. A common structure is written as "2/2/5": up to a 2% increase at the first adjustment, up to 2% at each adjustment after that, and a maximum of 5 percentage points above the starting rate over the life of the loan.
Key Differences at a Glance
| Feature | Fixed-Rate | 5/1 ARM |
|---|---|---|
| Rate structure | Locked for entire term | Fixed 5 years, then adjusts annually |
| Starting rate | Typically higher | Typically lower |
| Payment predictability | Fully predictable | Predictable for 5 years, then variable |
| Long-term rate risk | None — borne by lender | Borne by borrower after year 5 |
| Best fit | Staying long-term, want certainty | Selling or refinancing before adjustment |
Real Numbers: Fixed vs 5/1 ARM Payment Example
Using the same baseline as our other mortgage articles — a $400,000 loan at a 7% fixed rate over 30 years — the standard payment is $2,661/month and total interest over the full term is $558,036. Now compare that to a 5/1 ARM starting at a 6% introductory rate, a full percentage point lower.
At 6%, the ARM's initial payment is $2,398/month — about $263 less than the fixed loan every month for the first 5 years. Over those 60 months, the ARM borrower pays about $20,089 less in interest than the fixed-rate borrower, and the remaining loan balance after year 5 sits at roughly $372,217.
The real question is what happens after the intro period ends. We ran the remaining 25-year amortization under three adjustment scenarios, using a 2/2/5 cap structure so the rate can't exceed 11% (5 points above the 6% start rate) over the life of the loan.
| Scenario (after year 5) | New Payment | Total Interest, Full Loan | Vs. Fixed-Rate |
|---|---|---|---|
| Fixed-rate baseline (7%, unchanged) | $2,661 | $558,036 | — |
| ARM adjusts down to 5.5% | $2,286 | $429,614 | ~$128,400 saved |
| ARM adjusts to 7.5% | $2,751 | $569,088 | ~$11,050 more |
| ARM rises to lifetime cap (11%) | $3,648 | $838,338 | ~$280,300 more |
The spread between these scenarios is the entire risk-reward story of an ARM. If rates fall or stay near where they started, the ARM comes out ahead of the fixed loan on both monthly payment and lifetime cost. If rates climb toward the cap, the ARM can end up costing well over $100,000 more than the fixed-rate loan would have — even though it started cheaper.
Which Mortgage Is Better — Fixed or Variable?
There's no fixed answer that applies to everyone, but the decision usually comes down to two questions: how long you plan to keep the loan, and how much payment uncertainty you can tolerate.
If you plan to stay in the home for the full loan term, or you simply want to know your payment will never change, a fixed rate removes the guesswork entirely. If you're confident you'll sell or refinance within the intro period — say, you know this is a starter home or a short-term relocation — an ARM's lower initial rate can save real money with limited downside, since you'll be gone before any adjustment happens.
Is an ARM or Fixed Rate Better for You?
- Moving or refinancing within 5–7 years: An ARM likely saves you money, since you'll exit before the first adjustment.
- Staying 15+ years or unsure how long you'll stay: A fixed rate protects you from the scenario where rates rise and you're still holding the loan when it adjusts.
- Rates are currently high and expected to fall: An ARM lets you start lower now and potentially benefit if rates drop by your first adjustment.
- You need payment certainty for budgeting: A fixed rate is the safer choice regardless of where rates are headed.
Pros and Cons
- Payment never changes, for the full loan term
- Full protection if market rates rise later
- Simple to budget and compare across lenders
- No risk of "payment shock" at an adjustment date
- Lower starting rate and payment for the intro period
- Can save significantly if you sell or refinance early
- Payment can rise substantially after adjustment
- Best-case savings are smaller than worst-case costs
Run your exact numbers
Enter your loan amount and rate in the mortgage calculator to compare a fixed-rate payment against your own ARM scenario.