Most U.S. mortgages are fixed-rate, but adjustable-rate mortgages (ARMs) are worth understanding because they can be the right choice in specific situations. The decision comes down to how long you plan to stay and how much risk you accept in exchange for a lower starting payment.
How a fixed-rate mortgage works
A fixed-rate mortgage locks in your interest rate for the entire term. Your principal-and-interest payment never changes, which makes budgeting simple and protects you if rates rise. The trade-off is that the starting rate is usually higher than an ARM's introductory rate, and you do not benefit if market rates fall unless you refinance.
How an adjustable-rate mortgage works
An ARM has a fixed introductory period — commonly 5, 7, or 10 years — during which the rate stays the same. After that, the rate adjusts periodically based on a market index plus a margin. The most common structure is described as "5/1 ARM": a 5-year fixed period, then adjustments every 1 year.
- Introductory (fixed) period — the rate is locked, often lower than a fixed-rate loan.
- Adjustment period — the rate changes at set intervals after the intro period ends.
- Index — a published market rate the ARM is tied to.
- Margin — a fixed percentage the lender adds to the index.
- Caps — limits on how much the rate can rise per adjustment and over the life of the loan.
Comparing the two
| Feature | Fixed-rate | ARM |
|---|---|---|
| Payment stability | Never changes | Stable initially, then can change |
| Starting rate | Usually higher | Usually lower during intro period |
| Risk if rates rise | None — rate is locked | Payment can increase |
| Benefit if rates fall | Only via refinance | Payment may drop at adjustment |
| Best for | Long-term owners | Short-term owners |
When an ARM can make sense
- You plan to sell or refinance before the introductory period ends.
- You expect your income to rise and can absorb a higher payment later.
- The introductory savings are large enough to justify the risk.
When a fixed rate is the safer choice
- You plan to stay in the home long-term.
- You want predictable payments for budgeting.
- You cannot comfortably afford the payment if the rate rises to its cap.
For most buyers who plan to stay put, the predictability of a fixed rate outweighs the introductory savings of an ARM. But there is no universally correct answer — it depends on your timeline and risk tolerance.