A mortgage is usually the largest loan a household will ever take, and the biggest structural choice in it is how the interest rate behaves over time. You can lock one rate for the life of the loan, or accept a rate that starts lower and changes later. Each approach suits a different kind of borrower.
Key takeaways
- A fixed rate never changes; an ARM starts lower and can rise after the initial period.
- Caps set the worst case for an ARM, so always ask for the maximum payment.
- ARMs suit shorter stays; fixed rates suit long stays and tight budgets.
How a fixed-rate mortgage works
A fixed-rate mortgage keeps the same interest rate from the first payment to the last. Your principal and interest payment never changes, although taxes and insurance collected through escrow can still rise. Common terms include 15 and 30 years.
The appeal is certainty. You can plan decades ahead, and rising market rates do not affect you. The trade-off is that fixed rates are often higher at the start than adjustable ones, and if market rates fall you need to refinance, with its costs, to benefit.
How an adjustable-rate mortgage works
An adjustable-rate mortgage (ARM) starts with a fixed rate for an initial period, often several years. After that, the rate adjusts at set intervals. The new rate is based on a market index plus a fixed margin set in your loan agreement.
Caps limit how much the rate can change: one cap for the first adjustment, one for each later adjustment and one for the life of the loan. Your loan documents list these caps, and they tell you the worst case you could face.
Side-by-side comparison
| Fixed-rate | Adjustable-rate (ARM) | |
|---|---|---|
| Starting rate | Usually higher | Usually lower |
| Payment changes | Principal and interest never change | Can rise or fall after the initial period |
| Protection from rising rates | Full | Limited by caps |
| Benefit when rates fall | Only by refinancing | Rate can adjust down |
| Best for | Long stays, tight budgets | Shorter stays, flexible budgets |
Which one fits
- Fixed suits you if you plan to stay in the home for many years or could not absorb a higher payment.
- An ARM can make sense if you expect to sell or refinance before the initial period ends, and could handle the payment at its cap if plans change.
Tip: Ask every lender for the payment at the ARM’s maximum rate. If that number would break your budget, the lower starting rate is not worth the risk.
What lenders look at
Mortgage pricing depends on your credit history, income, existing debts, the size of your down payment and the property itself. Lenders often qualify ARM borrowers at a rate above the starting rate, to make sure you could handle an adjustment. A larger down payment can also remove the need for mortgage insurance on conventional loans, which changes the monthly cost as much as the rate choice does.
How to compare offers step by step
- Request a Loan Estimate from at least two or three lenders on the same day, for the same loan type and term.
- Compare the rate, the APR and the closing costs on page two of each estimate.
- For an ARM, find the initial period, the index, the margin and every cap.
- Work out the payment at the ARM’s maximum possible rate.
- Check whether points are included. Paying points lowers the rate but raises upfront costs.
Mistakes to avoid
Choosing an ARM because the first payment fits the budget, with no plan for the adjustment, is the classic error. So is assuming you will be able to refinance before the rate resets. Refinancing depends on future rates, your credit and your home’s value, none of which are guaranteed. On the fixed side, the most common mistake is overpaying in points for a lower rate when you are likely to sell or refinance within a few years, before the upfront cost pays for itself.
Common questions
What do ARM names like 5/1 or 7/6 mean?
The first number is the length of the initial fixed period in years. The second shows how often the rate adjusts afterwards, for example once a year or every six months.
Is the APR useful for comparing an ARM?
It helps, but an ARM’s APR is based on assumptions about future rates. Compare the initial rate, the caps and the worst-case payment as well.
Can I switch from an ARM to a fixed rate later?
Usually only by refinancing, which depends on your credit, home value and market rates at that time. Do not count on it as a guaranteed exit.
Are there fees to switch loans later?
Refinancing usually involves closing costs similar to your original mortgage. Include those costs when you estimate whether a later switch would save money.
Already own your home and want to borrow against it? Compare a home equity loan with a HELOC.