Fixed-Rate vs Adjustable-Rate Mortgage: How to Choose

The choice between a fixed-rate and an adjustable-rate mortgage (ARM) is really a choice about risk and time. A fixed rate gives certainty. An ARM offers a lower starting rate in exchange for the chance that your payment rises later.

How the two work

A fixed-rate mortgage keeps the same interest rate for the whole term, so principal and interest never change.

An ARM has a fixed rate for an initial period, then adjusts. A 5/6 ARM, for example, is fixed for five years and then adjusts every six months. At each adjustment the new rate is the market index plus a fixed margin, subject to caps. The initial rate is often lower than a comparable fixed rate, which is the appeal.

Understanding rate caps

Caps limit how far the rate can move and are usually written as three numbers. A 2/1/5 ARM means:

• the rate can rise at most 2 points at the first adjustment,
• at most 1 point at each later adjustment,
• and at most 5 points over the starting rate for the life of the loan.

Caps are what let you calculate a worst case before you sign.

A worked example

Take a $400,000 loan with a 5/6 ARM starting at 6.0%. The starting payment of principal and interest is $2,398.20 a month.

After five years, the balance has fallen to about $372,217. If the rate rose by the maximum 2 points at the first adjustment, to 8.0%, the payment on the remaining 25 years would be about $2,872.83, an increase of roughly $475 a month. With the lifetime cap the rate could climb to 11.0%, where the same balance would carry a payment of about $3,648, roughly $1,250 above the starting payment.

Rates could also fall or stay flat, and then the ARM would have cost less than a fixed rate. The ARM calculator shows the starting payment, the worst-case first adjustment, and the highest payment your caps allow, next to a fixed-rate comparison.

When a fixed rate fits

• You plan to stay in the home beyond the ARM’s fixed period.
• A payment increase would be hard to absorb.
• You value predictability, or you are on a tight budget.

When an ARM can make sense

• You expect to sell or refinance before the fixed period ends.
• You could comfortably afford the worst-case payment, not just the starting one.
• The initial rate is meaningfully lower than the fixed alternative, so you save real money during the fixed period.

Questions to ask a lender

1. What is the index, and what is the margin?
2. What are the initial, periodic and lifetime caps?
3. How often does the rate adjust after the fixed period?
4. Is there a prepayment penalty if I refinance?
5. What would my payment be at the maximum rate?

If you cannot answer the last question comfortably, a fixed rate is the safer choice.

Common questions

What happens at the end of the fixed period?

The loan begins to adjust. At each adjustment date, the lender looks up the index, adds the margin, and applies the caps to set your new rate. The payment is then recalculated so the remaining balance is paid off over the remaining term at the new rate.

Can I refinance before the rate adjusts?

Often yes, which is the plan many ARM borrowers have. The risks are that rates may be higher when you want to refinance, your home value may have fallen, or your finances may have changed so approval is harder. Do not rely on refinancing as the only plan.

Are ARMs riskier than they used to be?

Modern ARMs have caps and require lenders to qualify borrowers more carefully than the loans of the past, and the index is now generally a market rate such as SOFR. They still carry the basic risk that the payment can rise, so the key question remains whether you could afford the highest payment the caps allow.

Key takeaways

• A fixed rate gives certainty; an ARM trades a lower start for payment risk.
• Learn the caps, index and margin, and calculate the worst-case payment before you commit.
• Choose an ARM only if you can afford that worst case or are confident you will move first.