Choosing between a fixed rate and adjustable rate mortgage is one of the most consequential financial decisions you’ll make when buying a home. The wrong choice can cost you tens of thousands of dollars over the life of the loan. The right choice depends on how long you plan to stay in the house, your risk tolerance, and what interest rates are doing when you apply.
How fixed rate mortgages work
A fixed rate mortgage locks in your interest rate for the entire loan term, typically 15 or 30 years. If you get a 30 year fixed at 6.5%, your rate stays 6.5% whether market rates drop to 4% or spike to 9%. Your monthly principal and interest payment never changes.
The predictability is the main appeal. You can budget decades ahead knowing exactly what your mortgage payment will be. Property taxes and insurance may change, but the loan payment itself is set in stone.
The downside is that fixed rate mortgages usually carry higher initial interest rates than adjustable rate mortgages. Lenders charge a premium for the certainty they’re providing you. In a typical market, a 30 year fixed rate might be 0.5% to 1% higher than the initial rate on a comparable ARM.
Most homebuyers choose fixed rate mortgages, and for good reason. If you plan to stay in the home for 10+ years and want simplicity, a fixed rate is the safer choice. You’ll never wake up to a surprise increase in your housing payment.
How adjustable rate mortgages work
An adjustable rate mortgage (ARM) starts with a fixed rate period, usually 5, 7, or 10 years, then adjusts annually based on a market index plus a margin. A 5/1 ARM means the rate is fixed for 5 years, then adjusts once per year. A 7/6 ARM is fixed for 7 years, then adjusts every 6 months.
During the fixed period, the rate is typically lower than a comparable fixed rate mortgage. That initial savings can be substantial. On a $400,000 mortgage, the difference between 6.5% and 5.75% is about $180 per month, or over $10,000 during the first five years.
After the fixed period ends, the rate adjusts based on a benchmark index (commonly the Secured Overnight Financing Rate, or SOFR) plus a margin that’s set in your loan terms, usually 2% to 3%. If SOFR is 4% and your margin is 2.5%, your new rate would be 6.5%.
ARMs have caps that limit how much the rate can change. There are typically three caps: the initial adjustment cap (how much the rate can change at the first adjustment, often 2%), the periodic cap (how much it can change at each subsequent adjustment, often 2%), and the lifetime cap (the maximum the rate can ever reach, often 5% above the initial rate).
When an ARM makes sense
ARMs work well in specific situations. If you’re confident you’ll sell or refinance within the fixed period, you get the benefit of the lower rate without facing the adjustment. This is common for people who know they’ll relocate for work within 5 to 7 years, or first time buyers who expect to outgrow a starter home.
In a high rate environment, an ARM can be a reasonable bet if you expect rates to drop in the coming years. You take the lower ARM rate now and plan to refinance into a fixed rate when market rates come down. This is a bet on future rate movements, and bets can go wrong, but it’s not unreasonable.
ARMs also make sense for high income borrowers who can absorb payment increases without financial stress. If your mortgage payment is 15% of your income and a worst case rate adjustment would push it to 20%, that’s manageable. If it would push you to 35%, that’s dangerous.
When a fixed rate is the clear winner
If you plan to stay in the home for 15+ years, a fixed rate is almost always better. The math only favors the ARM if rates happen to be lower during the adjustment period, and over a 15 to 30 year window, rates are highly unpredictable.
If your budget is tight and a payment increase would cause real hardship, choose the fixed rate. The peace of mind alone is worth the slightly higher cost. Nobody wants to worry about their mortgage payment changing while they’re also dealing with job uncertainty or family expenses.
During periods of historically low rates (like 2020-2021, when 30 year fixed rates dipped below 3%), fixed rate mortgages are no brainers. You’re locking in a rate that’s likely lower than most ARMs will adjust to. Those homeowners are sitting on some of the cheapest money in history.
The refinancing factor
Many ARM borrowers plan to refinance before the adjustable period begins. This can work, but it’s not guaranteed. Refinancing requires you to qualify for a new loan at that future time. If your income has dropped, your credit has declined, or your home has lost value, refinancing may not be available on favorable terms.
Refinancing also has costs: typically 2% to 5% of the loan amount in closing costs. On a $400,000 loan, that’s $8,000 to $20,000. Factor these costs into any ARM versus fixed rate comparison.
The people who get burned by ARMs are usually those who planned to refinance but couldn’t. The 2008 housing crisis was driven in large part by ARM borrowers whose rates adjusted sharply upward while their home values fell, trapping them in unaffordable payments with no refinancing option.
Running the numbers
Here’s how to compare directly. Take the fixed rate payment and the ARM payment during the initial period. Calculate the total savings during the ARM’s fixed period. Then model what happens if the ARM adjusts to its maximum rate. How long would it take for the higher ARM payments to erase the initial savings?
If the break even point is well beyond your expected time in the home, the ARM might be worth considering. If it’s close to or within your expected ownership period, the fixed rate gives you more certainty for a minimal cost difference.
Most mortgage calculators can model ARM scenarios with different rate assumptions. Run at least three: rates stay flat, rates drop 1%, and rates rise to the cap. If you’re comfortable with the worst case, the ARM could make sense. If the worst case keeps you up at night, get the fixed rate and stop worrying.