ARM vs Fixed Rate: Which One Actually Fits Your Life

Ten years ago, almost nobody was shopping for an ARM. Fixed rates were so low that locking in for 30 years at 3 or 4 percent felt like a no brainer. Then rates jumped, and suddenly ARMs started showing up in loan quotes again. A guy I know bought a house last year with a 7/1 ARM at 5.9 percent while fixed rates were at 7.2 percent. His neighbor took a 30 year fixed at the same time. Who made the right call? The answer depends on things neither of them knew for sure.

An adjustable rate mortgage, or ARM, has an interest rate that can change over the life of the loan. The main appeal is a lower starting rate. The risk is that rate can rise later. Fixed rates, by contrast, lock in forever. Which one fits depends on your life more than on current market conditions.

How ARMs are structured

An ARM is named by two numbers like 5/1, 7/1, or 10/6. The first number is how many years the initial rate stays fixed. The second is how often the rate adjusts after that period ends. A 5/1 ARM has a fixed rate for five years, then adjusts every year after that. A 7/6 ARM has a fixed rate for seven years, then adjusts every six months.

Most ARMs today are 5/1, 5/6, 7/1, 7/6, or 10/1. The 1 or 6 at the end is how often rates change in the adjustment period.

The index and the margin

When your ARM adjusts, the new rate is set by adding a fixed margin to a market index. For current ARMs, the index is usually SOFR, the Secured Overnight Financing Rate, which replaced LIBOR a few years ago.

Margin is set at closing and does not change. A typical margin is 2.25 to 3 percent. If SOFR is at 4.5 percent at adjustment time, and your margin is 2.75, your new rate is 7.25 percent. The margin is the lender's spread. The index is market driven.

Rate caps protect you from the worst case

ARMs have caps that limit how much the rate can change. The structure is usually written as something like 2/2/5 or 5/2/5.

The first number is the initial cap, how much the rate can change at the first adjustment.

The second is the periodic cap, how much it can change at each subsequent adjustment.

The third is the lifetime cap, how much the rate can rise over the starting rate across the entire loan.

Example: a 7/1 ARM starting at 5.9 percent with 2/2/5 caps. At the first adjustment in year eight, the rate can rise by up to 2 percent, so max 7.9 percent. Each year after it can change by up to 2 percent. Lifetime max is 5.9 plus 5, equaling 10.9 percent.

The lifetime cap is the number that matters most. If you can afford the payment at the lifetime cap, you can survive the worst case. If you cannot, the ARM is too risky.

When ARMs make sense

Short time horizon in the house. If you know you will sell or refinance before the fixed period ends, the ARM gives you a lower rate for the whole time you have the loan. Military families moving every few years are classic ARM candidates.

Confidence that rates will fall. Nobody can predict rates with certainty, but if rates have risen sharply and the Fed is pivoting toward cuts, taking a 7/1 ARM when fixed rates are higher gives you downside protection. You get the lower starting rate, and if the market cooperates, your adjustment will be modest.

Significant income growth expected. If you are confident your income will be substantially higher in seven years, you can handle a potential rate hike at the adjustment. Early career professionals in high growth fields can play this.

Large additional savings to deploy. If you plan to pay down the loan aggressively before adjustments start, you reduce the balance that the higher rate applies to.

When fixed is clearly better

You plan to stay in the house a long time, more than ten years.

Your budget is tight and a rate hike would stress you out or force you to sell.

You want certainty for financial planning, especially around retirement.

Rates are historically low. If you can lock in a 30 year fixed at a rate you consider favorable, do it. The optionality is worth paying slightly more upfront.

The pain point most people miss

The ARM looks great on day one because the payment is lower. The real question is what happens on the first adjustment.

Run this stress test. Take your current ARM payment at the starting rate. Then recalculate it at the lifetime cap. Can you still make the payment comfortably? If yes, the ARM is a reasonable gamble. If no, you are hoping for luck.

For my guy's 7/1 at 5.9 percent on a 400,000 loan, his starting payment is 2,370. At the lifetime cap of 10.9 percent, the payment would be 3,775. That is a 60 percent increase. Could he still handle 3,775 a month in year nine? Honestly, he does not know. He is betting on rates coming down before adjustment.

The refinance escape hatch

The ARM strategy is often paired with the idea of refinancing before adjustment. Lock in a 7/1, enjoy seven years of lower payments, refinance into a fixed loan sometime around year six if rates are favorable.

The problem is you cannot control what rates will be in six years. If they are higher than they are now, refinancing locks in a worse rate. If they are lower, great. If they are the same, you refinanced for nothing and paid closing costs for the privilege.

Refinancing costs run about 3 percent of the loan amount, so 12,000 on a 400,000 loan. Build that into your math.

Interest only ARMs, mostly dead

A decade ago some ARMs had interest only features, where you did not pay down principal for the first several years. These helped cause the 2008 crisis and have mostly disappeared from mainstream lending. If a lender offers you an interest only ARM today, assume it is from a non traditional lender and look at it skeptically.

10/1 and 10/6 ARMs, the compromise

A 10/1 ARM gives you ten years of fixed rate before adjustments. For many buyers, this covers the full expected ownership period of a starter home or a career period. The starting rate is lower than a 30 year fixed, and you get long term certainty.

If you are drawn to an ARM but worried about the adjustment, a 10/1 is usually the sweet spot.

My own choice

I took a 30 year fixed when I bought mine. I like certainty. I also knew I did not want to be refinancing or stressing about rates five years in. I pay a little more each month than I would on an ARM, but I know what my payment will be in year twenty.

If you have the stomach for some uncertainty and your time horizon is clear, ARMs can save real money. If sleep matters to you, fixed is worth the small premium.