Fixed vs Adjustable Rate Mortgage

The choice between a fixed and an adjustable rate is the most consequential decision in a mortgage after the amount you borrow, and it is frequently made on a single number: whichever rate looks lower today. That number is the least useful part of the comparison, because an ARM's introductory rate is designed to look lower and says nothing about what you will pay in year seven.

This guide explains how each structure behaves, what the caps actually protect you against, and the specific circumstances where each wins. The mortgage calculator models a fixed rate, which is also how an ARM behaves during its introductory period.

How a fixed-rate mortgage works

The interest rate is set at closing and never changes. On a 30-year fixed loan, the principal-and-interest portion of your payment in month 360 is identical to month one.

The total payment can still move, because property taxes and insurance are collected into escrow and those change over time — a point covered in what is PITI. But the loan portion is genuinely fixed.

The consequence is that inflation works quietly in your favour. A payment that consumes a third of your income today consumes a smaller share in fifteen years if your income rises, because the payment does not.

How an adjustable-rate mortgage works

An ARM has two phases.

The introductory period. The rate is fixed for a set number of years — commonly five, seven, or ten — and typically starts below the prevailing fixed rate. This discount is the entire commercial appeal.

The adjustment period. After that, the rate resets periodically according to a formula: index + margin.

The index is a published benchmark that moves with market conditions; most US ARMs written today use SOFR. The margin is a fixed number of percentage points your lender adds, set at closing and unchanged for the life of the loan. If the index sits at 4% and your margin is 2.75%, the fully indexed rate is 6.75%.

The naming convention tells you the schedule. A 5/1 ARM is fixed for five years then adjusts annually. A 7/6 ARM is fixed for seven years then adjusts every six months. The first number is the fixed period, the second the adjustment frequency.

Understanding the caps

Caps are the guardrails, and reading them correctly is the difference between an informed decision and a gamble. They come as three numbers, often written like 2/2/5:

Initial adjustment cap. How much the rate can move at the first reset. In the example above, 2 percentage points.

Subsequent adjustment cap. How much it can move at each later reset. Again 2.

Lifetime cap. How far the rate can rise above the initial rate across the whole loan. Here, 5 percentage points.

That lifetime cap defines your worst case, and it is the number to plan around. A 5/1 ARM starting at 5.5% with a 5-point lifetime cap can reach 10.5%. On a $320,000 loan, that is the difference between roughly $1,817 and $2,927 a month — over $1,100 more, for the same debt.

Before signing an ARM, calculate the payment at the lifetime cap and ask whether you could carry it. If the answer is no, the ARM is a bet rather than a plan.

When each makes sense

Choose a fixed rate if you expect to stay beyond the introductory period, you value predictability, your budget has limited slack, or rates are low relative to recent history. A fixed rate is also the simpler product, and simplicity has real value in a thirty-year commitment.

Consider an ARM if your horizon is genuinely shorter than the fixed period — military relocation, a role with a known end date, a starter home you expect to outgrow. If you will be gone before the first adjustment, the introductory discount is free money and the caps never matter.

An ARM can also make sense when fixed rates are unusually high and you expect to refinance later. But note what that reasoning depends on: a forecast about future rates, and your ability to qualify for a refinance when the time comes. Both can fail, and they tend to fail together — rates stay high precisely when the economy makes qualifying harder.

Advantages of each

Fixed rate. Complete payment certainty for the loan portion. Immunity from rate increases. Simple to understand and to compare between lenders. Inflation erodes the real burden over time.

Adjustable rate. A lower initial rate, which means either a lower payment or a larger loan for the same payment. Genuine savings if you exit before adjustment. And if rates fall, the payment falls without refinancing or closing costs — something a fixed-rate borrower cannot achieve without paying for a refinance.

Disadvantages of each

Fixed rate. You are locked in if rates fall; capturing a lower rate requires refinancing and paying closing costs, which the refinance calculator can evaluate. You also pay a premium for the certainty, since fixed rates typically start above ARM introductory rates.

Adjustable rate. Payment uncertainty after the fixed period, with a worst case that can be genuinely unaffordable. More complex terms, and more places for a borrower to misunderstand what they signed. And the exit strategy — refinance or sell — may be unavailable exactly when you need it.

A worked comparison

Consider a $320,000 loan over 30 years.

A fixed rate at 6.5% gives a principal-and-interest payment of about $2,023, unchanged for the whole term.

A 5/1 ARM at 5.5% starts at about $1,817 — a saving of roughly $206 a month, or about $12,400 across the five fixed years.

What happens next decides everything. If you sell or refinance in year five, you banked the $12,400 and never faced an adjustment. If rates fell, your payment may drop further. If the rate adjusts upward toward the lifetime cap, the higher payments can erase that saving within a few years and keep going for the remaining twenty-five.

The ARM is not cheaper. It is cheaper first, and the question is whether you will still be there when the bill arrives.

Common mistakes

Comparing only the introductory rates. The relevant comparison is the fixed rate against the ARM's likely and worst-case rates across your actual holding period.

Assuming you will definitely refinance or sell. Plans change, and the ability to refinance depends on rates, home value, credit, and income at that future moment.

Ignoring the margin. The index moves, but the margin is fixed and permanent. A loan with an attractive introductory rate and a wide margin can be expensive after adjustment even if the index behaves.

Reading caps as protection rather than limits. A 5-point lifetime cap does not mean the rate will not rise much. It means it can rise five points.

Qualifying at the introductory payment. Some borrowers stretch to the maximum on the intro rate, leaving no room for any adjustment at all.

Practical tips

Ask for the index, the margin, all three caps, and the first possible adjustment date in writing. Those five facts fully describe the loan.

Calculate the payment at the fully indexed rate today — index plus margin, as if adjustment happened now. That is a more realistic view than the introductory rate.

Then calculate it at the lifetime cap and confirm you could carry it. If not, choose fixed.

Match the fixed period to your actual horizon with room to spare. If you think you will move in five years, a 7/1 gives a buffer that a 5/1 does not.

Compare the APR rather than the rate when weighing offers, since it folds in points and lender fees.

Where to go next

Model the payment at any rate in the mortgage calculator, and run it twice — once at the introductory rate, once at the lifetime cap. Use the refinance calculator to test whether escaping an ARM later would actually pay for itself. And to see how the interest-to-principal split behaves over the fixed period, the amortization calculator shows the schedule row by row.

This guide is educational and does not constitute financial advice. Consult a qualified professional or your lender about your own situation.