How a fixed-rate mortgage works
A fixed-rate mortgage locks the annual interest rate for the entire loan term at origination, so the monthly principal-and-interest payment calculated by the standard amortization formula, M = P[r(1+r)^n] / [(1+r)^n - 1], never changes for the life of the loan. Because the rate is fixed, only the split between interest and principal within each payment shifts over time as the outstanding balance declines -- the total payment amount itself stays constant from the first month to the last.
This structure gives a borrower complete certainty over the principal-and-interest portion of their housing cost for as long as they hold the loan, which simplifies household budgeting and removes exposure to future interest-rate increases. The trade-off is that a fixed-rate loan does not automatically benefit if market rates fall after origination; capturing a lower rate later generally requires refinancing, which carries its own closing costs and a breakeven calculation of its own.
How a variable-rate mortgage works
A variable-rate mortgage, including what is called an adjustable-rate mortgage (ARM) in the United States, ties the interest rate to a benchmark -- such as a central bank policy rate, a lender's standard variable rate, or an index like a Treasury or interbank rate -- plus a lender margin, and the rate resets at defined intervals rather than staying fixed for the full term. Many ARMs are structured as a hybrid, with an initial fixed period (commonly expressed as, for example, '5/1', meaning five years fixed followed by annual adjustments) before the rate begins moving with the benchmark.
Because the interest rate can rise or fall after origination, the monthly payment on a variable-rate loan is not fixed for the life of the loan the way a fixed-rate payment is. A variable rate that starts below the going fixed rate can produce real savings if benchmark rates stay flat or fall, but it also exposes the borrower to higher payments if the benchmark rises before the loan is paid off or refinanced.
Why the US market favors 30-year fixed loans
The 30-year fixed-rate mortgage is the dominant home loan product in the United States, a structure supported by a deep secondary mortgage market in which government-sponsored enterprises such as Fannie Mae and Freddie Mac purchase conforming loans from originating lenders, allowing lenders to offer a long-term fixed rate without holding all of the associated interest-rate risk themselves. Freddie Mac's weekly Primary Mortgage Market Survey (PMMS) is the standard benchmark cited for average US 30-year fixed rates, and the 15-year fixed loan is the main alternative, typically carrying a lower rate but a substantially higher monthly payment for the same principal.
Adjustable-rate mortgages remain available in the US market and can offer a lower initial rate than a comparable 30-year fixed loan, but they represent a minority of US purchase originations. Borrowers choosing a US ARM are effectively betting that they will sell, refinance, or otherwise no longer hold the loan before the initial fixed period ends and the rate becomes exposed to the benchmark.
Why the UK and Australia lean toward variable structures
UK mortgages are typically structured around a short initial fixed-rate deal -- commonly two or five years -- after which the loan automatically reverts to the lender's standard variable rate (SVR) unless the borrower proactively remortgages to a new deal. The SVR is set at the lender's discretion and is usually materially higher than available fixed or tracker deals, so most UK borrowers plan to remortgage before their fixed period ends rather than remain on the SVR; the Bank of England Bank Rate is the policy benchmark that most influences UK mortgage pricing over time.
Australian home loans are more commonly structured as variable-rate loans for the full term, often paired with an offset account -- a linked transaction account whose balance is subtracted from the loan balance before daily interest is calculated. The Australian Prudential Regulation Authority (APRA) has directed lenders to assess whether a borrower could still afford repayments at the loan's rate plus a serviceability buffer, set at 3 percentage points since late 2021, which is intended to test resilience against future variable-rate increases at the point of loan approval.
| Market | Dominant structure | What happens after the initial period |
|---|---|---|
| United States | 30-year fixed-rate mortgage | Rate never changes for the full term of the loan |
| United Kingdom | Short fixed deal (2 or 5 years) | Reverts to the lender's standard variable rate unless remortgaged |
| Australia | Variable rate, often with an offset account | Rate can change at any time the lender adjusts it |
Worked example: what a rate reset can cost
Consider a $300,000, 30-year loan that starts at a 5% annual rate. Using M = P[r(1+r)^n] / [(1+r)^n - 1] with r = 0.05/12 and n = 360, the initial monthly payment is $1,610.46. If this were a 5/1 ARM and the rate reset after 5 years (60 payments) to 7% for the remaining 25 years (300 payments), the outstanding balance at the reset point would be $275,486.20, and recalculating the payment at the new rate over the remaining term gives $1,947.08 -- an increase of $336.61 per month, or approximately 20.9% higher than the original payment.
This example illustrates the mechanism rather than a prediction: the direction and size of an actual reset depends entirely on how the relevant benchmark has moved by the time the fixed or introductory period ends, and a reset could just as easily lower the payment if benchmark rates have fallen. The key structural fact is that a variable-rate borrower does not know, at origination, what the post-reset payment will be, while a fixed-rate borrower does.
Weighing certainty against a potentially lower rate
The choice between a fixed and a variable rate is fundamentally a trade-off between payment certainty and the possibility of a lower average rate over the life of the loan. A fixed rate is generally favored by borrowers who value predictable budgeting, plan to hold the property and the loan for a long period, or are concerned about future rate increases; a variable rate can appeal to borrowers who expect to sell, refinance, or pay off the loan well before any reset, or who are comfortable absorbing payment volatility in exchange for a potentially lower starting rate.
In markets where variable structures dominate, such as the UK and Australia, borrowers effectively manage this trade-off by actively remortgaging or watching lender rate changes rather than by choosing a single rate structure for the full loan term the way a US 30-year fixed borrower does. Understanding which structure -- and which reset mechanism -- applies to a specific loan offer is a prerequisite for comparing it meaningfully against alternatives.
Часто задаваемые вопросы
What is the difference between a fixed and a variable mortgage rate?
A fixed-rate mortgage locks the interest rate, and therefore the principal-and-interest payment, for the entire loan term, so the payment never changes regardless of what happens to market rates. A variable-rate mortgage ties the rate to a benchmark -- a central bank rate, a lender's standard variable rate, or a market index -- that can move at defined intervals, meaning the payment can rise or fall over the life of the loan.
Why is the 30-year fixed mortgage so common in the United States?
The 30-year fixed-rate mortgage dominates the US market because government-sponsored enterprises such as Fannie Mae and Freddie Mac purchase conforming loans from lenders on the secondary market, allowing lenders to offer long-term fixed rates without retaining all of the associated interest-rate risk. Freddie Mac's Primary Mortgage Market Survey (PMMS) is the standard reference for average US 30-year fixed rates, and adjustable-rate mortgages remain a minority of US purchase loans by comparison.
What happens when a UK fixed-rate mortgage deal ends?
When a UK fixed-rate deal ends, the loan automatically reverts to the lender's standard variable rate (SVR) unless the borrower arranges a new fixed, tracker, or other deal beforehand. The SVR is set at the lender's discretion and is typically higher than available fixed or tracker deals, which is why most UK borrowers remortgage shortly before their existing fixed period expires rather than remain on the SVR.
Are Australian home loans usually fixed or variable?
Variable-rate loans are prevalent in the Australian market, often combined with an offset account -- a linked transaction account whose balance reduces the loan balance used to calculate daily interest. The Australian Prudential Regulation Authority (APRA) requires lenders to test loan applications against the rate plus a serviceability buffer of 3 percentage points, reflecting the expectation that a variable rate can change over the life of the loan.
How much can a monthly payment change when a variable rate resets?
The change depends entirely on how far the relevant benchmark has moved. In an illustrative example, a $300,000, 30-year loan starting at 5% has an initial payment of $1,610.46; if the rate reset to 7% after 5 years, the payment on the remaining balance over the remaining 25 years would rise to $1,947.08, an increase of $336.61 per month, or about 20.9%. A reset to a lower rate would instead reduce the payment.
Is a fixed or variable rate better?
Neither is universally better; the choice depends on how long the borrower expects to hold the loan and their tolerance for payment uncertainty. A fixed rate suits borrowers who value predictable payments and expect to hold the loan long-term, while a variable rate can suit borrowers who expect to sell or refinance before a reset, or who are comfortable with the possibility of a higher future payment in exchange for a potentially lower starting rate.
Источники
- Freddie Mac. Primary Mortgage Market Survey (PMMS) — weekly mortgage rate averages. freddiemac.com/pmms.
- Consumer Financial Protection Bureau (CFPB). Fixed-rate versus adjustable-rate mortgages. consumerfinance.gov.
- Federal Reserve Board. A consumer's guide to mortgage refinancing. federalreserve.gov.
- Financial Conduct Authority (FCA). Mortgages and Home Finance: Conduct of Business (MCOB) sourcebook. fca.org.uk.
- MoneyHelper (Money and Pensions Service). Mortgages: deposits, LTV and standard variable rates. moneyhelper.org.uk.
- Australian Prudential Regulation Authority (APRA). Guidance on serviceability standards for residential mortgage lending. apra.gov.au.
- Brueggeman WB, Fisher JD. Real Estate Finance and Investments. 15th ed. McGraw-Hill Education, 2019.