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You have two maximum mortgages, not one. There is the amount a lender will approve, which comes from a ratio applied to your gross income. And there is the amount your actual monthly budget survives, which is usually smaller, because it accounts for childcare, pension contributions, maintenance and the fact that you would like to eat out occasionally. Borrow against the second one.
The fastest way to find that second number is to run the calculation backwards. Instead of picking a house price and asking what it costs per month, pick the monthly payment you can genuinely carry and ask what loan it buys. That direction takes one multiplier and about thirty seconds.
Every £1 of payment buys a fixed amount of loan
At a given rate and term, the relationship between a monthly principal-and-interest payment and the loan it services is fixed:
Loan = monthly payment × (1 − (1 + i)^−n) ÷ i
where i is the annual rate divided by 12 and n is the term in months. That multiplier only depends on the rate and the term, so you can compute it once and reuse it for every scenario. Here it is for the two most common terms, expressed as the loan supported by each 1,000 of monthly payment.
| Interest rate | Per 1,000/month over 30 years | Per 1,000/month over 25 years |
|---|---|---|
| 4.0% | 209,461 | 189,452 |
| 4.5% | 197,362 | 179,910 |
| 5.0% | 186,282 | 171,060 |
| 5.5% | 176,122 | 162,843 |
| 6.0% | 166,792 | 155,207 |
| 6.5% | 158,211 | 148,103 |
| 7.0% | 150,308 | 141,487 |
| 7.5% | 143,018 | 135,320 |
Read it in whichever currency you use — the arithmetic is currency-blind. If you can carry $2,200 a month in principal and interest at a quoted 6.5% over 30 years, your loan ceiling is 2.2 × 158,211 = about $348,100. A UK buyer with £1,400 a month and a 4.5% offer over 25 years gets 1.4 × 179,910 = about £251,900.
Notice how much the rate matters, which is the part that gets asserted and never shown. Between 5.0% and 7.5% on a 30-year term, the same payment buys 23% less house. Nothing about your income changed.
This is the opposite direction from the mortgage calculator, which takes a price and produces a payment, shows the full formula and prints an amortisation schedule. Use this guide to find the payment; use the calculator to confirm it.
What "maximum" means depends on which country you are in
The number a lender approves comes from a ratio, and the ratio differs by market. Two questions get you what you need from any of them: what ratio do you apply, and what interest rate do you stress-test me at?
United States. Underwriting runs on two debt-to-income ratios. The front-end ratio is your total monthly housing cost divided by gross monthly income. The back-end ratio is every required monthly debt payment — housing, car loans, student loans, minimum card payments — divided by the same gross income. The limits vary by loan programme, so ask your loan officer for the specific figures rather than trusting a rule of thumb you read somewhere. The debt-to-income calculator works out where you currently sit.
The back-end ratio is why existing debt is expensive in a way most articles fail to quantify. Every dollar of monthly debt payment is a dollar that cannot service a mortgage, so it costs you the multiplier. A $400 car payment at 6.69% over 30 years removes 0.4 × 155,131 = about $62,000 of borrowing capacity. Clearing a small balance can be worth more than months of extra deposit saving.
United Kingdom. Lenders start from an income multiple, then run an affordability assessment on a stressed rate — a rate above the one you are actually being offered — to check the payment still works. The multiple is a starting point, not a rule, and the stress rate is set by the lender, so both are worth asking about directly. For scale, the median home in England cost 7.6 times the median full-time employee's earnings in 2025, £300,000 against earnings of £39,300, per the Office for National Statistics — which uses five times earnings as its broad affordability benchmark, so the typical English home now sits well above it. The Wales figure in the same release is 6.0 times earnings.
Canada. Lenders use gross debt service and total debt service ratios, the same idea as the US front-end and back-end pair. As a national reality check, Canadian households were already putting 14.75% of disposable income toward required debt principal and interest in the first quarter of 2026, according to Statistics Canada — and that is the average across all households, mortgaged or not.
Australia and Ireland. Australian lenders assess serviceability against the product rate plus a buffer, using a benchmark measure of household expenditure rather than only your stated spending. Irish lending sits under central loan-to-income and loan-to-value limits. In both cases, ask for the assessment rate — it, not the headline rate, sets your ceiling.
The price you are borrowing against, by market
What a typical home costs
- USUnited States$440,600median existing-home sale price, June 2026An all-time high and the 36th straight month of annual growth. Newly built homes are a separate, lower series at $398,300.National Association of Realtors, Existing-Home Sales
- UKUnited Kingdom£271,000average house price, May 2026Up 2.7% over the year, provisional. £292,000 in England, £215,000 in Wales, £196,000 in Scotland.ONS and HM Land Registry, UK House Price Index
- CACanadaC$696,078average residential sale price, June 2026Skewed upward by sales mix and by Toronto and Vancouver. CREA's own Home Price Index was down 3.6% year on year.Canadian Real Estate Association, National Statistics
- AUAustraliaA$1,111,100mean dwelling price, March quarter 2026A mean rather than a median, so it sits above the typical sale price. Average new owner-occupier loan was about A$735,000.Australian Bureau of Statistics, Total Value of Dwellings
The mortgage payment is not the housing cost
This is the single most common way an affordability estimate goes wrong. The loan payment is one line of a housing budget, not the whole of it.
In the US, the monthly cheque is PITI: principal, interest, taxes and insurance, with private mortgage insurance added below 20% equity and an HOA fee on top where one applies. On the calculator's own worked example — a $440,000 home, 20% down, 6.69% over 30 years — principal and interest is $2,269.04 but the actual monthly outflow is $2,859.04 once property tax and insurance are escrowed. Tax and insurance add 26% on top of the loan payment, and they keep rising after the loan payment has been frozen for thirty years. Housing already takes 33.4% of the average US household budget, $26,266 a year, according to the Bureau of Labor Statistics Consumer Expenditure Survey for 2024 — the largest category by a wide margin.
In the UK the extras are different but no smaller: Council Tax, buildings insurance, and on a leasehold flat a service charge and ground rent that the lender will factor into affordability and that can move sharply between years. ONS Family Spending puts mortgage interest, Council Tax and related payments at £65.30 a week for the average UK household in the year to March 2025, £32.30 of it Council Tax (ONS, Family spending workbook 1). That average includes outright owners who pay no mortgage at all, so treat it as a floor for anyone actually buying.
The practical rule: before you compare a mortgage payment with your current rent, add the tax, the insurance and any service charge to the mortgage side. Otherwise you are comparing a whole cost against a partial one. The budget categories list has the full set of lines a homeowner picks up.
Rates move, so your maximum moves with them
A borrowing ceiling calculated at today's quote is a snapshot, not a fact. The average quoted rate on a UK two-year fix at 75% loan-to-value moved more than a full percentage point inside seven months in 2026.
UK average quoted two-year fixed mortgage rate, 75% LTV
Monthly averages of advertised rates, January to July 2026
Source: Bank of England, Interactive Statistical Database, series IUMBV34
Two consequences. First, stress-test your own number: re-run it one percentage point above the rate you are quoted. On a $352,000 US loan, half a point takes the payment from $2,269.04 to $2,386.95 — $118 a month, every month, for thirty years. Second, if you are in a market where fixed deals expire, plan for the reversion. The average UK lender revert-to rate, the old standard variable rate, was 6.60% in July 2026 against 4.79% on a new two-year fix (Bank of England, series IUMTLMV). On a £270,000 loan over 25 years, that is the difference between £1,545.54 and £1,839.97 a month — £294 for doing nothing when your deal lapses.
Term is the other lever, and it is not free. That same £270,000 at 4.79% costs £1,545.54 a month over 25 years and £1,326.71 over 35 — £219 a month cheaper, at a cost of £93,558 in extra interest. Stretching the term to reach a price is a decision to pay for the house twice over rather than a way of affording it.
The cash you need that is not the deposit
The deposit is the visible number. It is not the only cash that has to be on the table on completion day.
In England and Northern Ireland, Stamp Duty Land Tax starts on the portion of the price above £125,000, and first-time buyers pay nothing up to £300,000 — but lose the relief outright above £500,000 (GOV.UK). It is a cliff, not a taper. A first-time buyer at £500,000 pays £10,000 in SDLT; at £510,000 the relief vanishes and the standard bands apply, producing a £15,500 bill. Ten thousand pounds more house costs £5,500 more tax. Scotland and Wales run their own transaction taxes with different thresholds entirely.
Work the tax through on a number of your own. On a £330,000 purchase, say, with first-time buyer relief, SDLT is 5% of the £30,000 above the £300,000 threshold — £1,500 — and that is before survey, conveyancing, searches and removals.
US buyers face a different list — appraisal, title insurance, origination and discount points, prepaid escrow — itemised on the Loan Estimate the lender must give you. Ask for it early: the number is specific to your lender and your state, and no national average will predict it.
The rule that matters in every market: do not fund any of this from your emergency fund. Buying a house is precisely when the boiler fails. Size the buffer separately, using how much emergency fund you need, and keep the deposit target in the savings goal calculator so the two never get confused.
Two names on the mortgage
Joint applications are the norm rather than the exception. Around 48% of US mortgages originated in June 2024 had a co-applicant, and 52% of US consumers with a mortgage share it with a linked household member, according to the Consumer Financial Protection Bureau.
The arithmetic cuts both ways. Two incomes combine, which raises the ceiling. But both sets of debts combine too, and one partner's obligations reduce what the pair can borrow by exactly the same multiplier as if they were your own. A partner's £300-a-month car finance costs a joint UK application 0.3 × 174,697 = about £52,400 of borrowing at 4.79% over 25 years. If you are within a year of applying, that finance agreement is worth more cleared than a comparable amount of extra deposit.
Credit files stay individual in both markets, and the weaker file often drives the rate offered. If one of you has repair work to do, do it before the application, not during: strategies for a higher credit score for US readers, improving a bad credit rating for UK ones. And settle how you will split the payment before you sign, not after — how to split bills with a partner and combining finances as a couple both cover the unequal-incomes case, which is the one that causes arguments.
Your maximum, not the lender's
Here is why the two numbers diverge. A lender assesses gross income against required debt payments. It does not deduct your pension contributions, your childcare, your commuting costs, the sinking fund for a new roof, or the saving you would like to keep doing.
Worked example: what is left for principal and interest
A US household on the 2024 median household income of $83,730 a year ($6,977 a month gross). Every deduction below is an assumption, not a measured figure.
Source: US Census Bureau, Income in the United States: 2024, Median household income, $83,730
That result lines up with the published measure of the same squeeze. The National Association of Realtors put its Housing Affordability Index at 102.3 in June 2026, up from 95.5 a year earlier, meaning a median-income family had only just enough income to qualify for a mortgage on a median-priced home (NAR). Just enough is not comfortable.
Three checks before you accept a number:
- The one-point test. Recalculate at your quoted rate plus one percentage point. If the payment breaks the budget, the loan is too big regardless of what the offer says.
- The single-income test. Could you cover the payment on one income for three months? If not, the emergency fund has to be bigger before the deposit gets any larger.
- The 12-month test. Run the payment as a real line in your budget for a few months before you commit, moving the difference between your rent and the projected payment into savings. If it holds, the number is real. The step-by-step household budget guide sets that up, and the 50/30/20 rule is a quick sanity check on whether the payment leaves room for anything else.
When this method does not work
The multiplier assumes a fully amortising loan at a fixed rate for the whole term. Several common situations break that assumption.
Irregular income. Lenders typically average self-employed income over several years, so a strong recent year may not count for what you expect. Build the affordability number off your worst recent twelve months, not your best — the approach in the irregular income budget guide.
Interest-only and offset mortgages. An interest-only payment is not amortising, so the multiplier above overstates nothing and understates everything: the balance never falls, and a repayment vehicle has to be funded separately.
Deals that expire. Any UK, Canadian or Australian mortgage where the rate is fixed for a period shorter than the term is priced on the deal rate and lived on the reversion rate. The multiplier tells you what the deal-period payment buys, not what the loan costs over 25 or 30 years.
Leasehold and HOA-heavy properties. Where a service charge or HOA fee is large and rises independently, the affordability calculation has a variable in it that no fixed-rate mortgage protects you from.
Very high-cost regions. Where prices run far above local earnings, no ratio-based method produces a comfortable answer, and the honest conclusion is often that the purchase needs a bigger deposit or a different postcode rather than a cleverer calculation.
Where to go next
- mortgage calculator — the formula in full, an amortisation schedule and every assumption it makes
- debt-to-income calculator — where you sit before you apply
- saving for your first home — building the deposit and the costs alongside it
- how much emergency fund do I need — the buffer that should never become deposit
- combining finances as a couple — how joint applications change the arithmetic
- loan calculator — for the debts worth clearing before you apply
Test the payment before you commit to it
Put the projected payment into a real budget and run it for a few months. If it survives alongside everything else you spend, the mortgage is affordable. If it does not, better to find out now.
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