What "affordability" means here
Affordability is a lender's estimate of the most you could safely borrow, based on income, debt and a target percentage of your budget going on housing. It is not the same as pre-approval, and it is not a promise: it is the standard maths that sits behind a lender's own, more detailed underwriting. This tool runs that standard maths two ways. The US approach checks two ratios against gross income, known as the 28/36 rule. The UK approach starts from a straight multiple of income instead, the shorthand figure most UK lenders quote before running a full affordability assessment.
The US 28/36 rule, front-end and back-end
The front-end ratio caps the whole monthly housing payment, principal, interest, taxes and insurance together, known as PITI, at 28% of gross monthly income. The back-end ratio caps every recurring monthly debt combined, housing plus car loans, student loans, credit cards and anything else, at 36% of gross monthly income. Because the back-end figure already includes housing, the room left for housing under the back-end rule is 36% of income minus whatever debts already exist. Whichever ceiling comes out lower is the one that actually binds, since a lender using this guideline will not approve a payment above either limit.
Take the calculator's own defaults: $90,000 a year is $7,500 a month. The front-end limit is $2,100. The back-end limit is $2,700 minus $500 of debts, or $2,200. The front-end figure is lower, so it binds, and $2,100 is the most this guideline allows for the whole housing payment before a cent of it is split between the loan, taxes and insurance.
From a monthly payment to a house price
A monthly housing budget is not a house price on its own; it has to be split and then converted. This tool takes a flat monthly allowance for taxes and insurance, subtracts it from the bound ceiling to leave a principal and interest payment, then runs that payment back through the standard fixed-repayment loan formula in reverse to find the largest loan it can support. Add the down payment and the result is the maximum house price.
A flat monthly allowance is used for taxes and insurance rather than a percentage of the house price, because the price is the very thing this calculator is solving for; a percentage-of-price figure would need the answer before it could be worked out. A flat amount avoids that circular step and is easy to check against a real tax bill or insurance quote nearby. Continuing the default case: $2,100 minus a $300 allowance leaves $1,800 for principal and interest. At 6% over 30 years, that supports a loan of roughly $300,225. Add the $40,000 down payment and the maximum price comes out at roughly $340,225.
The UK approach: a straight income multiple
UK mortgage lending is not built around the 28/36 ratios at all. The figure most UK lenders quote first is a straight multiple of gross annual income, commonly around 4.5 times as a mainstream default, though several lenders now go higher for specific profiles or higher earners, and the FCA allows individual lenders some flexibility in how much lending sits above that 4.5× mark. Multiply income by the multiple, add the deposit, and the result is an indicative budget.
The calculator's UK default, £60,000 of income at 4.5× plus a £30,000 deposit, comes to £300,000, of which £270,000, or 90%, is loan and the rest is deposit. That loan-to-value figure matters on its own: UK lenders price deals in bands around common thresholds, commonly 60%, 75%, 80%, 85%, 90% and 95%, and a smaller loan against the same deposit usually reaches a better-priced band.
A multiple alone is only ever a starting point. Under FCA rules, a lender still has to run a full affordability assessment on top, checking actual income, committed spending and other debts, and it must stress-test the mortgage against a higher interest rate for at least five years ahead under its own affordability rules, to check the payment would still be manageable if rates rose. Two applicants on the same income and the same multiple can be offered different amounts once that full assessment runs.
Why a small change moves the budget so much
Under the US approach, the interest rate does not change the monthly budget at all, since that comes straight from income and debts, but it changes how much loan that budget can support. A higher rate means more of every payment goes on interest, so the same $1,800 buys a smaller loan. The rate sensitivity table above runs this through directly, holding the monthly budget and term fixed while only the rate moves.
Under the UK approach, the multiple plays the equivalent role. There is no rate in the sum, so the whole budget moves in direct proportion to whatever multiple is used, which is why the gap between a cautious 3.5× and a stretched 5.5× figure on the same income is so large. Neither table endorses the higher end; both exist to show how sensitive the final figure is to an assumption that is easy to change but hard to notice changing.
What the deposit and debts actually change
A bigger down payment or deposit adds pound-for-pound or dollar-for-dollar onto the maximum price in both approaches, since it sits outside the ratio or multiple maths entirely; it does not change the monthly budget, only what that budget is added to. Debt works differently in each. Under the UK multiple, debt is not part of the sum shown here, though a lender's own separate affordability check still accounts for it. Under the US rule, existing debt directly narrows the back-end ceiling, and enough of it, at or above 36% of gross income, leaves no room for a housing payment under this guideline, whatever the front-end ratio might otherwise allow. The Down Payment Calculator explores how the size of a down payment itself changes the closing costs and monthly payment that follow from it.
Reading the comfort bands
The 28% front-end and 4.5× income figures used as the default in each lens are common starting points, not universal rules; actual lending criteria vary by lender, credit history and country. The comfort bands further up show what a more conservative or more aggressive assumption would do to the same income: 25%, 28% and 33% front-end on the US side, and 3.5×, 4.5× and 5.5× income on the UK side. Higher percentages and higher multiples are shown so the shape of the trade-off is visible, not because a higher figure is being recommended. A lender's own underwriting, not this page, decides what is actually offered.
Questions people ask
Is the 28/36 rule a legal limit?
No. It is a lending guideline, not a law. Lenders are required to check a borrower's ability to repay, and many use ratios close to 28% and 36% as a starting point, but actual approved debt-to-income ratios on conventional loans can run higher, sometimes up to around 45%, and government-backed loan programmes such as FHA, VA and USDA often allow more flexibility again. The figures here show the standard guideline, not any one lender's actual criteria.
Why does the calculator use a flat taxes and insurance figure instead of a percentage?
Because the house price is the answer this tool is solving for, not an input. A percentage-of-price allowance would need the price before it could be calculated, creating a circular step. A flat monthly amount sidesteps that, and it is easy to sense-check against an actual property tax bill or insurance quote for a similar home in the area being considered.
What counts as a debt in the back-end ratio?
Any recurring monthly obligation reported to credit bureaus or otherwise committed: car finance, student loan payments, credit card minimums, personal loans and similar. Everyday living costs such as groceries, utilities or childcare are not included in the standard ratio, even though they clearly affect what a household can really afford; that is one reason the ratios are a guideline rather than the full picture a household budget gives.
Does UK mortgage lending use anything like the 36% back-end ratio?
Not by that name. UK affordability assessments are individual to each lender and check income, committed spending and existing debt directly rather than through a fixed 36% figure, alongside the income multiple and an interest rate stress test. The underlying idea, that existing debt reduces what can be safely borrowed, is the same; the exact figure a UK lender lands on for a given case is not something a fixed ratio can reproduce.
Why is my back-end limit sometimes lower than my front-end limit?
Because the back-end limit starts from a higher percentage of income, 36% against 28%, but then has existing monthly debts subtracted from it before comparing the two. Enough debt closes that gap and can push the back-end figure below the front-end one, or even to zero if debts alone reach or exceed 36% of gross income, in which case this guideline leaves no room for a housing payment until debt comes down or income rises.
Can I actually borrow 5.5 or 6 times my income in the UK?
Sometimes, but it is not the mainstream default. Most high street lenders work from around 4 to 4.5 times income; higher multiples, into the 5× to 6.5× range, tend to be reserved for specific circumstances, such as higher combined incomes or particular professional schemes, and are subject to the same affordability assessment and rate stress test as any other application.
How is this different from the Mortgage Calculator?
The Mortgage Calculator starts from a house price you already have in mind and works out the monthly payment, schedule and payoff date. This tool runs the opposite direction: starting from income, debts and a deposit, it works out a maximum price worth considering in the first place, before a specific property is on the table.
These figures are estimates built from published lending guidelines and are not financial or lending advice, and not a pre-approval or an offer. Real affordability depends on credit history, other financial commitments, property type, insurance and tax specifics, and each lender's own criteria and current rates. Speak to a mortgage lender, broker or adviser before making a decision.