Lenders do not lend against a salary. They lend against what is left after debts, tested as a ratio, priced by a credit file, and backed by a deposit. This guide works that test end to end, so the number the calculator gives you is a number you understand.

How lenders count income

A mortgage calculator asks for one income number. A lender asks where that number came from, how steady it is, and how much of it will still be there in a year. That difference is the whole qualification test. Gross income is the starting point: pay before tax and before deductions, from every source the lender is willing to count. Salary is the cleanest source, because it arrives on a schedule and can be verified from payslips and bank statements. Overtime, bonuses and commission count only to the extent that they are regular — a lender will usually average variable pay over a period rather than taking the best month at face value, and a bonus that appears once in three years is unlikely to be counted at all. Self-employed income is averaged too, typically over more than one tax year, because one good year does not prove a stable income. Rental income from an existing property can count, usually after the lender deducts the costs of holding that property and sometimes after applying a haircut for vacancies. Income that cannot be documented does not count, however real it feels to the person earning it: if it is not on a payslip, a tax return or a bank statement the lender can verify, it is invisible to the test.

The practical consequence is that two households with the same headline salary can qualify for very different loans. One earns $90,000 as a fixed salary with no other debts. The other earns $90,000 in a good year that includes a large one-off bonus, carries $600 a month of car and card payments, and has a short employment history. The first household’s income is counted in full and its debts are small. The second household’s counted income is lower, its debts are higher, and its file looks less predictable. None of that is a judgement about either household. It is the arithmetic of a lender that has to be repaid from income that arrives month after month, not from income that might arrive.

DTI: the ratio that sets the ceiling

Debt-to-income ratio, or DTI, is the single most useful number in qualification, and it is simple to work out. Add every monthly debt payment a lender counts — the proposed housing payment plus car loans, student loans, credit card minimums and any other regular debt — and divide the total by gross monthly income. The result, as a percentage, is the back-end DTI. A second ratio, the front-end DTI, divides the housing payment alone by gross monthly income. Lenders have traditionally used both: the front-end ratio checks that housing itself is affordable, and the back-end ratio checks that housing plus everything else leaves room to live.

The classic yardstick is the 28/36 rule: housing costs at most 28% of gross income, and all debts at most 36%. It is a useful starting point and it is genuinely how many lenders have thought about the problem, but it is not a law. Different loan programmes publish different limits, individual lenders set their own tests, and a strong deposit or a long credit history can move the answer. Treat 36% as a planning assumption and ask the lender what test it actually applies. The formula itself, though, is exact: DTI equals total monthly debts divided by gross monthly income, multiplied by 100. On a $90,000 salary, gross monthly income is $7,500. Total monthly debts of $2,700 give a DTI of 36%. Total monthly debts of $3,000 give 40%. The same income, the same house, and a different debt load produce a different answer — which is why paying down a car loan before applying can change the outcome more than a pay rise of the same size.

DTI also explains why a mortgage calculator based on income alone is only half a tool. A calculator that asks for income and produces a loan amount without asking about debts is assuming the debts are zero. Enter the debts honestly and the answer moves, sometimes a lot. The qualifying test is not income minus nothing. It is income minus the debts you already carry, minus the living costs the lender assumes, tested at a rate that may be higher than the rate you will actually pay. Every input that is left out makes the estimate look better than the lender’s version of the same household.

Why salary multiples mislead

Three times your salary. Four times your salary. The multiples are everywhere in mortgage advice, and they survive because they are easy to remember. They are shorthand for the DTI test, and like most shorthand they hide the parts that matter. A multiple knows your salary and nothing else. It does not know your debts, your deposit, your credit file, your other commitments, or the rate you will pay. Two borrowers on the same salary can therefore be told the same multiple and face very different realities: one with no debts and a 20% deposit, another with $800 a month of debts and a 5% deposit. The multiple treats them as identical. The lender does not.

That does not make multiples useless. They are a reasonable first check, a way to sense-check whether a price is in the right neighbourhood before doing the full test. The table below shows the multiples for three salaries, as pure arithmetic, so the shorthand can be compared with the worked DTI example that follows. Read the table as a starting point, not as an answer: the lender’s answer comes from the DTI test, the credit file and the deposit, applied together.

Notice what the table cannot show. At $90,000, the gap between three and four times salary is $90,000 of borrowing — a whole extra room, or a whole extra decade of payments, depending on how it is used. Which end of that range a borrower actually reaches depends on debts and deposit, not on the salary alone. A borrower with no debts and a large deposit may reach the top of the range or beyond it. A borrower with heavy debts may not reach the bottom. The multiple is a question — roughly how much? — and the DTI test is the answer.

Credit: score, history and price

Credit enters qualification in two ways: it affects whether a lender will lend at all, and it affects the price of the loan. A credit file is a record of how reliably a borrower has repaid debts: payments made on time, balances carried, accounts opened and closed, and any defaults or court judgements. Lenders summarise that record in a credit score, and they price risk from it: a stronger file generally earns a lower rate, which lowers the payment and raises the amount the same income can support, while a weaker file earns a higher rate or, at the extreme, a decline.

There is no single minimum score that every lender uses, and any article that names one as a universal rule is inventing it. Different programmes set different expectations, individual lenders add their own overlays, and a score is always read beside the rest of the file: income, debts, deposit and employment history all sit in the same decision. A borrower with a modest score but a large deposit and no debts is a different proposition from a borrower with the same score, a small deposit and heavy debts. What can be said generally is that the file improves with the same habits that improve any credit record: pay every bill on time, keep card balances low relative to their limits, avoid opening several new accounts in the months before applying, and do not take on new debt — a car loan, a store card — in the run-up to an application, because new debt changes the DTI test at exactly the wrong moment. Check the credit file itself before applying, so errors can be corrected while there is still time: a wrongly recorded missed payment is worth disputing before it is priced into a mortgage offer.

Deposit, reserves and closing costs

The deposit does three jobs at once. It reduces the amount that must be borrowed, which reduces the payment and the total interest. It reduces the loan relative to the value of the home, which is the direction lenders prefer and the reason a 20% deposit usually avoids private mortgage insurance. And it proves, in cash, that the borrower can save — which is evidence a lender reads as discipline. A larger deposit therefore helps qualification even though it does not change income or DTI directly: it changes the loan the income has to support.

Reserves matter for the same reason. Lenders like to see money left over after the purchase, because the first year of homeownership brings costs that no calculator predicts: a repair, a broken appliance, a period without overtime. A file with reserves looks safer than a file that has spent everything on the deposit, and some programmes effectively require it. Closing costs sit beside the deposit in the cash calculation: lender fees, appraisal, title and legal costs, prepaid tax and insurance, and similar charges, which are paid at completion and come out of the same savings as the deposit. Budget for them separately, in cash, rather than discovering them at the point where the deposit is already committed. A buyer who can show deposit, closing costs and a reserve is showing the lender a household that can absorb a surprise — which is, in the end, what qualification is trying to establish.

Pre-approval, pre-qualification and the calculator estimate

Three different things travel under the name of an answer, and confusing them causes most of the disappointment in house hunting. A calculator estimate is arithmetic: given the numbers entered, this is the loan the formula produces. It involves no document checks and no credit check, and it is only as honest as its inputs. Pre-qualification is a lender’s estimate from the figures the borrower states, again without document checks: stronger than a calculator because a human has looked at the shape of the file, but still unverified. Pre-approval is a conditional decision after the lender has verified income, debts, assets and credit, usually with a stated maximum loan amount and an expiry date. It carries real weight with sellers, and it is the closest thing to an answer that exists before a specific property is appraised.

None of the three is a final approval. The final decision depends on the property itself, the appraisal, and a last document check, and it can still change if the borrower’s circumstances change between pre-approval and completion — a new debt, a job change, a large unexplained deposit. Use the three in order: the calculator to explore, pre-qualification to sense-check, pre-approval to shop with. And remember that using a mortgage calculator does not affect a credit score at all: it performs no credit check, asks for no personal details, and leaves no trace on a credit file. The hard credit inquiry that can affect a score comes later, with a real application.

Worked household: from salary to price

Put the pieces together on one household, with every assumption stated and every figure labelled illustrative. The household earns $90,000 a year, so gross monthly income is $7,500. It carries $400 a month of other debts. The lender’s illustrative back-end test is 36%, so the total monthly debt budget is $7,500 multiplied by 36%, which is $2,700. Subtract the $400 of other debts and $2,300 a month remains for housing. That $2,300 is the housing budget on the DTI test — but it is not yet a loan amount, because housing includes property tax, insurance, PMI and HOA dues on top of principal and interest, and those vary by place and property.

To see the loan that $2,300 of principal and interest supports, use the amortisation formula in reverse at an illustrative 6.5% fixed rate over 30 years: a payment of $2,300 a month supports a loan of about $363,885. Add a deposit of $40,000 and the purchase price that loan plus deposit can reach is about $403,885, before closing costs and before the tax, insurance and PMI that must fit inside the same $2,300 housing budget — which in practice means the price the household can actually afford is lower than $403,885, because part of that $2,300 is consumed by tax and insurance rather than by principal and interest. The exact price depends on local tax rates, insurance costs and whether PMI applies, and the lender’s own test, not this illustration, decides the real answer. The value of the walk-through is not the final number. It is the order of operations: income, minus debts, tested as a ratio, converted to a loan at a stated rate and term, plus deposit, minus the other costs of owning. Change any one input and the answer moves — which is exactly what the calculator above is for.

If the number comes back lower than you hoped

A low qualification number is information, not a verdict, and the levers that move it are the same inputs the test uses. Reducing monthly debts raises the housing budget dollar for dollar on the DTI test: clearing a $300 monthly car payment frees $300 of housing budget, which at an illustrative 6.5% over 30 years supports roughly $47,000 more loan. Increasing the deposit lowers the loan the income must support and can remove PMI. Lengthening the term lowers the payment and raises the amount the same income supports, at the cost of much more total interest — a trade to make with open eyes, not by default. Improving the credit file takes longer, but it lowers the rate, which lowers the payment for the same loan. None of these levers is a promise: the lender’s test still governs, and the cheapest way to a higher number is rarely the best way to a home that stays affordable. A loan that passes the test but leaves no room for tax rises, insurance rises or a period of lower income is a loan that passes on paper and fails in life. Leave room in the budget for the costs the test does not see.

Frequently asked questions

Is a mortgage really 3 times my salary?

No. Three or four times salary is shorthand, not a lender rule. Lenders test your debts against your income with a debt-to-income ratio, alongside your credit history, deposit and reserves. On a $90,000 salary, three times salary is $270,000 and four times is $360,000, but a borrower with $400 a month of other debts qualifies very differently from a borrower with none. Use the DTI test in this guide instead of a multiple.

How do I work out my DTI?

Add every monthly debt payment a lender counts, including the proposed housing payment, and divide by gross monthly income before tax. For example, $2,700 of monthly debts on $7,500 of gross monthly income is a 36% DTI. The classic yardstick is 28% for housing alone and 36% for all debts, but lenders set their own tests, so use the ratio as a planning figure and ask the lender what test it applies.

What credit score do I need for a mortgage?

There is no single minimum score that every lender uses. Different loan programmes set different expectations, lenders add their own requirements, and the score is always read beside income, debts, deposit and employment history. A stronger file generally earns a lower rate. Check your credit file before applying, correct any errors, and avoid new debt in the months before an application.

Does using a mortgage calculator affect my credit score?

No. This calculator asks for numbers only, runs in your browser and performs no credit check, so it cannot change your score. A real mortgage application is different: a lender’s hard credit inquiry can affect your score, which is why it pays to estimate here first and apply once you know the shape of the loan you want.

How do lenders count self-employed or variable income?

Usually by averaging it over more than one year, so that one good year does not stand in for a stable income. Overtime, bonuses and commission count to the extent that they are regular and documented, and rental income counts after the costs of the property are deducted. Income that cannot be verified from payslips, tax returns or bank statements generally does not count.

Can I qualify for a mortgage if I already have debts?

Yes, if the debts fit inside the lender’s DTI test. Debts reduce the housing budget rather than disqualifying you outright: on a $90,000 salary at an illustrative 36% test, $400 a month of other debts leaves $2,300 a month for housing. Reducing debts before applying is often the fastest way to raise the amount you can borrow.