How Much House Can You Afford?

Two Different Questions That Get Treated as One

There's a moment early in every home search where the question "how much house can I afford?" quietly turns into "how much will a bank hand me?" Those feel like the same question. They are not. One is a lending decision made by an underwriter looking at ratios, credit history, and risk models. The other is a personal decision about how you want your income to be spent for the next 15 to 30 years, including the parts of your life that have nothing to do with your mortgage.

Lenders will, in almost every case, approve you for more than what leaves you financially comfortable. That's not a conspiracy — it's just that their model of "affordable" is built around default risk, not around whether you'll still be able to fund a retirement account, take a vacation, or handle a surprise car repair without stress. Knowing the actual math lenders use is useful precisely because it lets you see where their ceiling is, so you can deliberately choose to stop well short of it.

The Front-End Ratio: Housing Costs Alone

The front-end ratio (also called the housing ratio) measures your total monthly housing payment — principal, interest, property taxes, homeowners insurance, and any HOA dues, often abbreviated PITIA — against your gross (pre-tax) monthly income. Most conventional lenders want this at or below roughly 28%, though the exact ceiling varies by loan program.

In formula form:

• Front-end ratio = Monthly housing payment ÷ Gross monthly income

This ratio exists because housing is usually the largest single line item in a household budget, and lenders want assurance that it doesn't consume so much of your paycheck that a minor income disruption puts you at risk of missing a payment.

The Back-End Ratio: Everything You Owe

The back-end ratio is the broader measure, and it's the one that usually ends up being the binding constraint. It's your total monthly debt obligations — housing payment plus car loans, student loans, minimum credit card payments, and any other recurring debt — divided by gross monthly income. This is essentially your debt-to-income ratio (DTI).

Back-end ratio limits typically fall between 36% and 43%, and can stretch higher — sometimes into the high 40s — for certain loan programs (FHA loans, for instance, tend to allow more room than a standard conventional loan) or when you have strong compensating factors like a large down payment or excellent credit. But as a general rule of thumb, once your total debt load crosses somewhere around 43% of gross income, most conventional underwriting gets noticeably harder to clear.

Here's the important part: the back-end ratio is why two people with identical incomes can qualify for very different loan amounts. If you have a $600 monthly car payment and $400 in student loan payments, that's $1,000 a month already committed before housing even enters the picture — and it directly shrinks the housing payment a lender will let you take on.

A Worked Example

Let's put real numbers on this. Say a household has a combined gross monthly income of $9,000 ($108,000/year), and $500 per month in existing debt (a car loan).

Step 1 — Front-end ceiling (28%): $9,000 × 0.28 = $2,520 maximum monthly housing payment, on housing alone, with no reference to other debt.

Step 2 — Back-end ceiling (36%): $9,000 × 0.36 = $3,240 maximum for housing plus all other debt combined. Subtract the existing $500 car payment: $3,240 − $500 = $2,740 available for housing.

Step 3 — Take the lower of the two: $2,520 (front-end) is lower than $2,740 (back-end), so $2,520 is the binding constraint in this case. That $2,520 monthly housing payment — at, say, a 6.75% 30-year mortgage rate, with property tax and insurance folded in — works out to roughly a $330,000–$360,000 maximum home price, depending on the down payment, local tax rate, and insurance costs.

Now change one input: give that household an extra $700/month car payment and a $400/month student loan instead of the $500 car payment. Back-end ceiling stays at $3,240, but now $3,240 − $1,100 = $2,140 available for housing — which is now the binding constraint, well below the front-end's $2,520. Same income, meaningfully lower home price, purely because of existing debt. This is exactly why lenders — and you — need to run both ratios, not just one.

You don't need to do this arithmetic by hand every time you see a new listing. Our home affordability calculator runs both ratios against your actual income and debts and gives you a realistic price range in seconds — worth doing before you fall in love with a listing that's outside your range.

Why "Approved For" and "Should Spend" Are Different Numbers

A pre-approval letter tells you the largest mortgage a lender is willing to extend based on the ratios above, your credit profile, and the loan program's rules. It says nothing about your other financial goals — retirement contributions, an emergency fund, childcare, saving for a car, or simply having breathing room in your monthly budget.

This gap matters because the front-end and back-end ratios are ceilings calibrated to default risk, not to your personal financial plan. A household maxed out at 36% back-end DTI can be current on every payment and still have very little left over for anything else — no cushion for a job change, a medical bill, or a slow month for a commission-based income. Stretching to the top of your approved range works fine until something in life doesn't go according to plan, and something eventually doesn't.

A more conservative approach many financial planners suggest: treat the lender's maximum as a ceiling you deliberately stay under, not a target. Aiming for a front-end ratio closer to 20–25% rather than the full 28%, for example, leaves room for the costs below — which the ratios above don't fully capture.

The Costs the Ratios Don't Show You

The front-end ratio typically includes principal, interest, taxes, insurance, and HOA dues (PITIA) — but it stops there. It does not include the ongoing cost of actually owning and maintaining a home, which is real money that has to come from somewhere in your budget every single year.

• Maintenance and repairs: A commonly used rule of thumb is budgeting 1–2% of the home's value per year for maintenance — a roof, water heater, HVAC system, or appliance failure doesn't wait for a convenient month. On a $400,000 home, that's roughly $4,000–$8,000 a year, or $330–$670 a month, that no mortgage calculator will show you.
• Utilities: A bigger home (or a move from renting, where some utilities may have been included) often means a real increase in electricity, gas, water, and trash costs.
• Property tax reassessment: Taxes are frequently based on purchase price, not the seller's prior assessed value — meaning your actual property tax bill can jump above what a listing's estimated payment shows.
• Private mortgage insurance (PMI): If your down payment is below 20% on a conventional loan, PMI adds a real monthly cost on top of principal and interest until you reach sufficient equity.
• Closing costs and moving costs: Typically 2–5% of the purchase price, due up front, separate from your down payment.

None of this means homeownership is a bad idea — it just means the "affordable" number needs to account for more than the mortgage payment on a rate sheet. Our mortgage payment calculator can help you see the principal-and-interest piece clearly, and it's worth padding whatever number it gives you with a realistic maintenance and insurance estimate before treating it as your true monthly cost.

Check Your Debt-to-Income Ratio First

Since the back-end ratio is so often the real constraint — and since paying down even one debt can meaningfully raise your home-buying power — it's worth knowing your current DTI before you start house hunting, not after a lender runs it for you. Our debt-to-income calculator gives you that number directly, and it can be a useful prompt to pay off a car loan or consolidate credit card debt before applying, if that would meaningfully expand your options.

The Bottom Line

Lenders use the front-end and back-end ratios to answer one narrow question: can this household make the payments without an unacceptable risk of default? That's a useful ceiling to know, but it's not the same question as "what should I actually spend?" The second question depends on your other goals, your appetite for risk, and the real cost of maintaining a home — none of which show up in a pre-approval letter. Run the numbers yourself, pad them for maintenance and the unexpected, and treat the bank's maximum as a boundary, not a destination.