How Much House Can You Actually Afford? (The Formula Lenders Use vs. What Makes Sense for You)

The bank will tell you one number. Your gut will tell you another. The actual answer — the one that lets you sleep at night while building equity — is usually somewhere in between, and almost nobody explains how to find it.

Mortgage affordability is one of those areas where the official guidance sounds precise but routinely leads people into homes they can technically afford on paper and genuinely struggle with in practice. Understanding the formula lenders use, why it's designed the way it is, and what it deliberately leaves out is the starting point for making a home-buying decision you won't regret.

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The formula lenders use: DTI and why it exists

When a bank decides how much to lend you, they're primarily looking at one number: your debt-to-income ratio, or DTI. This is your total monthly debt payments — including the proposed mortgage — divided by your gross monthly income.

The calculation looks like this:

DTI = Total monthly debt payments ÷ Gross monthly income

Most conventional lenders want to see a DTI of 43% or lower for the total debt figure (what lenders call "back-end DTI"). Some government-backed loan programs will go higher — FHA loans, for example, sometimes allow DTIs up to 50% — but 43% is the standard ceiling for most conventional mortgages.

What this means in practice: if your gross monthly income is $7,000, lenders will consider you for mortgages where your total debt payments — mortgage principal, interest, property taxes, homeowners insurance, and any other monthly debt like car payments or student loans — don't exceed $3,010 a month.

There's also a front-end DTI figure that some lenders check, which looks only at housing costs (mortgage + taxes + insurance) rather than all debt. The standard ceiling here is typically 28%.

The DTI formula exists because it's a reliable predictor of default risk at a population level. Borrowers with high DTIs default more often. For a lender issuing thousands of mortgages, it's a sensible filter. For you as an individual, it tells you the maximum you can borrow — not the amount you should borrow.

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What DTI deliberately doesn't account for

The DTI formula uses gross income — your income before taxes. This is the first and most significant way the lender's number can diverge from your reality.

If you earn $7,000 a month gross but take home $5,200 after federal and state taxes, retirement contributions, and health insurance premiums, your actual spending money is not $7,000. But the lender's formula treats it as if it were. A mortgage that represents 40% of your gross income represents roughly 54% of your take-home pay — and that's before utilities, groceries, transportation, childcare, or anything else.

The DTI also ignores lifestyle costs entirely. It doesn't know whether you have children, whether you live in a high cost-of-living city, whether you have significant medical expenses, or whether you travel frequently. Two households with identical DTIs can have radically different financial breathing room, and the formula has no way to capture that.

It doesn't account for what happens to your cash flow after the down payment. Depleting your savings to hit a 20% down payment might get you a better rate and eliminate PMI, but if it leaves you with no emergency fund, you're a single appliance failure away from credit card debt.

And it doesn't account for the true ongoing costs of homeownership, which are consistently underestimated. Property taxes increase over time. Maintenance and repairs average 1–2% of the home's value annually — on a $400,000 home, that's $4,000 to $8,000 a year, or $333 to $667 a month that the DTI formula never sees.

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The rule of thumb alternatives: what they get right and wrong

Beyond DTI, several rules of thumb circulate in personal finance discussions. It's worth understanding each one.

The 28% rule says housing costs shouldn't exceed 28% of gross income. Like DTI, it uses gross income, which overstates your real spending power. It also doesn't account for how much other debt you're carrying, which is a significant omission if you have car payments or student loans.

The 2.5x income rule says you shouldn't buy a home worth more than 2.5 times your annual gross income. This is a quick sanity check — if you earn $80,000 a year, it suggests a ceiling of $200,000. It's useful as a rough filter but breaks down quickly in markets where median home prices are 5–8x median income, which describes most major cities today.

The 30% of take-home rule is arguably more useful than the gross-income versions because it starts from money you actually have. It says housing costs (mortgage + taxes + insurance + maintenance) shouldn't exceed 30% of your net monthly income. This is a more conservative number, and in high cost-of-living markets it may mean accepting a smaller home or a longer savings timeline — which is often the honest answer.

None of these rules account for your specific situation. They're population-level averages applied to individual decisions, which is where they consistently fall short.

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Building your own number

The more useful question isn't "what will a lender approve?" It's "what mortgage payment would let me continue living the way I want to live, maintain an emergency fund, make progress on other financial goals, and handle homeownership costs without stress?"

That question requires a bottom-up calculation, not a top-down ratio.

Start with your actual take-home income — what actually lands in your bank account each month. Then list every non-housing expense you expect to continue: food, transportation, utilities, insurance, subscriptions, childcare, debt payments, entertainment, savings contributions, and anything else that's genuinely part of your life. Be honest rather than optimistic.

What's left is your maximum housing budget — the number you can genuinely afford, as opposed to the number a lender will allow.

Then factor in the costs DTI ignores: property taxes (which vary enormously by location, sometimes adding $500–$1,500 a month to the cost of a $400,000 home), homeowner's insurance, maintenance reserves, and any HOA fees. These can easily add 30–50% to the base mortgage payment.

The result is usually a more conservative number than what a lender would approve — and a more honest one.

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The down payment decision

The conventional advice is to put 20% down to avoid private mortgage insurance (PMI), which typically costs 0.5–1.5% of the loan amount annually. On a $400,000 home, that's $2,000 to $6,000 per year in PMI — a real cost worth avoiding if you can.

But 20% of a $400,000 home is $80,000. For many buyers, particularly first-time buyers in expensive markets, accumulating that amount takes years. The question of whether to wait and save more versus buy sooner with less down depends on several factors: the trajectory of home prices in your target market, the opportunity cost of what you'd earn on that capital invested elsewhere, how long you plan to stay in the home, and the state of your emergency fund after the purchase.

There's no universal answer. But the calculation is worth doing with real numbers rather than the general principle of "20% is better." Sometimes it is. Sometimes putting 10% down and keeping a healthy cash cushion is the genuinely smarter choice.

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Stress-testing your number

Once you have a target monthly payment in mind, stress-test it against scenarios that are realistic, not catastrophic.

What happens if one income disappears for three months? If you're a two-income household relying on both incomes to make the mortgage comfortable, a period of one income — due to job loss, illness, or career transition — should be something you can survive, not something that immediately threatens your housing.

What happens if interest rates rise and you're on a variable-rate mortgage? What happens if property taxes increase at the next assessment? What happens if the roof needs replacing two years after you move in?

These scenarios aren't reasons not to buy — they're reasons to buy at a payment level that gives you room to absorb them. The home that feels financially comfortable is rarely the most expensive home you could get approved for.

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Building the plan backward from your goal

If you know you want to buy a home in three to five years, the useful question shifts from "what can I afford now?" to "what do I need to do between now and then to be ready?"

This is where a financial planning approach is more valuable than a mortgage calculator. You need to know: what's the realistic price range for homes in your target area? What down payment will you need, and what closing costs should you budget for? How much do you need to save each month between now and your target date, given your current savings level? What changes to your spending or income would accelerate the timeline?

Cashowa's financial planner does exactly this kind of backward planning. Give it a goal — "buy a $450,000 home in 5 years" — and it builds a step-by-step plan with a monthly savings target, milestone projections, and the specific trade-offs involved in getting there sooner or later. Every figure in the plan is computed from your actual inputs, not population averages. And if your situation changes — a raise, a large expense, a revised target — you can update the inputs and regenerate the plan rather than starting from scratch.

The formula lenders use tells you the ceiling. Your own numbers — run honestly — tell you the floor. The right answer for your home purchase lives somewhere in that range, and finding it requires the bottom-up calculation, not the top-down ratio.

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Frequently asked questions

How much does my credit score affect what I can afford?

Significantly. Your credit score determines the interest rate you'll be offered, and small differences in rate produce large differences in total cost over a 30-year loan. On a $350,000 mortgage, the difference between a 6.5% rate and a 7.5% rate is roughly $230 per month — or about $82,000 over the life of the loan. Improving your credit score before applying for a mortgage is one of the highest-return actions you can take in the year before you buy.

Is it better to get pre-approved before or after I decide how much I want to spend?

Decide how much you want to spend first, using your own bottom-up calculation. Then get pre-approved to understand what you'll qualify for. This order matters: if you get pre-approved first, you're anchored to the lender's maximum rather than your honest number, and it's psychologically difficult to buy below your approval ceiling once you know what it is.

Should I include my emergency fund in my down payment calculation?

No. Your emergency fund should remain intact after the purchase. A common and painful mistake is draining savings to hit 20% down and then having no buffer for the inevitable early expenses of homeownership — a broken appliance, an unexpected repair, a period of reduced income. Budget your down payment from savings earmarked specifically for the home purchase, separate from your emergency reserves.

Does it make sense to buy in a market where prices are very high relative to renting?

Not always, and the math is worth running. If the total monthly cost of owning a home — mortgage, taxes, insurance, maintenance — significantly exceeds what you'd pay to rent a comparable place, renting and investing the difference can be financially superior, particularly if you might move within five to seven years. The "always better to own" assumption doesn't hold in all markets or at all price points.

How do I estimate property taxes before I've found a specific home?

Property tax rates are public information, set by local municipalities and typically expressed as a percentage of assessed value. The effective rate varies dramatically by location — from under 0.5% in some states to over 2.5% in others. For a quick estimate, look up the property tax rate for the county or city you're targeting and apply it to your target home price. Keep in mind that assessed value doesn't always equal purchase price; some jurisdictions assess at a percentage of market value.

What's the real cost of a 30-year mortgage versus a 15-year mortgage?

The monthly payment on a 15-year mortgage is significantly higher — typically 30–50% more than the equivalent 30-year payment — but the total interest paid is dramatically less. On a $300,000 loan at 7%, a 30-year term costs roughly $418,000 in total interest over the life of the loan. A 15-year term at a slightly lower rate (typically 0.5–0.75% less than 30-year) costs roughly $156,000 in total interest. The 15-year is better if you can comfortably afford the higher payment. The 30-year gives you flexibility — you can always pay more, but you're not required to.