How to Calculate How Much House You Can Afford

Every major real estate site has a house affordability calculator. Enter your income, debt, and down payment, and it spits out a maximum purchase price. The problem is that these calculators tell you the maximum a lender might approve — not the maximum you should borrow. Those two numbers can differ by $50,000 to $150,000, and the gap between them is where buyers become house-poor.

Here's how to properly calculate how much house you can afford — not just how much a lender will give you.

What Affordability Calculators Actually Measure

Lender-based affordability tools model the mortgage approval thresholds established by Fannie Mae, Freddie Mac, FHA, and VA. They calculate how large a loan you can qualify for based on:

• Your gross (pre-tax) annual income
• Your existing monthly debt obligations
• Your estimated down payment
• Current interest rates
• Your credit score (sometimes)

The result is a maximum loan amount based on conventional underwriting guidelines — typically the amount that would bring your back-end debt-to-income (DTI) ratio to 43-45%.

This is useful for understanding lender limits. It is not a budget. A lender approving you for $450,000 doesn't mean you can comfortably carry a $450,000 mortgage. It means they're confident you won't default.

The 28/36 Rule: A Better Starting Point

A more conservative and historically reliable rule of thumb is the 28/36 guideline:

28%: Your monthly housing costs (mortgage principal and interest, property taxes, homeowners insurance, and HOA) should not exceed 28% of your gross monthly income.

36%: Your total monthly debt payments (housing costs plus all other debts — car loans, student loans, credit cards) should not exceed 36% of your gross monthly income.

Worked example with a $100,000 annual household income:

• Gross monthly income: $8,333
• 28% housing limit: $2,333/month for all housing costs
• 36% total debt limit: $3,000/month for housing + all other debts

If you're currently paying $400/month in car loan and $200/month minimum on student loans ($600 total), your housing costs should stay below $2,400 under the 28% rule and below $2,400 under the 36% rule (since $600 existing debt + $2,400 housing = $3,000 = 36%).

Compare that to a lender's calculation: they might approve you up to a 45% DTI, which would allow $3,750/month in total debt ($8,333 × 45% = $3,750 − $600 existing = $3,150 for housing). A lender might approve you for a payment $750/month higher than what the 28/36 rule would suggest is safe.

From Monthly Payment to Maximum Purchase Price

Once you know your comfortable monthly housing cost limit, you can back-calculate the maximum purchase price:

Monthly payment → loan amount conversion

For a 30-year loan at 6.75% interest, each $100,000 in loan amount costs approximately $648/month in principal and interest.

If your comfortable housing payment is $2,333/month (from the example above), and property taxes + insurance will add another $450/month, you have $1,883 for principal and interest.

$1,883 / $648 × $100,000 = approximately $290,600 in loan amount.

Add your down payment. If you're putting down $25,000, your maximum comfortable purchase price is approximately $315,600.

A lender might approve you for $380,000 based on 45% DTI. The difference — $64,000 — represents the financial breathing room between what you can technically borrow and what you should.

The Four Real Variables in Affordability

1. Interest rate: The most volatile variable. A 1% change in rate changes your monthly payment by approximately $60 per $100,000 borrowed on a 30-year term. At 7% versus 6%, the difference on a $350,000 loan is roughly $210/month — which translates to roughly $32,000 less in purchasing power if you hold the monthly payment constant.

Run your affordability calculation using today's rates, not the promotional rates from 2020 or 2021.

2. Property taxes: These vary enormously by location and are chronically underestimated. A 1.5% property tax rate on a $400,000 home is $6,000/year or $500/month — as much as a car payment. A 0.5% rate on the same home is $2,000/year or $167/month. Research the specific municipality's tax rate before assuming an average.

3. HOA fees: Condo and planned community HOA fees range from $150/month in modest communities to $800-$1,500/month in high-end developments. These fees directly reduce your borrowing capacity and are often excluded from quick affordability calculations.

4. Insurance: Homeowners insurance averages around $150-$250/month for a typical home, but in hurricane-prone Florida, wildfire-risk California, or flood-risk Louisiana, premiums can be $300-$800/month or higher. In some markets, insurance availability itself has become a barrier to purchase.

What Affordability Calculators Don't Account For

The online calculators at Zillow, Bankrate, and NerdWallet are useful for a first approximation. They don't account for:

Maintenance costs: The 1% rule — budget 1% of the home's value per year for maintenance — is a minimum for newer construction and an underestimate for older homes. On a $350,000 home that's $3,500/year or $292/month. This never appears in an affordability calculator, yet it's as inevitable as your mortgage payment.

Utilities: Homeowners pay their own utilities. Transitioning from an apartment where utilities are partially shared or where a smaller space costs less can increase utility costs by $150-$400/month.

Income after tax: Affordability tools use gross income. Your mortgage payment comes from take-home pay. High earners in high-tax states (California, New York, New Jersey) should validate that their net income actually supports the calculated payment after federal, state, and FICA deductions.

The down payment timeline: Knowing what you can afford is only half the problem. Knowing how long it will take to accumulate the down payment and closing costs for that house — given your current savings rate — is the other half.

Running the Full Calculation

A comprehensive affordability assessment looks like this:

Step 1: Calculate 28% of your gross monthly income. That's your maximum housing cost budget.

Step 2: Research the actual property tax rate in your target area. Subtract the monthly property tax estimate from your housing cost budget.

Step 3: Add a homeowners insurance estimate ($200/month is a reasonable starting point; adjust for your specific market). Subtract from remaining budget.

Step 4: If targeting a community with HOA fees, subtract those.

Step 5: Whatever remains is your P&I (principal and interest) budget. Use a mortgage calculator to convert that to a loan amount at current rates.

Step 6: Add your planned down payment to get your maximum purchase price.

Step 7: Check that the total monthly payment (including P&I, taxes, insurance, HOA) leaves adequate room for maintenance, savings, and your other financial goals.

Step 8: Calculate how long it will take to save the required down payment and closing costs for that purchase price.

Affordability and the Savings Phase

The affordability calculation tells you the target. The savings plan tells you how to reach it.

Many buyers discover that their affordable purchase price requires a larger down payment than they currently have — sometimes significantly larger. The savings phase between now and purchase is where the real work happens: accumulating the down payment, closing costs, and reserves in the right account types, at the right yield, with the right behavioral systems to ensure consistency over a 1-5 year timeline.

The Down Payment Savings Plan & Strategy Guide bridges the gap between knowing what you need and building the savings system to get there — including dynamic calculators that adjust your monthly savings target as your target home price appreciates over your savings horizon.

The Bottom Line

A house affordability calculator gives you a lender's maximum. What you actually need is your personal maximum — the payment that leaves room for maintenance, emergencies, retirement savings, and a life that doesn't revolve around your mortgage. Work backward from what you can comfortably sustain, not forward from the largest loan a bank will give you.