How much house can I afford? The math behind the number

“How much house can I afford” feels like it should have a single answer, and the listing sites are happy to hand you one by multiplying your salary by some round number. That number is almost always too high, because it skips the part that actually drains your account every month: the full payment, not the price tag.

TL;DR: Affordability is a monthly payment question, not a sticker price one. Cap housing near 28 percent of gross monthly income and total debt near 36 percent, build the payment from principal, interest, tax, insurance and any HOA or PMI, then work backwards to a price.

Start from the monthly payment, not the price

A house has a price, but you do not pay the price. You pay a monthly amount for decades, and that amount is what has to fit your life. So the useful question is not “what price can I reach,” it is “what payment can I carry every month without the rest of my budget buckling.”

That flip matters because the same price means very different payments depending on your rate, your down payment, and your local taxes. A 400,000 home in a low-tax county with a big down payment can cost less per month than a 320,000 home in a high-tax one. Price alone hides all of that.

The 28/36 rule, in plain terms

Lenders lean on a long-standing guideline that gives you a fast sanity check.

The first number, 28, is your housing ratio. Your full housing payment should sit at or below 28 percent of your gross monthly income, meaning income before tax. If you earn 6,000 a month before tax, that puts your housing ceiling around 1,680.

The second number, 36, is your total debt ratio. Every required debt payment you have, housing plus car loans, student loans, minimum card payments, should sit at or below 36 percent of gross monthly income. On that same 6,000 a month, all debt together should stay under about 2,160. If your car and student loans already eat 600, your housing room shrinks to roughly 1,560 even though the 28 percent line said 1,680.

Lenders will often approve you above these lines, sometimes well above. The ratios are not a legal cap, they are a comfort cap. Borrowing right up to a 43 or 45 percent total ratio is allowed and routinely regretted.

What the monthly payment actually contains

People price a house on principal and interest, then get blindsided at closing. A real monthly housing payment is usually four things, and sometimes six.

Piece	What it is	Notes
Principal	Pays down the loan balance	The part that builds equity
Interest	The lender’s charge on the balance	Largest early, shrinks over time
Property tax	Annual tax, billed monthly into escrow	Varies wildly by location
Homeowners insurance	Required coverage on the home	Also escrowed in most loans
PMI	Private mortgage insurance	Only if you put down under 20 percent
HOA dues	Community or condo fees	Not in the loan, but a real monthly cost

The shorthand for the core four is PITI: principal, interest, taxes, insurance. The 28 percent ceiling is meant to cover PITI plus HOA and PMI, not just the loan. Skip the tax and insurance lines and your “affordable” payment can be 300 to 500 a month light.

Work an example backwards

Say you bring home 6,000 a month gross, and you have a 350 car payment and 150 in student loans, so 500 in existing debt.

Your total-debt ceiling is 36 percent of 6,000, which is 2,160. Subtract the 500 you already owe and 1,660 is left for housing. Your housing-only ceiling is 28 percent of 6,000, which is 1,680. The lower of the two wins, so plan around 1,660 for the entire PITI-plus payment.

Now reserve part of that for the non-loan pieces. If taxes, insurance and any HOA run about 460 a month in your area, only about 1,200 is left for principal and interest. That 1,200 is the figure that maps to a loan size at today’s rates, and the loan plus your down payment is your real price ceiling. A mortgage calculator at finance.hivly.net lets you punch in the rate, term and tax estimate and read the price that lands on a payment you chose, instead of guessing.

The costs the ratios ignore

The 28/36 rule keeps the bank safe. It does not keep your life comfortable, because it counts none of the spending that has no loan attached. Groceries, utilities, childcare, commuting, retirement contributions, and the simple fact that a house breaks. Roofs, water heaters and appliances fail on their own schedule, and a rough rule is to set aside about 1 percent of the home’s value each year for upkeep.

This is why the approved maximum is a ceiling, not a target. Buying a payment at the top of your ratio leaves nothing for the rest, and the rest does not go away. Many people who stay financially calm after buying chose a payment a notch or two below what they qualified for, precisely so the house did not crowd out everything else.

A quick path to your own number

Run these in order and you will have a grounded figure in a few minutes.

Take your gross monthly income and multiply by 0.28 for your housing ceiling, then by 0.36 for your total-debt ceiling. Subtract your current required debt payments from the second number. Use whichever result is smaller as your full housing budget.

From that budget, carve out an estimate for property tax, insurance, and any HOA or PMI, since those are not negotiable once you own. What remains is your principal-and-interest room, and that is the input that becomes a loan amount and, with your down payment added, a price.

Then do the honest part: ask whether a payment at that ceiling still leaves room to save, absorb a surprise, and live. If it feels tight on paper, it will feel tighter in a house. Pick the payment you can carry on a normal month, and let that, not the listing price, decide what you can afford.