How Much House You Can Afford, and Why the Bank's Max Isn't Your Budget

Most people figure out their home budget backwards. They open a listings app, fall for a place, and only then ask the bank what they can borrow. The number comes back smaller — or sometimes bigger — than they guessed, and the search starts over from zero. The faster path is to settle the math first, then go shopping for houses you can actually finance.

The good news is that lenders don't use a mysterious black box. Conventional mortgage underwriting runs on a simple pair of ratios, and once you know them you can reproduce the bank's answer at your kitchen table. The home affordability calculator does exactly this in your browser, but the logic is worth understanding so the output isn't a surprise.

The 28/36 Rule in Plain Numbers

Here is the whole rule, and it really is just two percentages:

Front-end ratio (the 28): your total monthly housing payment — principal, interest, property taxes, homeowner's insurance, and any HOA dues — should stay at or under 28% of your gross monthly income.
Back-end ratio (the 36): that same housing payment plus every other recurring debt — car loan, student loans, minimum credit-card payments, personal loans — should stay at or under 36% of gross monthly income.

Gross means before tax. Whichever of the two caps produces the smaller allowable payment is the one that decides your loan. If you carry almost no other debt, the 28% housing rule usually binds. If you're paying down a car and student loans, the 36% back-end rule kicks in first and pulls your number down.

That second point trips up a lot of buyers. The 36% cap is a shared pool: every $100 a month going to a car payment is $100 that can't go to a mortgage. Clear the debt and the same income suddenly approves a larger loan.

A Worked Example on a $90,000 Income

Let's run a real one. Say you earn $90,000 a year — that's $7,500 in gross monthly income.

28% front-end cap: 0.28 × $7,500 = $2,100 a month for all housing costs.
36% back-end cap: 0.36 × $7,500 = $2,700 a month for housing plus other debt.

Now suppose you have a $350/month car payment and a $250/month student loan — $600 in monthly debt. The back-end cap leaves $2,700 − $600 = $2,100 for housing. Both rules land on the same $2,100 here, so housing is the limit either way.

Out of that $2,100, set aside roughly $450 for property tax, insurance, and HOA. That leaves about $1,650 for principal and interest. At a 6.5% rate over 30 years, $1,650 a month supports a loan of roughly $261,000. Add a $50,000 down payment and you're looking at a home price near $311,000.

Now wipe out that $600 of monthly debt. The back-end pool reopens, but the 28% front-end cap still holds you at $2,100 — so in this case the affordable price barely moves. Bump the debt up to $1,200 a month instead, and the back-end cap drops housing to $1,500; the loan falls to about $214,000 and the price to roughly $264,000. That $47,000 swing is debt alone, with the same salary and the same down payment.

How the Down Payment Actually Works

Here's the part that feels counterintuitive: a bigger down payment does not raise how much the bank will lend you. Your loan ceiling is set entirely by income and those two DTI caps. What the down payment changes is the price, dollar for dollar:

Max home price = max loan + down payment.

So if your income supports a $261,000 loan, then $50,000 down buys a $311,000 home and $90,000 down buys a $351,000 home — the loan is identical in both cases. The extra cash also lifts your down-payment percentage, which matters at the 20% line: cross it on a conventional loan and you drop private mortgage insurance, shaving real money off the monthly payment. If you're weighing how much to save versus how to deploy it elsewhere, a compound interest calculator is a useful companion for seeing what that cash earns if it stays invested instead.

Why the Bank's Maximum Is Not Your Budget

I learned this one the expensive way. Years ago a lender pre-approved me for a number that, on paper, fit the 28/36 rule perfectly. What the rule never asked about was the rest of my life: a kid starting daycare, a car nearing replacement, and the simple fact that I sleep better with a few months of expenses in the bank. The approved payment would have technically worked every month and quietly strangled everything else. I bought well under the cap, and I have never once regretted it.

The 28/36 rule is a lender's risk tolerance, not your comfort level. It says nothing about retirement contributions, childcare, medical costs, travel, or how steady your income feels. Underwriting treats a 36% back-end ratio as acceptable; plenty of financially calm households deliberately sit at 20–25%. The maximum is a ceiling, not a target.

A practical move: take the affordable price the calculator gives you, then run the actual monthly payment back through your real after-tax budget and check that the rest of your life still fits. If the number leaves you breathing room, it's a budget. If it only "works" on a perfect spreadsheet, it's a trap.

Putting It Together Before You Shop

The whole sequence takes about two minutes:

Enter gross annual income and your real recurring monthly debt — and leave out your current rent, because it vanishes the day you buy.
Add down payment, rate, and term, plus a realistic figure for taxes, insurance, and HOA.
Read which cap is binding. If it's the back-end rule, ask whether retiring a debt first unlocks meaningfully more house.
Sanity-check the monthly payment against your own comfort, not the lender's ceiling.

From there you'll know the right tier of listings to browse and walk into a lender meeting already holding the number. Do this once before you fall in love with a place, and the rest of the process gets a lot less stressful.

Made by Toolora · Updated 2026-06-13