A one percentage point difference in your mortgage rate costs $235 a month on a $350,000 loan. Over 30 years, that is $84,600 — enough to buy a decent car every decade you own the house. Yet most buyers spend more time comparing kitchen backsplashes than comparing the numbers that decide whether the house is affordable at all.
The good news: the math lenders use is not secret, and it is not complicated. You can work it out on the back of an envelope, and you should — before a lender does it for you.
The 28/36 rule, in plain terms
The 28/36 rule is the standard most lenders start from. It has two parts:
- Your housing payment should not exceed 28% of your gross monthly income. That is the "front-end" ratio, and it covers PITI: principal, interest, property taxes, and insurance.
- Your total debt payments — housing plus car loans, student loans, credit card minimums — should not exceed 36% of gross monthly income. That is the "back-end" ratio.
Gross means before taxes. That trips people up, because your take-home pay is what actually hits your checking account, and it is meaningfully smaller.
A worked example: the $85,000 salary
Say you earn $85,000 a year. That is $7,083 in gross monthly income.
- Front-end cap (28%): $1,983 per month for housing, all-in.
- Back-end cap (36%): $2,550 per month for all debt combined.
Now watch what existing debt does. Suppose you have a $400 car payment and $200 in student loans — $600 a month total. Your back-end cap of $2,550 minus $600 leaves $1,950 for housing. That is now your binding limit, because it is lower than the $1,983 front-end cap. Every dollar of monthly debt you carry reduces the house you can buy, roughly dollar for dollar against your payment budget.
So how much house does $1,950 a month get you? Subtract taxes and insurance first — call it $500 a month, a reasonable figure for a mid-priced home in a mid-tax state. That leaves about $1,450 for principal and interest. At 6.5% on a 30-year loan, $1,450 a month supports a loan of roughly $229,000. Put $40,000 down and you are shopping around $270,000 — probably less house than you assumed an $85K salary would buy. Running your own income and debts through a mortgage calculator takes about two minutes and replaces guesswork with an actual number.
How lenders actually compute DTI
Lenders call this debt-to-income ratio, or DTI, and their version has a few wrinkles worth knowing:
- They use minimum payments on credit cards, not your balance or what you actually pay.
- Student loans in deferment still count — often at 0.5% to 1% of the balance per month, depending on the loan program.
- Income must be documented and stable. Bonuses and side-gig income usually need a two-year history before they count.
- Conventional loans can stretch to 45% or even 50% back-end DTI with strong credit and reserves. That a lender will approve it does not mean your budget will survive it.
That last point matters most. The 28/36 rule is a ceiling, not a target. Lenders are estimating the risk that you default. They are not estimating whether you can still afford daycare, a 401(k) contribution, and the occasional vacation.
The down payment question
You do not need 20% down. The average first-time buyer puts down far less, and conventional loans go as low as 3%. But the trade-offs are real.
Put 3% down on a $300,000 house and you are borrowing $291,000 and paying private mortgage insurance. PMI typically runs 0.5% to 1.5% of the loan balance per year; at 0.8%, that is about $194 a month on top of everything else — money that builds no equity and buys you nothing except the loan itself. PMI drops off once you reach 20% equity on a conventional loan, but that can take years.
Put 20% down and you skip PMI, borrow less, and usually get a slightly better rate. The catch is the $60,000 in cash, plus closing costs of 2% to 5% of the purchase price, plus the emergency fund you should not drain to get there. A smaller down payment on a house you can comfortably carry beats an empty savings account and a maxed-out budget.
What one percentage point really costs
Here is the rate sensitivity math on a $350,000, 30-year loan, computed with the standard amortization formula:
- At 6.5%: $2,212 per month in principal and interest. Total interest over the life of the loan: about $446,000.
- At 7.5%: $2,447 per month. Total interest: about $531,000.
That single point costs $235 a month and roughly $84,600 over 30 years. It also shrinks your buying power: the same monthly payment that carries a $350,000 loan at 6.5% only carries about $316,000 at 7.5%. This is why shopping multiple lenders — and paying attention to when you lock your rate — moves the needle more than almost any negotiation you will do on price. An amortization calculator will show you exactly how each rate scenario splits between principal and interest, month by month.
The costs nobody puts in the listing
The mortgage payment is the floor, not the total. Budget for:
- Property taxes: commonly 0.5% to 2% of home value per year depending on the state. On a $300,000 home at 1.1%, that is $275 a month.
- Homeowners insurance: often $1,500 to $3,000 a year, and climbing fast in wildfire and hurricane states.
- Maintenance: the standard planning figure is 1% to 2% of home value per year. On a $350,000 house, that is $3,500 to $7,000 annually. Not every year — some years it is a $200 gutter repair, some years it is a $12,000 roof. The average is what matters.
- HOA dues, if applicable, which can run from $50 to several hundred dollars a month and are not optional.
A $2,200 mortgage payment can easily become a $3,000 monthly housing cost once all of this lands.
The mistakes that hurt the most
- Shopping at the top of your pre-approval. Pre-approval is the maximum a lender will risk, not a recommendation. Set your own ceiling below it.
- Ignoring the back-end ratio. Buyers fixate on the housing payment and forget the car loan quietly eating $400 of their monthly capacity.
- Draining savings for the down payment. Houses generate surprise expenses immediately. Closing with less than three months of expenses in reserve is how new homeowners end up carrying credit card debt by month six.
- Skipping the rate shop. A half-point difference between lenders is common and, as the math above shows, worth tens of thousands of dollars.
- Forgetting that taxes and insurance rise. Your P&I is fixed on a fixed-rate loan; the escrow portion is not, and reassessments after a sale can jump your tax bill in year one.
The 28/36 rule will not tell you what house to buy. It tells you where the danger zone starts. Work the numbers from your actual income and debts, price in the hidden costs, and buy the house that leaves room in the budget for the life you plan to live in it.