House Affordability Calculator Guide
The 28% and 36% figures behind an affordability estimate aren't arbitrary — they're two separate ratios underwriters calculate independently, and they don't always point to the same answer. Understanding how front-end and back-end debt-to-income (DTI) ratios actually work explains why two buyers with identical incomes can qualify for very different loan amounts.
Front-End DTI vs. Back-End DTI
Front-end DTI measures only your proposed housing payment — principal, interest, taxes, insurance, and HOA dues, often abbreviated PITIA — against your gross monthly income. Back-end DTI adds in everything else you owe: car payments, student loans, minimum credit card payments, personal loans, even court-ordered support. Conventional lenders generally like to see front-end DTI at or under roughly 28% and back-end DTI at or under roughly 36%, though many conventional programs will stretch back-end DTI into the low-to-mid 40s for borrowers with strong credit, a bigger down payment, or extra cash reserves. Back-end is the ratio that actually decides most approvals — a lender who sees your total obligations eating half your paycheck isn't reassured by a modest housing payment on its own.
Why FHA Math Runs Looser
FHA-backed loans are built for thinner margins. Where conventional underwriting treats 28/36 as a soft ceiling, FHA guidelines commonly allow front-end DTI up to around 31% and back-end DTI up to around 43%, and automated underwriting can push back-end well past 50% when compensating factors — residual income, minimal payment shock, a longer credit history — are strong enough. That flexibility is exactly why FHA loans are popular with buyers whose income looks fine on paper but who are carrying real debt: a wider back-end ceiling directly translates into a higher approved loan amount, even at the same income and interest rate. The tradeoff is mortgage insurance that, unlike conventional PMI, typically doesn't cancel once you cross 20% equity — something to factor in before assuming FHA is the cheaper path over the life of the loan.
Why the Back-End Number Wins
It comes down to what actually predicts default. A borrower can carry a low housing payment and still be stretched thin by a $700 car loan, $400 in student loan payments, and $300 in credit card minimums — none of which show up in front-end DTI at all. Back-end DTI captures that full obligation picture, which is why it functions as the real ceiling in almost every underwriting model; front-end DTI mostly just confirms the housing piece isn't disproportionate on its own. This is also the mechanical reason paying down a car loan or credit card balance before applying can raise your approved amount more than it looks like it should: every dollar of monthly debt payment eliminated frees up close to a dollar of qualifying housing payment, run back through the interest rate and loan term to produce a larger loan amount.
If you want to see how a specific back-end DTI ceiling translates into an actual home price given your income and debts, the house affordability calculator above runs that math directly. For the payment breakdown once you have a target price, including PMI, run it through the mortgage calculator. And if you're weighing whether to pay off debt first or save a bigger down payment first, the credit cards payoff calculator can show how quickly clearing a balance frees up back-end DTI room. None of this replaces an actual conversation with a loan officer, who can pull your real credit file and tell you which ratio, and which loan program, you'll actually qualify under.