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Buying a home is the largest financial decision most people make in their lifetime. Yet many buyers approach it backwards — they find a home they love, then hope the financing works out. The smart approach is the opposite: calculate exactly how much you can afford first, then shop within that budget. This calculator uses the same debt-to-income formulas that mortgage lenders apply during underwriting.
The most widely used affordability benchmark is the 28/36 rule. The front-end ratio (housing DTI) says your total monthly housing costs — principal, interest, property taxes, and homeowner's insurance, collectively called PITI — should not exceed 28% of your gross monthly income. The back-end ratio (total DTI) says all monthly debt payments combined should not exceed 36% of gross monthly income.
In practice, conventional lenders approve loans up to 43–45% total DTI, and FHA loans allow up to 50% with compensating factors. But just because a lender will approve you at 45% DTI doesn't mean it's wise to borrow that much. At 45% DTI, nearly half your take-home pay goes to debt before a single bill is paid. Most financial planners recommend staying under 36% total DTI to maintain financial breathing room.
First-time buyers often compare mortgage payment to their current rent — a misleading shortcut. On a $350,000 home at 7% interest with 1.2% property taxes and $1,800/year insurance, the PITI on a 30-year loan is approximately $2,790/month — significantly more than the $2,329 P&I payment alone. Many buyers shock themselves by looking only at P&I, then discovering they can't comfortably afford the property after taxes and insurance are factored in.
The traditional wisdom says put 20% down: you avoid PMI (typically $60–$250/month), get better rates, and start with equity. On a $400,000 home, the difference between 20% and 5% down produces approximately $430/month in payment difference — plus eliminates $150–$200/month in PMI. Over 5 years, that's nearly $37,000.
However, keeping the extra down payment invested in a diversified index fund earning 8% annually may outperform the PMI savings over a 5-year period. The right choice depends on your specific opportunity cost, risk tolerance, and how long you plan to stay.
A 1% rate increase reduces buying power by roughly 10–12%. On a $400,000 loan: at 5% the monthly P&I is $2,147; at 6% it's $2,398; at 7% it's $2,661; at 8% it's $2,935. Someone who could afford a $500,000 home at 4% can only afford approximately $370,000 at 7% — a 26% reduction in purchasing power from the same income and debt profile.
On a $300,000 loan, the 30-year at 7% costs $418,527 in total interest. The 15-year at 6.5% costs $176,657 — saving $241,870. Your monthly payment is higher by about $650/month, but you own your home free and clear in half the time. The right choice depends on income stability, other investment opportunities, and how much financial flexibility you need.
On a $320,000 loan at 7% for 30 years, your first monthly payment is $2,129. Of that, $1,867 goes to interest and only $262 reduces your balance. After 5 years of payments you've paid $127,740 but your balance has only decreased by $13,200. This is why selling within 3–5 years of purchase often produces disappointing financial results: you've paid mostly interest and built very little equity.