What Is Home Affordability and How Lenders Actually Calculate It
Lenders calculate home affordability using your debt-to-income (DTI) ratio: your total monthly debt payments, including the new mortgage, divided by your gross monthly income. Most lenders cap that ratio at 36–43%, though some approve higher with strong credit — which is why a bank's maximum approval is often higher than what actually feels affordable.
"How much house can I afford" and "how much will a bank lend me" are two different questions with two different answers, and mixing them up is the single most common mistake in home shopping. Lenders answer the second question with a formula. Only you can answer the first.
This guide explains the formula lenders actually use, the two rules of thumb that consistently show up in real mortgage discussions, and why treating a bank's maximum as your personal budget is a common and expensive mistake.
The formula lenders use: debt-to-income ratio
Debt-to-income ratio, or DTI, is your total monthly debt payments — the new mortgage plus existing debts like car loans, student loans, and minimum credit card payments — divided by your gross (pre-tax) monthly income.
Most conventional lenders cap total DTI somewhere between 36% and 43%, meaning your mortgage plus every other debt payment combined shouldn't exceed that share of your income. Some loan programs allow higher ratios for borrowers with strong credit and reserves, which is exactly why two people with the same income can get approved for very different loan amounts.
The 28/36 rule and the 2–2.5× income rule
Two rules of thumb come up constantly in real home-buying discussions, and both are legitimate — they're just answering slightly different questions. The 28/36 rule says housing costs alone shouldn't exceed 28% of gross income, and total debt (including housing) shouldn't exceed 36%.
The older 2 to 2.5× income rule is blunter: a home price of roughly two to two-and-a-half times your gross annual household income. On a $75,000 household income, that points to a home in the $150,000–$190,000 range — often noticeably more conservative than what a 36% DTI calculation would approve, which is a large part of why it persists as popular advice even though lenders don't use it directly.
Why your approval amount and your comfortable amount aren't the same number
A lender's DTI formula has no idea about your other financial goals — retirement contributions, an emergency fund, travel, childcare, or simply how much cash flow flexibility you want month to month. It only knows your income and your listed debts.
This is why a very common outcome is getting pre-approved for meaningfully more than feels comfortable to actually spend. The fix isn't to distrust the pre-approval number; it's to run your own numbers separately, with your actual target down payment and a housing budget that reflects your whole financial picture, not just your debt ratio.