Home Affordability Calculator

Affordability underwriting runs on two debt-to-income ratios rather than on price. The front-end or housing ratio caps principal, interest, taxes, and insurance - the PITI figure, plus HOA dues where they exist - at 28 percent of gross monthly income. The back-end ratio caps PITI plus every other recurring obligation reported on the credit file, including auto notes, student loans, and minimum card payments, at 36 percent. Whichever constraint binds first sets the housing budget, which is why a borrower earning 150,000 dollars with 2,000 dollars of monthly debt is limited by the back end while a borrower with little other debt is limited by the front end. This calculator solves in the direction lenders do: it derives the maximum housing payment, subtracts escrowed taxes and insurance, then inverts the annuity formula to recover the largest supportable loan and adds the down payment. The property tax term creates a circularity - taxes depend on price, and price depends on how much payment remains after taxes - so the calculation iterates until the two agree. Effective property tax rates commonly run from about 0.3 percent in Hawaii to over 2 percent in New Jersey and Illinois, and homeowners insurance typically falls between 1,200 and 2,500 dollars a year. Below a 20 percent down payment, private mortgage insurance of roughly 0.3 to 1.5 percent of the loan annually applies and is modeled here at 0.5 percent. The 28/36 rule is a guideline, not a legal ceiling: Fannie Mae and Freddie Mac accept back-end ratios up to 45 or 50 percent with compensating factors, FHA commonly allows 43 percent or higher, and the qualified mortgage standard historically anchored at 43 percent. The model also excludes maintenance, which typically consumes 1 to 3 percent of home value each year, and utilities.

Ratio-based housing underwriting descends from the Federal Housing Administration created by the National Housing Act of 1934. Before insured lending, informal rules of thumb prevailed - a house priced at twice annual income, or a payment of about one week's pay per month. FHA's Underwriting Manual of 1935 and its successors formalized the practice of measuring a borrower's payment against income and became the template that private lenders copied. Conventional lenders through the 1950s and 1960s often used a rougher standard of 25 percent of gross income for housing. The specific 28/36 pairing became standard as the secondary market industrialized. The Federal National Mortgage Association was chartered in 1938 and made private in 1968, the Government National Mortgage Association was split off that same year, and the Federal Home Loan Mortgage Corporation was created in 1970; selling loans to these buyers required uniform, testable underwriting rules, and the 28 and 36 percent thresholds were embedded in their seller-servicer guides. Automated underwriting arrived in the mid-1990s with Fannie Mae's Desktop Underwriter and Freddie Mac's Loan Prospector, which weighed reserves, credit score, and loan-to-value alongside the ratios and permitted higher DTI with compensating factors. After the 2007 to 2009 crisis the Dodd-Frank Act of 2010 imposed a statutory ability-to-repay duty; the Consumer Financial Protection Bureau's 2013 qualified mortgage rule, effective January 2014, set a 43 percent back-end limit that was later replaced in 2021 with a price-based standard, though the 28/36 heuristic survives as consumer guidance.