House Affordability

The House Affordability Calculator is grounded in the established principles and formulas described below. A mortgage is a secured loan used to purchase real estate, where the property itself serves as collateral. Understanding how mortgage payments are calculated helps borrowers compare offers, plan budgets, and potentially save hundreds of thousands of dollars over the life of a loan. The standard monthly mortgage payment for principal and interest is determined by the amortization formula: M = P[r(1+r)^n] / [(1+r)^n - 1], where M is the monthly payment, P is the loan principal (home price minus down payment), r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments (loan term in years times 12). This formula produces level payments over the life of the loan, but the proportion allocated to interest versus principal changes with each payment. In the early years, the majority of each payment covers interest because the outstanding balance is large. As the balance decreases, more of each payment reduces principal. This gradual shift is called amortization. For example, on a $300,000 loan at 6.5 percent for 30 years, the monthly principal and interest payment is approximately $1,896. In the first month, roughly $1,625 goes to interest and only $271 to principal. By year 15, the split is roughly equal, and in the final year, nearly the entire payment reduces the balance. The total monthly housing payment typically includes four components, often abbreviated PITI: Principal, Interest, Taxes, and Insurance. Property taxes are assessed annually by local governments, usually ranging from 0.5 to 2.5 percent of assessed value, and are divided into monthly escrow payments collected by the lender. Homeowners insurance protects against damage and liability, and lenders require coverage at least equal to the loan amount. Private Mortgage Insurance (PMI) is an additional cost required when the down payment is less than 20 percent of the purchase price. PMI protects the lender against default, not the borrower, and typically costs between 0.3 and 1.5 percent of the original loan amount annually. PMI can be removed once the loan-to-value ratio reaches 80 percent through regular payments or appreciation, and is automatically terminated by law at 78 percent LTV. Fixed-rate mortgages lock the interest rate for the entire loan term, providing predictable payments. The most common terms are 30 years (lower monthly payment, more total interest) and 15 years (higher monthly payment, substantially less total interest). On a $300,000 loan at 6.5 percent, choosing a 15-year term over a 30-year term saves approximately $200,000 in total interest, but requires a monthly payment roughly 50 percent higher. Adjustable-rate mortgages (ARMs) offer a lower initial rate for a fixed period (commonly 5, 7, or 10 years), after which the rate adjusts periodically based on a market index plus a margin. ARMs carry rate caps that limit how much the rate can increase per adjustment and over the loan's lifetime. ARMs can be advantageous for borrowers who plan to sell or refinance before the adjustment period begins. Mortgage points are fees paid at closing to reduce the interest rate. One discount point costs 1 percent of the loan amount and typically reduces the rate by approximately 0.25 percent. Points make financial sense when the borrower plans to hold the mortgage long enough for the monthly savings to exceed the upfront cost, usually a break-even period of 4 to 7 years. Lenders evaluate borrowers using the debt-to-income (DTI) ratio. The front-end ratio compares monthly housing costs to gross monthly income and should generally be below 28 to 31 percent. The back-end ratio includes all monthly debt obligations and should typically remain below 36 to 43 percent. Credit score, employment history, and assets also significantly influence approval and the interest rate offered.

The House Affordability Calculator builds on a long history of ideas and practice, traced below. The concept of the mortgage dates to ancient civilizations. In Roman law, the hypotheca allowed a debtor to pledge property as security without surrendering possession. The English word mortgage derives from the Old French mort gage, meaning dead pledge, because the arrangement ended (died) either when the debt was repaid or when the lender foreclosed on the property. In medieval England, mortgages were typically short-term arrangements requiring a lump-sum repayment. The modern long-term amortizing mortgage did not emerge until the twentieth century. Before the 1930s, American home loans were commonly five-year balloon mortgages requiring renewal or full repayment, which created catastrophic risk for borrowers when the Great Depression caused banks to refuse renewals. The US federal government transformed mortgage lending during the 1930s. The Federal Home Loan Bank System was created in 1932 to provide liquidity to mortgage lenders. The Federal Housing Administration (FHA), established in 1934, introduced the long-term, fixed-rate, fully amortizing mortgage — the format that dominates American housing finance today. By insuring lenders against default, the FHA made low-down-payment loans viable and standardized underwriting practices nationwide. The GI Bill of 1944 (Servicemen's Readjustment Act) provided zero-down-payment VA-guaranteed home loans to returning veterans, fueling the suburban housing boom of the 1950s and 1960s and dramatically expanding homeownership rates. The creation of Fannie Mae (1938) and Freddie Mac (1970) established the secondary mortgage market, allowing lenders to sell mortgages to investors and free up capital for new lending. The first mortgage-backed securities in the 1970s further expanded available capital for home loans. The Savings and Loan crisis of the 1980s resulted from maturity mismatch — thrift institutions funded long-term fixed-rate mortgages with short-term deposits — combined with deregulation and fraud. Approximately 1,000 institutions failed, costing taxpayers an estimated $160 billion. Adjustable-rate mortgages gained popularity partly as a response to this crisis, shifting interest-rate risk from lenders to borrowers. The 2008 financial crisis was triggered by the collapse of the subprime mortgage market. The originate-to-distribute model incentivized lenders to approve risky loans and sell them into securitization vehicles, leading to widespread defaults when housing prices fell. Millions of foreclosures followed, and the near-collapse of the global financial system prompted the Dodd-Frank Act of 2010, which established qualified mortgage standards, ability-to-repay requirements, and created the Consumer Financial Protection Bureau (CFPB) to oversee mortgage lending practices. Today, the 30-year fixed-rate mortgage remains uniquely American — most other countries primarily use adjustable-rate or shorter-term mortgages. Conforming loan limits, set annually by the Federal Housing Finance Agency, determine the maximum loan size eligible for purchase by Fannie Mae and Freddie Mac. In 2024, the limit for most US counties was $766,550, with higher limits in designated high-cost areas.