The housing affordability index (HAI) is a single number that answers a fundamental question: can a household earning the median income in a metro area afford a median-priced home? When the index is 100, the median household just qualifies. Below 100, housing is out of reach for the typical household; above 100, they can afford more than the median.

The Standard Calculation

HAI = (Qualifying Income Threshold / Median Household Income) × 100 Qualifying Income = Monthly PITI × 12 / 0.28 PITI = Principal + Interest + Taxes + Insurance (assumes 20% down, 30-year fixed mortgage, current rate)

ZipMarketData's Affordability Endpoint

The /affordability endpoint returns the HAI for 900+ US metro areas using Census ACS median household income, Redfin median prices, and current mortgage rate assumptions. You can also supply a custom income parameter to personalise the score.

GET /affordability?metro=Austin-Round+Rock&income=85000 { "metro": "Austin-Round Rock, TX", "median_home_price": 475000, "median_household_income": 82000, "affordability_index": 68, "interpretation": "housing costs significantly exceed what typical households can afford" }

Why HAI Matters for Investors

Markets with very low HAI (< 70) face an affordability ceiling on price growth — eventually buyers run out of income to service larger mortgages. Markets with HAI above 130 have significant runway for price appreciation and population growth. HAI is one of the best long-range forecasting tools for identifying sustainable growth markets vs. bubble-prone ones.