Data Sources

The Affordability Index uses two primary public datasets to calculate affordability metrics, including the Affordability Score (0-100) and detailed affordability ratios for every US city and ZIP code with available data.

Zillow Home Value Index (ZHVI)

Source:
Zillow Research zillow.com/research/data/
Update Frequency:
Monthly (typically released mid-month for prior month)
Coverage:
Census Places (cities/towns) and ZIP Code Tabulation Areas (ZCTAs)
Description:
ZHVI represents the typical home value for a given geography. It is a smoothed, seasonally adjusted measure that reflects the middle tier of the housing market.

We use the "Single-Family Residences + Condos, Middle Tier" ZHVI dataset, which provides the most comprehensive coverage across US geographies.

US Census American Community Survey (ACS)

Source:
US Census Bureau census.gov/programs-surveys/acs
Table:
B19013 - Median Household Income in the Past 12 Months
Dataset:
ACS 5-year estimates (most stable and comprehensive)
Update Frequency:
Annual (5-year estimates released in December, typically lag 1-2 years)
Coverage:
All Census Places and ZCTAs with sufficient sample sizes

Important: ACS estimates include margins of error (MOE). Smaller geographies (small towns, rural ZCTAs) have higher margins of error.

Geographic Coverage

What We Cover

  • Census Places: ~19,000+ cities, towns, and census-designated places (CDPs) nationwide
  • ZIP Code Tabulation Areas (ZCTAs): ~32,000+ ZIP areas with available data
  • States: All 50 states plus District of Columbia

Coverage Gaps & Limitations

Not all areas have complete data. Here's why some locations may be missing or have partial coverage:

  • Zillow ZHVI Gaps: Rural areas, small towns, and ZIPs with limited housing transactions may lack Zillow home value data. Commercial/industrial ZIPs may also be excluded.
  • ACS Data Suppression: Very small areas (under a few thousand residents) may have income data suppressed for privacy purposes or have high margins of error.
  • ZIP vs ZCTA: Some USPS ZIP codes don't have corresponding ZCTAs, particularly PO Box-only ZIPs and rural routes.

Data Quality Indicators

We provide visual indicators to help you assess data reliability:

  • Full Coverage: Both Zillow and Census data available. High reliability.
  • Partial Coverage: One data source missing (e.g., home values available but income data missing). Use with caution.
  • Small Population Warning: Areas under 5,000 residents get a warning banner due to higher margins of error in Census estimates.

How We Use This Data

Calculating the Affordability Ratio

Affordability Ratio = Home Value / Median Household Income

We combine Zillow's ZHVI (typical home value) with Census ACS median household income to create this ratio. A lower ratio means homes are more affordable relative to local incomes.

Example: If the median home costs $300,000 and median household income is $75,000, the ratio is 4.0 (homes cost 4× annual income).

Temporal Mismatch

Important caveat: Our ratio combines data from different time periods. Zillow ZHVI is updated monthly (current), while ACS income is a 5-year average (lags 1-2 years). This means we're comparing current home values to income from 1-2 years ago.

Why this matters: If incomes in your area have changed significantly (up or down) in the past 1-2 years, the affordability ratio may not fully reflect current conditions. We update income data annually when new ACS estimates are released (typically December).

Update Schedule

  • Zillow ZHVI: Monthly refresh when new data is published (mid-month for prior month)
  • Census ACS: Annual refresh when new 5-year estimates are released (typically December)
  • Timestamps: Each data point includes an "as of" date showing when it was last updated

Data Attribution

This project uses publicly available data from the following sources. We provide attribution on every page:

Zillow Research Data

Attribution: Data provided by Zillow Research (zillow.com/research/data/)

Zillow Home Value Index (ZHVI) is used under fair use for research and educational purposes. Zillow data is refreshed monthly.

US Census Bureau Data

Attribution: US Census Bureau, American Community Survey (census.gov/programs-surveys/acs)

ACS 5-year estimates are public domain data. Income data is updated annually when new estimates are released.

Data Updates

We refresh our data on the following schedule:

  • Zillow ZHVI: Monthly refresh when new data is published
  • Census ACS: Annual refresh when new 5-year estimates are released (typically December)

Each data point includes a timestamp indicating the "as of" date for that measurement.

Attribution

This project uses publicly available data from: