Methodology

1. Geography: ZIP codes vs. ZCTAs

A ZIP code is a USPS delivery route, not a geographic area, and the Census Bureau does not publish statistics for it directly. Every demographic figure on this site is reported for the corresponding ZIP Code Tabulation Area (ZCTA), the Census Bureau's approximation of a ZIP code built from whole census blocks. For most residential ZIPs the two match closely; they diverge for post-office-box, single-building, campus and industrial ZIPs. County assignment follows the county containing the ZIP's centroid; a handful of rural ZIPs straddle county lines, and their page names the county the USPS uses.

2. ZIPs without demographics

946 of the 2,710 California ZIPs in this directory are PO Box or unique ZIPs: they serve post office boxes, a single large mailer or an institution rather than street addresses, so they have no ZCTA and no Census population, income or housing estimate. A further 206 standard delivery ZIPs have a resident population too small, or too institutional (campuses, military installations, hospitals), for the ACS to publish usable income and home-value medians. All of these pages state that plainly instead of showing zeros, point to the delivery ZIPs of the same city that do carry data, and are excluded from every median, rank and report. That leaves 1,558 ZIPs with full housing data, which is the population every "of 1,558" rank on this site refers to.

3. Primary source: ACS 5-Year Estimates

Population, density, median age, sex ratio, median household income, per-capita income, median home value, median gross rent, owner- and renter-occupancy, total housing units, high-school and bachelor's attainment (population 25 and over), poverty rate, unemployment, and land and water area come from the U.S. Census Bureau's American Community Survey (ACS) 5-Year Estimates, retrieved from data.census.gov at the ZCTA level. The 5-year product is used because 1-year estimates are only published for geographies of 65,000 or more people, which excludes almost every ZIP. A 5-year estimate describes the average condition across the collection window rather than a single point in time. Each field maps to a standard ACS detailed table — B01003 for population, B19013 for median household income, B19301 for per-capita income, B25077 for median home value, B25064 for median gross rent, B25003 for tenure, B17001 for poverty, B15003 for educational attainment, B23025 for employment status and B01002 for median age — so any figure can be checked directly on data.census.gov by table and ZCTA.

4. The $2,000,001 and $250,001 caps

The ACS top-codes median home value at $2,000,001 and median household income at $250,001. 53 California ZIPs sit at the home-value cap (concentrated on the Peninsula and in the South Bay, in Marin, and along the Los Angeles and Orange County coast) and 10 at the income cap. Their true medians are higher, so any ratio computed from a capped value is a floor. Our reports and per-ZIP profiles mark these with "at least" or a plus sign; ZIP pages show the capped value as the Census reports it.

5. Figures we compute ourselves

• Price-to-income ratio = median home value ÷ median household income. 3.0 is the long-run U.S. norm; ratios above 5.0 are conventionally described as severely unaffordable. The median California ZIP is currently 6.7.
• Rent-to-income ratio = (median gross rent × 12) ÷ median household income, as a percentage. Above 30% is "cost-burdened" and above 50% "severely cost-burdened" under the U.S. Department of Housing and Urban Development definitions. Both ratios use ZIP-level medians and describe the area, not any individual household.
• Statewide, county and city benchmarks are medians across the ZIPs in scope with each ZIP weighted equally, not population-weighted. City and county populations are the sum of their ZIPs' ACS populations.
• Ranks ("N of 1,558") sort every ZIP with housing data on a single ACS field, 1 = highest. County affordability ranks sort the county's ZIPs on price-to-income, 1 = lowest ratio.
• Nearby ZIPs, nearest cities and neighboring counties are the closest entries in the dataset by straight-line distance between centroids, corrected for longitude at California's latitude.
• Density bands in the written profiles (apartment-district, mixed urban, single-family suburban, low-density, rural) are fixed thresholds on residents per square mile: 18,000, 8,000, 2,500 and 300.

6. Editorial content

The description, FAQs and "Affordability & Profile" section on each ZIP page, the "at a glance" overview on each city and county page, and every report are generated by the California Zip Codes Data Team from that area's specific figures and reviewed before publication; each sentence is conditional on the numbers, so two areas with different data read differently. We do not accept sponsored rankings, paid placements or real-estate referral fees, and no page's content is influenced by advertisers. Crime, school and walkability scores are not published on this site: the ACS does not collect them, and ZIP-level versions from third parties are not comparable across California's 58 counties.

7. Known limitations

• ACS estimates are survey-based with margins of error that grow as population shrinks. Rural mountain and desert ZIPs with a few hundred residents carry wide margins; small differences between such ZIPs are not meaningful, and reports restrict rankings to ZIPs above a stated population floor.
• Median home value is the owner's estimate from the ACS questionnaire, not a transaction price, and is capped at $2,000,001.
• Median gross rent includes utilities and covers all occupied rental units, including long-tenured and rent-controlled leases, so it runs below current asking rents in the coastal metros.
• ZIPs dominated by students (around UC and CSU campuses), military bases or prisons show medians that describe residents, not the buyer or renter pool.
• USPS city names are not municipal boundaries: "Los Angeles" ZIPs include unincorporated areas, and some incorporated cities appear under a neighboring post office's name.
• ZIP-level medians describe areas, not households.

8. Update schedule

The Census Bureau releases new ACS 5-Year Estimates each December. We rebuild the dataset after each release, re-run every derived figure, profile and report, and re-review pages whose numbers moved materially. Between annual rebuilds, corrections are applied as they are found.

9. Corrections

If a figure on this site does not match its Census source, or a city or county assignment, ZIP type, nearby-ZIP list or description is wrong, tell us with the ZIP code and the field in question. We verify against the primary source and correct the page; substantive corrections are noted on the affected page and on the About page.