PlainZIP
Methodology and Sources
First published February 22, 2026 · revised October 9, 2026
Every figure on PlainZIP comes from a public file named on the page where it appears, with the file's date. ZIP code pages link each figure's table at its publisher (the Census Bureau's table for that ZIP code, FHFA, the IRS), and the sources below link every publisher, so you can check a figure at the source. The files are loaded into one database by a scripted pipeline that refuses to continue when an input is missing, and each page reads its figures from that database when you open it. Survey figures are printed with the survey's own years (the American Community Survey 2020-2024), never with the date the site was built, and where the survey's margin of error is large the page shows it.
Sources
To check a figure yourself, start from the publishers' own tables: the Census Bureau's data.census.gov (search a ZIP code as a ZCTA), FHFA's house price index files, the IRS's ZIP code income statistics and the Bureau of Labor Statistics' consumer price index.
Download the data
Every ZIP code PlainZIP knows, 41,706 rows, is one CSV file: plainzip-zip-codes.csv. Each row carries the ZIP code, its type (residential, PO Box or unique, military, territory), the USPS town name, the state, the county with the largest share of the ZIP code and its FIPS code, population and the medians of household income, home value and rent (ACS 2020-2024), land area, the FHFA 10-year home price change, and the address of its page. Medians the Census Bureau publishes only as a cap are written the same way ("250000+", "2500-"); a blank cell means the source has no figure. Each city, county, state, prefix and ranking page offers its own list in the same columns, next to its table.
The figures are those on the pages, from the files above. You may reuse them with a link to PlainZIP (terms); the GeoNames and simplemaps parts keep their CC BY 4.0 attribution.
How the database is checked
The pipeline loads each file into its own table, then builds the tables pages read. It stops when an expected input is missing or when an index year falls short of its publisher's schedule, and it records each source file's address and download time. ACS values that the Census Bureau publishes as caps ($250,000 or more for household income, $2,000,000 or more for home value) are kept as caps: pages print them in words and rankings treat them as ties, never as exact amounts. ACS figures with a large margin show it, and a change between the 2015-2019 and 2020-2024 surveys is stated only when it is larger than the combined margin of the two estimates at 90% confidence (1.645 standard errors of the difference) and the ZIP code's area kept at least 90% of its land between the 2010 and 2020 Census maps (19,304 ZIP codes qualify). Incomes from 2015-2019 are converted to 2024 dollars with the CPI-U (1.22699).
Before each release, figures on sample pages of every type are compared with the publisher's own tables (for example the Census Bureau's API for a ZIP code's population and income), and automated tests check that every list page links each of its ZIP codes, that every former address answers with one redirect to the page that now answers it, and that no page shows a build date as a data date; they also recompute income ranks from the county and state lists, check every county share against the Census relationship file and every sectional center against USPS labeling list L005. The checks run on the release database before each release, and a release ships only when all pass; the last full run passed on October 9, 2026.
| Pages | What is checked | How, by whom, last check |
|---|---|---|
| ZIP code pages | population and median household income of sample ZIP codes against the Census Bureau's API, in the database and on the page; capped values printed in words; income ranks recomputed from the county and state lists; dates printed only from source files | automated (scripts/qa/v3-checks.mjs), on the release database before each release, on a sample of every class; last passed October 9, 2026 |
| List pages | city, county, state and prefix lists: every ZIP code linked or anchored; every county share against the Census relationship file; every sectional center against USPS labeling list L005, parsed independently of the pipeline | automated (scripts/qa/v3-checks.mjs), on the release database before each release; last passed October 9, 2026 |
| Former addresses | every earned and a sample of other former URLs answer with one redirect to a page that carries the section it points to | automated (scripts/qa/v3-checks.mjs), on the release database before each release; last passed October 9, 2026 |
| Rankings | the eligibility rules (1,000+ people living in households, margin at most 25% of the figure, income per person included; 200+ returns and at most 1.5 a household for IRS income per return); ties at the Census caps stated and ordered by income per person | rules in the pipeline; reviewed before release by separate AI review agents (a search-quality rater, a spam analyst, a competitor editor, an answer engine and a claims reviewer); no person reviews each page |
| Research | each finding recomputed from the database; method and limits stated; counts compared with what chance alone gives | reviewed before release by separate AI review agents (a search-quality rater, a spam analyst, a competitor editor, an answer engine and a claims reviewer); no person reviews each page |
Which pages stand alone, and why
A residential ZIP code has its own page when the American Community Survey counts at least 100 residents in it, or the Census Bureau counts at least five business establishments, or readers already find the page in search. Smaller ZIP codes (9,440 of them, including every ZIP code without a residential area) appear as rows on the page of their town, county or, for military and island-territory codes, the ZIP code types guide, and their address leads there. A city with two or more residential ZIP codes has a list page (1,362 cities); a town with one ZIP code is found on that ZIP's page. A county with two or more residential ZIP codes has a list page (3,121 counties), and a three-digit prefix with three or more has one (711 prefixes). One set of ZIP codes is listed once: where a county or a prefix holds nearly the same residential ZIP codes as a city (80% or more of the two sets combined), as Kings County and the 112 prefix do with Brooklyn, it is a named section of the city's page, an independent city such as Richmond city is a section of the city of the same name when most of its ZIP codes carry that name, and the District of Columbia's county and city lists are sections of its state page. The same rules decide the pages, the sitemaps and the redirects, and they are re-applied every time the data is reloaded, so a ZIP code that gains residents gains its page.
How the pages are made
PlainZIP's owner (named on the About page) uses AI tools to write the code, the page templates and the explanatory text, including the research write-ups and these pages. The AI does not decide the numbers: every figure comes from a database query, the sentences on ZIP, city and county pages are built from that place's own figures by fixed rules, and each analysis states its method and limits. Before publication, every template, rule and analysis is checked against the source tables by the automated checks above and read by separate AI review agents for accuracy, sourcing and wording (a search-quality rater, a spam analyst, a competitor editor, an answer engine and a claims reviewer). No person reviews each page; PlainZIP's owner answers for what is published.
Limits
A ZIP code is a Postal Service delivery code, not a boundary: the Census Bureau's ZIP Code Tabulation Areas approximate the residential ones from census blocks, and every "by ZIP code" figure is really by ZCTA. ZIP codes for PO boxes, single organizations and the military have no area and no Census figures. ACS estimates are averages over five years of samples and lag the present; small areas carry wide margins. FHFA describes its ZIP-level price indexes as developmental, built from homes with mortgages bought or guaranteed by Fannie Mae and Freddie Mac; where the index and the Census survey's home values move in opposite directions, the ZIP page says so. The IRS files each return under the address on it, which can be an office or a mailbox rather than the home: income per return is an average over returns, not over residents, and ZIP codes with more than 1.5 returns per household are not ranked by it. Neighborhood names are GeoNames points placed in a ZIP code's area, not boundaries. Business counts are by the business's ZIP code, which can differ from where people live. Town names are the Postal Service's names, which often cover more than a city's legal limits.
What PlainZIP does not cover
PlainZIP describes ZIP codes as postal areas. Profiles of cities and counties as places (their own population and history) are left to other publishers; PlainZIP's city and county pages list the ZIP codes in them and show each ZIP code's own figures. PlainZIP does not look up street addresses or ZIP+4 codes: the USPS ZIP Code Lookup does.
Terms used on PlainZIP
What is a ZCTA?
A ZIP Code Tabulation Area: the Census Bureau's map of a residential ZIP code, built from the census blocks whose addresses mostly use that ZIP code. Census figures "by ZIP code" are by ZCTA. PlainZIP uses the 2020 ZCTAs (33,774 of them).
What does the margin of error mean?
ACS figures are estimates from a sample of households. The Census Bureau publishes a 90% margin with each: the true value lies within it nine times in ten. PlainZIP shows the margin where it is wide, and calls a change real only when it is larger than the combined margin of the two estimates (1.645 standard errors of the difference).
Why do some incomes read "$250,000 or more"?
The ACS publishes medians above its top code as the code itself (household income 250,001; home value 2,000,001). PlainZIP prints those as "or more" and ranks them as a tie (118 ZIP codes report household income at the top code).
When is a ZIP code comparable between surveys?
When its 2020 ZCTA keeps at least 90% of the land of its 2010 ZCTA (Census 2010-2020 relationship file). 19,304 ZIP codes qualify; changes are stated only for them.
What is a ZIP code's center?
The Census Bureau's internal point of the ZCTA (2023 Gazetteer): a point inside the area, used for distances, the radius tool and the national map. A ZIP code without an area has no center: GeoNames gives it its town's coordinates, so PlainZIP measures no distances to it.
What do the first three digits mean?
The sectional center: the mail processing facility that sorts for the ZIP code. PlainZIP names it from USPS labeling list L005 (effective October 1, 2026), which assigns each three-digit prefix to its facility; the town names most common among a prefix's ZIP codes are shown separately. The first digit alone names one of ten national areas, from 0 in the Northeast (with Puerto Rico and the Virgin Islands) to 9 on the Pacific (with the Pacific island areas).
Who answers for it
PlainZIP's owner, named on the About page, answers for every page. Write to hello@plainzip.com or see the corrections page; a confirmed error is fixed at its source in the next data load and listed on that page.