Geography reference
Census Tracts
The neighbourhood unit of American statistics. The American Community Survey, the decennial census and most federal programme data are published against these, so a column of eleven-digit codes is what an analyst usually has — and a tract map is what they usually cannot make.
The 242 census tracts of San Francisco County, drawn from the exact boundaries Vizzie joins your data to — not a picture of them.
How the codes are built
A tract GEOID is eleven digits that nest: two for the state, three for the county, six for the
tract itself. 06075010100 is California, San Francisco County, tract 101. The
first five digits are the county FIPS code on its own, which is why a tract file can be split
by county without a lookup table.
The leading zero is the thing that breaks. Every state below FIPS 10 — Alabama through
Connecticut, and California at 06 — starts with a zero, and a spreadsheet will silently strip
it the moment the column is read as a number. 06075010100 becomes
6075010100, ten digits that match nothing. If a join comes back near-empty on US
data, check the width of the codes before anything else.
30049000504 | Census Tract 5.04 |
06037911100 | Census Tract 9111 |
12127090305 | Census Tract 903.05 |
17115001802 | Census Tract 18.02 |
What it rolls up into
- Census block — the smallest unit published
- Block group — a few hundred to a few thousand people
- Census tract — around 4,000 people, between 1,200 and 8,000
- County
- State
Which years exist
Tracts are redrawn for each decennial census. Where population has grown they are split, where it has fallen they are merged, and the new pieces get new numbers — so a 2010-coded dataset will not join cleanly to 2020 boundaries in exactly the places that changed most, which are usually the places being studied.
Vizzie holds the 2020 set. A dataset published on 2010 tracts reports a partial match rather than a silent one: the unmatched codes are the tracts that moved.
Map a spreadsheet of these
Paste a column of tract codes into Vizzie with whatever you measured beside it. Vizzie recognises the code format, joins it to the boundaries above and draws the map — no shapefile, no download, no GIS install.
Boundaries: U.S. Census Bureau, 2020 Census — cartographic boundary files (public domain).
Licence: US-Census-Public-Domain. Source: US Census Bureau.
Boundary counts and sizes measured from the data Vizzie holds, 2026-09-19.