How the FCC's coverage maps are made

Since 2022 every facilities-based mobile carrier in the United States has had to file coverage maps with the FCC under the Broadband Data Collection, the program created by the Broadband DATA Act of 2020. The maps are filed twice a year, showing the network as of June 30 and December 31, and the FCC publishes them on its National Broadband Map about five months later. Everything on this site is read from those published files.

One map per technology and speed tier

A carrier does not file a single "coverage" map. It files one polygon map for each combination of technology and minimum speed the FCC defines:

  • 4G LTE at 5 Mbps down and 1 Mbps up.
  • 5G-NR at 7 Mbps down and 1 Mbps up.
  • 5G-NR at 35 Mbps down and 3 Mbps up.
  • 3G at 0.2 Mbps down and 0.05 Mbps up, now rarely filed.

Inside each map the carrier marks where the speed holds for a phone outdoors at a standstill, and separately where it also holds inside a moving vehicle. The in-vehicle area is always the smaller one. There is no indoor map at all. What the two environments mean.

The map is a model, run on the FCC's terms

The polygons come from the carrier's radio-frequency propagation model: software that takes every cell site, its antenna heights, frequencies and power, the terrain and land cover between the site and the ground, and predicts the signal at each point. The FCC fixes the assumptions so filings can be compared. The speed must be met with 90 percent cell-edge probability, meaning the model has to be confident nine times in ten at the edge of the area, and with the network 50 percent loaded, meaning half of each cell's capacity assumed already in use. Carriers also file the model name, the clutter categories and the link budget behind each map, and the FCC publishes those too.

Two things follow. The map is a prediction, not a record of phones that connected. And it is a prediction for an average phone outside: a different handset, a different antenna, a phone in a pocket or on a dashboard will do better or worse.

From polygons to hexagons

The FCC does not publish the raw polygons. It lays a grid of H3 hexagons over each map at resolution 9, hexagons of about 0.105 km², roughly 350 m across, and marks a hexagon as covered when the carrier's polygon covers the hexagon's centre point. The published file for a carrier, a state and a technology is that list of hexagons, each with the speed tier, the environment flag and the lowest signal strength the model predicted inside it.

The address lookup on this site finds the resolution-9 hexagon an address falls in and reads every carrier's entry for it. An address near a hexagon edge sits in a different cell from its neighbour across the street, and a carrier's polygon can cover one hexagon centre and miss the next, so a result is best read as "the carrier models service in this 350 m cell", not as a verdict on one rooftop.

The summaries behind the county pages

Alongside the hexagon files the FCC publishes summary tables: for each state, county, congressional district and census place, the share of land area each carrier files as covered outdoors and in a vehicle, and the share covered by any carrier at each technology tier, split into urban and rural land using the Census Bureau's definitions. The state and county pages are those tables, with nothing added.

Who is and is not in the files

The filings cover carriers that own their radio networks: the three national carriers, the regional carriers, and a number of small rural operators. A reseller that runs on another carrier's network does not file; its coverage is its host's map, with any limits the reseller's plan adds. Roaming agreements are not in the maps either: a carrier's map shows its own network only, even where its customers get service through a partner.

Sources

  • FCC, Broadband Data Collection: Specifications for Data Downloads from the National Broadband Map, the file and field definitions the ingest reads.
  • FCC, Broadband Data Collection mobile filing requirements, for the speed tiers, cell-edge probability and loading assumptions.
  • Uber H3 documentation, for hexagon sizes by resolution.