Data sources and measurements

Distance and circle geometry

Circles use a spherical Earth with a mean radius of 6,371,008.8 meters. Distance uses the haversine formula; circle points use great-circle bearings. Map outlines and geographic exports approximate each circle with 180 segments.

Radii range from 100 meters to 1,000 kilometers. Centers must be between 85° S and 85° N because the basemap uses Web Mercator, which distorts shapes near the poles. One mile equals exactly 1,609.344 meters.

Area, diameter and overlap

Area is A = 4πR² sin²(r / 2R), close to πr² for local circles. Diameter is twice the radius; circumference is 2πR sin(r / R). Measurements describe each circle separately. Overlaps are not removed or combined into a unique coverage area.

US ZIP coverage

Results use 2020 Census ZCTAs from TIGERweb’s ACS2025 service that intersect the circle’s 180-segment polygon. ZCTAs approximate ZIP delivery areas and may omit PO Box, organization-only or recently changed USPS codes. Lookup supports up to 100 miles (160.9344 km) and returns up to 2,000 areas. A plus sign marks truncated results.

Fully inside means the entire ZCTA polygon is contained in the circle polygon; partly inside means they intersect. Distance is to the ZCTA centroid, which can be outside the circle. Display boundaries are simplified. Copy and CSV follow the current coverage filter and sort order. ZIP data is US-only; circle drawing works worldwide.

Cities and towns within a radius

City results use a bundled snapshot of GeoNames cities15000, licensed under CC BY 4.0. It covers places above 15,000 residents and capitals, excluding city subdivisions. Smaller places may be absent. Selection uses reference points, not city boundaries or population inside the circle.

The table shows the nearest 500 matches and the map the nearest 100. City CSV contains all matches. The download date is shown beside the results. Each query uses the selected circle; overlapping circles are queried separately.

Concentric circles

Presets and custom distances add circles around the selected center in the current unit. Existing circles remain; matching center and radius combinations are skipped. Each ring counts toward the 12-circle limit. Rings can later be moved or resized independently; their centers are not linked.

Search and map data

Photon searches addresses and postal codes using OpenStreetMap data. Choose the correct result or use coordinates. OpenStreetMap provides the basemap. Maps, search and Census queries require internet access. City queries and city CSV use the server’s dataset; geometry and other exports run in the browser.

Exports

GeoJSON and KML use WGS84 longitude/latitude circle polygons. GeoJSON includes centers, radii in meters and areas in square meters. Circle CSV uses the chosen unit. PNG captures the visible map with a legend and attribution; use Fit all circles first to include every circle. ZIP CSV follows the selected filter; city CSV includes every match and its source.

Saving and sharing

Browser storage saves the workspace on this device. Share links encode names, coordinates, radii and colors in the URL fragment. Anyone with the link can reopen those circles. Read the privacy policy before sharing sensitive locations.

Measurement limits

RadiusMap is for planning and visualization. A radius is a straight-line surface distance, not a driving distance, travel-time boundary, legal survey or guarantee of service availability. Population estimates are not provided.