How to use it
- Set the centre: search, use your location, or click the map.
- Set the radius.
- Read the estimate. In the US it comes from the ZIP codes inside the circle; elsewhere from the cities and towns inside it.
5 practical uses
- Estimate the market around a shop, a clinic or a restaurant.
- Size a catchment for a school, a gym or a church.
- Support a business plan or a grant application with a figure.
- Compare candidate sites by the people within reach.
- Estimate the audience of a local advert or a leaflet drop.
The maths behind it
The estimate counts the population of every ZIP code (or place) whose centre point lies inside the circle, by the haversine distance from the centre. The density is that population divided by the area of the circle, πr². The error comes from ZIP codes cut by the circle's edge, which is why the estimate is best when the radius is much larger than the ZIP codes.
How good the estimate is
It is an estimate. The ZIP code count is good in towns and cities, where ZIP codes are small: a 10-mile circle around a city centre holds dozens of them and the errors at the edge average out. In the countryside a ZIP code can be 30 miles across, and it counts as all in or all out. The city count only knows places with 5,000 or more people, so it misses villages and the countryside entirely, and it uses municipal populations, not metropolitan areas. For a ring of a few miles around a shop, use the ZIP codes; for "how many people live within 100 km of Paris", the cities.
ZIP code positions and populations are the 2020-census ZIP Code Tabulation Areas from SimpleMaps' free database (CC BY 4.0), 33,000 of them. Each is one point, the centre of the area, so a ZIP code counts as inside or outside the circle by that point; a big rural ZIP code is only partly inside in truth. Post-office-box and military ZIP codes are included where they have a position.
Cities and towns come from GeoNames (CC BY 4.0): every populated place with 5,000 or more people, 46,000 of them, with the population GeoNames records, which for a city is usually the municipality rather than the metropolitan area. Distances are to the centre point of each place.