Measurement tool
Map radius tool
Draw a circle of 1 to 500 miles on the map and read its area and circumference on the sphere.
Short answer
- 100 mi radius
- 31,414sq mi
- 500 mi radius
- 784,354sq mi
Circle area on a sphere versus flat
| Radius | Area on sphere | Flat π r² | Flat overstates by |
|---|---|---|---|
| 10 mi | 314 sq mi | 314 sq mi | 0.00% |
| 50 mi | 7,854 sq mi | 7,854 sq mi | 0.00% |
| 100 mi | 31,414 sq mi | 31,416 sq mi | 0.01% |
| 250 mi | 196,284 sq mi | 196,350 sq mi | 0.03% |
| 500 mi | 784,354 sq mi | 785,398 sq mi | 0.13% |
How to use it
- Click the map or search a place to set the center.
- Drag the Radius slider from 1 to 500 miles. The circle redraws as you move it.
- Read the card: area in square miles, circumference in miles, and the center's latitude and longitude, all measured on the sphere.
- Click somewhere else to move the circle while keeping the radius.
A true circle on a curved surface
Every point on the drawn edge is the same great-circle distance from the center. On a web map, which uses the Mercator projection, that circle looks slightly egg-shaped at high latitudes, taller than it is wide. That is the projection stretching north-south distances, not an error in the circle.
A flat map circle drawn with a fixed pixel radius would be the wrong size away from the equator. The Mercator guide explains the stretch.
Why the sphere gives a smaller area
On a curved surface, a circle of radius r encloses less area than π r². The exact figure is 2πR² (1 − cos(r/R)), where R is the Earth's radius. The difference is tiny for a town-sized circle and grows with the radius, as the table shows. The circumference follows the same rule: 2πR · sin(r/R), a little less than 2πr.
The tool reports both values on the sphere, so they stay right at the 500 mile end of the slider.
What is inside the circle
This tool draws and measures the circle only. To list what falls inside it, use population within a radius, ZIP codes within a radius, or cities within a radius. Each uses the same geodesic circle.
Common radius questions
People use a radius to check delivery or service areas, to see which towns fall within a set distance of home, or to picture a 50 mile rule from an event or a lease. A circle answers "how far as the crow flies". It ignores roads, rivers, and borders.
For "how far can I get in 30 minutes", use the drive time map instead.
Questions people ask
Why does my circle look like an oval?
The web map uses Mercator, which stretches north-south distances away from the equator. The circle is round on the ground.
What is the largest radius?
500 miles.
Is the area land only?
No. It is the full area of the circle, land and water.
Sources and data as of
Data as of: computed on a sphere of radius 6,371.0088 km.
How every number is computed: methodology. Spotted a wrong figure? Report an error.