Click anywhere on the map to drop a centre point, then set the radius. You get the covered area and two export formats — a circle definition and an approximated GeoJSON polygon.
A radius geofence stores two things: a centre coordinate and a distance. Testing whether a device is inside is a single distance calculation, which is why circles are the cheapest possible geofence to evaluate and why almost every tracking platform supports them.
GeoJSON, however, has no circle type. RFC 7946 defines points, lines, and polygons — that is all. So when a circle has to travel through a GeoJSON-based pipeline, it gets approximated as a many-sided polygon. This tool exports both, because which one you need depends entirely on what is consuming it:
| Format | Use when |
|---|---|
| Circle — centre + radius | The receiving system supports radius geofences natively. Exact, compact, cheapest to test. |
| GeoJSON polygon — 64-sided approximation | The receiving system only speaks standard GeoJSON. Accurate to within ~0.1% of the true circle area. |
Geoblip accepts both, so a zone drawn here can be used either way without conversion loss.
The radius must be larger than the position error of whatever is reporting into it. Otherwise a stationary device will appear to cross the boundary repeatedly as its reported position drifts — the classic symptom of a geofence that "keeps alerting for no reason".
| Radius | Roughly covers | Reliable with |
|---|---|---|
| 50 m | A single building | Nothing — below GPS noise floor |
| 150 m | Building plus car park | Good open-sky GPS |
| 500 m | A school campus, a small village centre | Most phones and trackers |
| 1 km | A neighbourhood | Anything, including cell-tower positioning |
| 5 km+ | A town, a delivery region | Anything |
If you find yourself wanting a tight radius around an irregular site, that is usually the signal to switch to a traced polygon instead — the geofence builder follows the actual outline rather than forcing a circle to cover corners it does not need.
Web maps use the Web Mercator projection, which stretches distances increasingly as you move away from the equator. A circle that is genuinely round on the ground is drawn as an ellipse at high latitudes. This is a rendering artefact, not an error in the geometry — the exported coordinates describe a true circle on the Earth's surface. The effect is invisible in the tropics and obvious in Scandinavia.
The area shown uses spherical geometry rather than the flat-plane
πr², which slightly overstates large circles. The difference
is negligible below a few kilometres and grows from there. To measure an
irregular shape instead, use the
polygon area calculator, and
to check the distance between two specific points use the
distance calculator.
A radius on a map is a description. To get notified when a device enters or leaves it, you need something watching. In Geoblip you draw the zone, assign a phone or GPS tracker, choose the hours it should be active, and pick how you want to hear about it — email, push, or in-app.
This tool draws the boundary. Geoblip watches it — assign a phone or GPS tracker, set the hours it should be active, and get an email or push notification the moment it crosses the line.