Radius guide
How to draw a radius around a location (catchment areas explained)
Drawing a circle around a point answers a simple question: what is within a certain distance of here? It is the quick way to size a store's trade area, a delivery zone, or how far you could realistically travel from home. This guide shows you how to draw one, what the radius, area and perimeter actually mean, and the one honest caveat that catches most people out.
What you'll need
Just a browser. Map Measurer runs on any phone, tablet or computer, with no account and no install. You need a centre point in mind — a shop, a station, your front door — and a rough sense of the distance you care about. The tool handles the maths.
Step by step
- Open the map measuring tool and select the Radius tool in the toolbar.
- Find your centre. Use the search box to jump to a place, address or postcode, or drag and zoom the map until your point is in view.
- Click once to set the centre. This anchors the middle of your circle — the location everything will be measured from.
- Click again to set the edge. The distance from the centre to this second click becomes your radius, and a circle is drawn around the centre to match.
- Read the panel. The tool shows the radius, the area enclosed and the perimeter (the distance all the way around the circle).
- Switch between km and miles with the unit toggle, and the area between square metres, square kilometres and square miles — acres and hectares are there for smaller circles. Every figure updates instantly.
If your edge click lands in the wrong place, use Undo to step back, or Clear to start fresh and reposition the centre.
Tip: if you already know the radius you want — say exactly 5 km — set the centre, then zoom in and nudge your second click until the readout shows the figure you're after. Working to the number rather than eyeballing the edge gives you a circle you can trust.
Worked example: a 5 km catchment around a shop
Suppose you're sizing the trade area around a new shop and you want everything within 5 km. Set the centre on the shop, then place the edge so the radius reads 5 km. The tool reports an area of about 78.5 km² — that's π × 5² — and a perimeter of roughly 31.4 km around the edge of the circle. If you prefer imperial, a 1-mile radius works out to about 3.14 square miles by the same formula. Those numbers give you an honest sense of how much ground the circle covers before you read anything else into it.
How it works and what the numbers mean
The radius is the straight-line distance from the centre to the edge — the same as-the-crow-flies distance you'd get from clicking two points in distance mode, just measured outward in every direction at once.
From that one number, the rest follows by simple geometry:
- Area = π × r². Double the radius and the area roughly quadruples, which is why a "10 km radius" covers four times the ground of a "5 km radius", not twice. It's worth keeping in mind when you compare zones.
- Perimeter = 2 × π × r. This is the circumference — the distance you'd travel going once around the edge of the circle.
Map Measurer shows all three together so you don't have to reach for a calculator, and recomputes them the moment you change the radius or switch units.
The honest part: a radius is not a travel-time area
This is the caveat worth slowing down for. A radius circle measures straight-line distance equally in all directions. The real world is not so even. Roads bend and detour, rivers and coastlines block the way, hills slow you down, and one-way systems send you the long way round. So the area you can actually reach from a point — in, say, ten minutes' driving — is almost never a neat circle. It's a lopsided blob that stretches along fast roads and pinches in wherever the going is slow.
That blob has a name: an isochrone — a line joining everywhere you can get to within a set travel time. It's a different and more complex calculation than a radius, and it changes with traffic, time of day and mode of transport. A radius answers "within X distance as the crow flies"; an isochrone answers "within X minutes' travel". They are not the same question, and confusing the two leads to overconfident maps.
So when is a circle the right tool? When straight-line distance is genuinely what you mean. That covers a lot: retail and store trade areas, real-estate searches for homes "within X of a school or station", delivery or service zones, drone range (fly out and return on a battery), emergency-planning buffers around a site, and the everyday "how far could I get from home?". For a quick first pass at any of these, a radius is fast, clear and easy to explain. If your decision really hinges on travel time, treat the circle as a starting sketch, not the final answer. The wider gap between straight lines and real routes is covered in straight-line vs driving distance.
Limitations and accuracy
The geometry itself is exact — given your radius, the area and perimeter are correct to the metre. Two practical things still move your result:
- Where you click the edge. A few pixels off, especially when zoomed out, can shift the radius by tens or hundreds of metres, and because the area depends on the radius squared, that error gets amplified. Zoom in and work to the number when precision matters.
- Map projection. Flattening a round planet onto a screen subtly stretches the picture, more so far from the equator. For a modest local radius it's negligible, but it's honest to know it's there.
And, as above, the biggest "error" of all is conceptual: reading a circle as a travel-time area when it isn't one. For more on where measured figures drift, see how accurate online map measurements really are.
FAQ
Does a radius circle show how far I can drive in a given time?
No. A radius measures straight-line distance equally in every direction, so it answers "within X distance as the crow flies". A real travel-time area follows roads and is a lopsided shape called an isochrone, not a circle. For most purposes the circle is a quick approximation, not a route.
How do I work out the area inside a radius?
The area of a circle is pi times the radius squared. A 5 km radius gives about 78.5 square kilometres, and a 1-mile radius gives about 3.14 square miles. Map Measurer calculates this for you and shows the area alongside the radius and perimeter.
Can I change the units of the radius and area?
Yes. You can switch the radius between kilometres and miles, and the area between square metres, square kilometres and square miles. For smaller circles you can also read the area in acres or hectares. Every figure updates instantly when you toggle units.