Perth radius map: 5, 10, 25 and 50 kilometre circles

A quick anchor for the scale. Ten kilometres out, there’s nothing on the GeoNames list at all. The furthest name in these tables is Rockingham, 38.2 km away to the south-south-west, and it still sits inside the outer circle. People underestimate that big ring every time, which is most of why this page exists.

One caution before you lean on the table. A GeoNames centre is a single point, usually somewhere near the historic core, and a city isn’t a point. Kwinana is listed at 31.8 km because that’s the gap between two reference points. Its nearer edge is closer than that, probably by a good margin.

Place names, coordinates and populations come from GeoNames, which the site uses under CC BY 4.0.

Here’s the thing people get wrong about a radius. Double it and you quadruple the ground underneath. Going from 5 kilometres to 50 multiplies the area by a hundred, so 79 km² becomes 7,854 km². The outermost band on its own, the gap between the 25 and 50 km circles, accounts for 5,890 km². Three quarters of everything inside the big circle, in one band.

Those areas are worth holding against the ring counts. 7,854 km² inside the outer circle, and GeoNames puts 2 named settlements in all of it. A density figure off that would be meaningless, so there isn’t one here. What the areas are good for is sizing: knowing the 25 km ring covers 1,963 km² tells you how much ground a service area at that radius has to reach across.

Every circle here is a straight-line radius, measured crow-flies. Rockingham shows up at 38.2 km south-south-west, and the road out there will be longer. Always is. In a town laid out on a grid, reckon on 20 to 40 per cent more actual travel than the radius says. Put a river, a harbour or a decent hill in the way and that rule of thumb stops meaning much at all.

Plenty of sites draw a plain circle, label it drive time and leave it there. Those are worth ignoring. A proper isochrone needs a road graph, turn restrictions and some idea of traffic, and it changes shape by the hour. What a circle gives you instead is a bound that doesn’t move: nothing outside the 25 km ring is within 25 kilometres of Perth, whatever the roads do.

Where it all comes from. Names, coordinates and populations are GeoNames cities15000, which covers settlements of roughly 15,000 people and up. Anything smaller is simply absent. City subdivisions get stripped out on purpose, so a page about Perth doesn’t fill with its own neighbourhoods instead of its neighbours. Distances are haversine on a 6,371.0088 km sphere, areas are spherical caps.

One oddity you’ll notice if you compare pages. Population counts come from GeoNames, which records what the national statistics office last published, so some are a few years old and a couple are more. They’re good for ranking and for rough sizing. The figure carried for Perth is 2,384,371, which for some cities means the municipality and for others the whole urban area, so compare it with care.

What people actually use these rings for, roughly in order of how often it comes up: delivery and service areas, finding somewhere to live within a sensible distance of work, sizing a catchment for a shop or a clinic, checking how far an event’s noise and traffic will carry. Straight-line distance is the right first pass for all of them.