London radius map: 5, 10, 25 and 50 mile circles

If you want one landmark to calibrate against, use Willesden. It’s 4.9 mi west-north-west of the centre, so it sits almost exactly on the inner circle. At the other end, Cambridge is 48.9 mi out to the north-north-east, near enough the edge of the 50 mi ring. Everything in between scales from those two.

Read the distances as point to point. GeoNames gives every settlement one coordinate, roughly central, and these figures are the gaps between those coordinates. A sprawling suburb whose centre lands at 10 mi might well start at half that. Where the boundary actually matters, draw the circle and look at where the line crosses.

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 miles to 50 multiplies the area by a hundred, so 79 mi² becomes 7,854 mi². The outermost band on its own, the gap between the 25 and 50 mi circles, accounts for 5,890 mi². Three quarters of everything inside the big circle, in one band.

Cross the two tables and something useful falls out. The 25 mi disc holds 16,279,968 listed people across 1,963 mi², about 8,293 per mi². Push out to 50 and you buy 5,890 mi² more for only 5,011,597 extra people. Most of that last band is empty, which is what you’d expect and still surprises people when they see it drawn.

These are straight-line rings. That’s the right tool for a coverage question and the wrong one for travel time. A 30-minute drive out of London comes out as a lopsided blob, an isochrone, racing down a motorway and stopping dead at a bridge. Drawing one properly takes a routing server. This site hasn’t got one, deliberately, because everything runs in your browser with no account and nothing uploaded.

If travel time is genuinely what you need, use these rings to narrow the field first and check the survivors properly afterwards. The circle is cheap, instant, and honest about what it measures. It also can’t tell you that the 10 mi to the north crosses water while the same 10 mi to the south is dual carriageway the whole way.

Two things about the method. Distance is measured over the curve of the Earth, not across the flat map image, which is where a surprising number of simpler tools quietly go wrong. And the Earth gets treated as a sphere when it’s really a bit squashed, costing a fraction of a per cent. Fine for a catchment. No use for settling a boundary, so don’t try.

All of it was computed when the site was built, then written into a file that ships with the page. Nothing here is fetched while you read. That’s why the tables are in the HTML the server sends rather than appearing a second later, and it’s why the figures can’t drift: rerun the build and every number on all fifty of these pages is recalculated from the same dump.

Most of the traffic to a page like this is practical. Somebody wants to know which towns near London fall inside a service area, or how many people live within 25 miles of a site they’re weighing up. The table answers the first one outright. For the second, take the ring populations as a floor, since everything under 15,000 people is missing from them.