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

If you want one landmark to calibrate against, use Wheat Ridge. It’s 5.3 mi west-north-west of the centre, so it sits almost exactly on the inner circle. At the other end, Greeley is 49.5 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.

Area climbs with the square of the radius, and it catches people out every time. The 5 mi disc around Denver covers 79 mi². The 25 mi one covers 1,963 mi², twenty-five times as much for five times the distance. By 50 mi you’re looking at 7,854 mi² of Colorado and whatever lies past its edges.

Cross the two tables and something useful falls out. The 25 mi disc holds 2,489,586 listed people across 1,963 mi², about 1,268 per mi². Push out to 50 and you buy 5,890 mi² more for only 353,039 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 Denver 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.

On the arithmetic. Distances are great-circle distances on a sphere of radius 6,371.0088 km, worked out with the haversine formula, and each bearing is the initial heading you’d take leaving Denver. Areas use the spherical cap rather than πr², which shaves a hair off the flat answer. About one part in seventy thousand at 50 miles. Invisible, and true anyway.

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 Denver 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.