San Jose radius map: 5, 10, 25 and 50 mile circles

Numbers on their own are hard to feel. Five miles from the centre of San Jose gets you about as far as Campbell, which the table puts at 4.7 mi south-west. Stretch it to 25 and you’re out past Hayward, 24.9 mi north-north-west. Hold those in your head and the rest of the map scales itself.

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 San Jose 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 California and whatever lies past its edges.

Cross the two tables and something useful falls out. The 25 mi disc holds 2,624,770 listed people across 1,963 mi², about 1,337 per mi². Push out to 50 and you buy 5,890 mi² more for only 3,762,518 extra people. Most of that last band is empty, which is what you’d expect and still surprises people when they see it drawn.

Every circle here is a straight-line radius, measured crow-flies. Richmond shows up at 48.1 mi north-north-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.

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 San Jose. 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 San Jose 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.