Population in a drawn area

Worked example: the Salt Lake Valley

The tool opens on a six-corner outline of the Salt Lake Valley floor. On the sphere, the shape covers about 329 square miles. Its two totals are:

• Place population: 1,174,176 people in 18 incorporated places of 1,000+, led by Salt Lake City (217,783) and West Valley City (138,144).
• County population: 1,216,274, all of Salt Lake County, the only county whose internal point is inside.

Dividing the place total by the area gives about 3,570 people per square mile. The county total is 42,098 higher. Part of that gap is Magna (31,381), which lies in the county but outside the shape's northwest edge; the rest is unincorporated residents and canyon communities the place list does not cover.

How to use it

1. The map opens on a sample shape around the Salt Lake Valley. Press Clear to start your own, then pan and zoom to your area.
2. Click the map to add corners in order around the edge, or type a place or coordinates in Add a corner. After the third corner the shape closes and fills.
3. Press Undo to remove the last corner. The count next to the buttons shows how many corners the shape has.
4. Read the card: the place population with its place count, the county population with its county count, the drawn area, and people per square mile.
5. The table below lists the places largest first, and the line under it names the counties counted. Download places CSV saves the places. Copy the page address to share the shape.

Why there are two totals

Census population estimates are published for areas such as places and counties, not for shapes you draw. So the tool has to decide which areas to include, and it shows two answers.

Place population adds every incorporated place of 1,000+ whose internal point is inside your shape. It misses people in unincorporated areas and towns under 1,000, and it counts a whole city even when part of it is outside.

County population adds every county whose internal point is inside. It includes rural and unincorporated residents but is all or nothing: a county whose center is just outside adds zero. For a small shape inside one county, it is often zero or far too high, so read the place total first. For a shape spanning several counties, the county total is usually the better guide.

How the area and density are measured

The area is computed on a sphere of radius 6,371.0088 km, with the corners joined in click order and the last corner joined back to the first. It matches the map area calculator for the same corners. Shapes are assumed smaller than half the globe.

People per square mile divides the place total by that drawn area. Because places are counted whole, a shape that clips the edge of a large city can show a density higher than anything on the ground, and a shape mostly of farmland or water will read low. Treat it as a rough guide to how built up the area is.

Limits

Census-designated places are absent, so areas with large unincorporated suburbs undercount in the place total; Hawaii has a single entry, Urban Honolulu. Containment is tested on plain longitude and latitude, as the shape appears on the map, with each edge taking the short way across the 180th meridian. An outline that crosses itself gets a notice instead of totals. The drawn corners are saved in the page address, so a link reopens the same shape for anyone.