How population density is calculated

The formula, worked on Cook County

Take the Census Vintage 2024 population estimate and divide by the 2024 Gazetteer land area. Water is left out, so a county on a big lake is not made to look emptier than it is.

Cook County, IL residents  = 5,182,617
Cook County, IL land       = 944.92 sq mi
density = 5,182,617 ÷ 944.92 = 5,484.7 per sq mi
per sq km = 5,484.7 ÷ 2.589988 = 2,117.7

One square mile is 2.589988 square kilometers, the square of 1.609344 km per mile, so divide a per square mile figure by that to get people per square kilometer.

Why the national figure misleads

The national density of 96.3 describes almost no one. Counties with fewer than 100 people per square mile cover 83.6% of US land but hold only 16.3% of residents. Most people live in the dense counties that cover a sliver of the map.

One way to capture that is population-weighted density: the density of the county the average resident lives in. Weighting each county's density by its population gives 2,173 people per square mile, about 23 times the simple national figure. Counties are coarse units, so a tract-level version would be higher still.

Counties by density band

The densest and sparsest counties

New York County, NY is the densest county-equivalent at 73,286 people per square mile. Yukon-Koyukuk, AK is the sparsest at 0.03. The full order is in the rankings of the most densely populated counties and the least densely populated counties.

Common density mistakes

• Using total area instead of land area. Coastal and lake counties then look emptier than they are.
• Averaging county densities to get a state density. Divide the state's total residents by its total land instead; an average of ratios weights a tiny county the same as a huge one.
• Comparing a city's density with a county's. City limits and county lines enclose very different amounts of empty land.

State density vs the typical county

A state's density and its typical county's density can tell very different stories. In Nevada, the state as a whole has 29.7 people per square mile, but its median county has only 3.1, because most residents live in a few dense counties. That is the largest ratio between the two among states with more than one county.

Both numbers are correct. The state figure answers how crowded the state is overall. The median county answers what most of the state's map looks like. Use the one that matches the question, and say which one you used.

Converting to people per square kilometer

Most countries report density per square kilometer. Divide a per square mile figure by 2.589988. The US figure of 96.3 per square mile becomes 37.2 per square kilometer, and the median county's 45.0 becomes 17.4. Going the other way, multiply by 2.589988.

Density for your own area

Every county page shows its density with a national rank, and every state page lists its counties by density. For an area that does not follow county lines, the population within a radius tool counts residents of places inside a circle, and dividing by the circle's area gives a density for that circle.