Choose the measurement before comparing places: land for land extent, total for the whole statistical geography, and water share for its composition.
A county’s size can mean the amount of land inside it, the water assigned to its statistical boundary, or the two combined. Before comparing numbers, decide what you want to learn. US County Area keeps all three measurements visible so the definition travels with the answer.
Start with the question
- How much land is in a county? Use land area. This is the basis of our main county rankings.
- How large is the entire statistical geography? Use total area, including water.
- How much of its measured area is water? Divide water area by total area and multiply by 100.
None of these values measures usable building land, agricultural acreage, or publicly accessible space. Those questions require additional information about land use, ownership, and terrain.
A worked example: Harris County, Texas
The 2026 Gazetteer gives Harris County 1,707.3 square miles of land and 69.7 square miles of water. Add the published figures to obtain 1,777 square miles in total. Dividing water by total gives a water share of 3.9%.
If you label the total as “land area,” you overstate the land measurement by the full water component. Likewise, comparing one county’s land area with another’s total area gives an inconsistent result, even when both numbers come from reliable sources.
Changing units changes the number, not the county
Multiply square miles by 2.58998811 for square kilometers, or by 640 for acres. For example, a hypothetical 100-square-mile area is about 259 square kilometers and exactly 64,000 acres. Converting both counties with the same factor preserves their relative size.
Try a consistent comparison
Our Harris County and Cook County comparison keeps land, water, and total in separate rows. Switch units or choose another county to see how the relationship changes. The bar chart uses land area alone.
Source definitions: 2026 Census Gazetteer record layouts. Calculations and examples are by US County Area.
“Twice as large” and “100% larger” describe the same ratio
If county A has 200 square miles of land and county B has 100, A is twice as large by land area. It is also 100% larger than B: the difference of 100 is divided by B’s 100. Reversing the comparison makes B 50% smaller than A, because the denominator is now 200. The two percentage statements are asymmetric even though the difference is the same.
This becomes important when writing a headline from the comparison tool. State which county is the baseline, retain the measurement, and avoid mixing a ratio with a percentage. “Two times larger” is ambiguous in ordinary language; “twice the land area” is clearer. An area ratio also says nothing about whether the shape of one county would fit inside another.
Calculate first and round for the reader afterward
A source figure rounded to a tenth of a square mile is suitable for a quick summary but can conceal differences. Keep the original values for arithmetic and round the final result. If you add several already-rounded displayed figures, the sum may differ from a total calculated at full precision. Neither display necessarily identifies the underlying values incorrectly.
For a new analysis that needs the finest retained precision, convert original square meters directly: divide by 1,000,000 for square kilometers. To reproduce this site’s published square-mile-based conversions, start from its square-mile fields instead. Explain the route you used. Small discrepancies between those routes are a rounding issue, not evidence that the physical county changes when units change.
Make the measurement part of the table, not a footnote
Use headings such as “Land area (mi²)” and “Water share (%)” rather than a generic “Size.” Add the release year and the comparison universe near the title. When two tables are placed together, check that both include the same kinds of county equivalents and the same geographic coverage.
For a short article, one worked pair can be more informative than a long list of unsourced superlatives. Present the components, explain the formula, and link the source records. Readers can then apply the method to a county they know. The goal is a comparison that keeps its meaning when copied into a spreadsheet, quoted in a classroom, or shared without the rest of the page.
Sources and calculation notes
County examples and calculations use our pinned 2026 Census Gazetteer dataset. Ratios use published square-mile values; land rankings use original square meters. Examples labeled hypothetical are teaching examples, not population estimates.
How we calculate and verify the data · Download and reproduce the examples · Suggest a correction