County outline
Where Boulder County is located
Area breakdown
Land accounts for 98.1% of Boulder County’s measured area and water for 1.9%. Its combined area converts to 1,917.8 square kilometers or approximately 473,901 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 726.416 | 1,881.409 | 464,906.2 |
| Water | 14.055 | 36.402 | 8,995.2 |
| Total | 740.471 | 1,917.811 | 473,901.4 |
How Boulder County compares
By original Census land area, Boulder County ranks #52 of 64 in Colorado and #1222 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 0.507 times the Colorado county-equivalent median of 1,431.5 square miles. It accounts for 0.7% of the combined county land area in the state.
See the complete Colorado ranking · What counts as a county equivalent?
Population and density of Boulder County
The Census Bureau estimates that 328,560 people lived in Boulder County on July 1, 2025. That ranks #230 of 3,222 county equivalents nationally and #9 of 64 in Colorado. Since the April 1, 2020 estimates base of 330,761, the population has declined by 2,201 (−0.7%), compared with +4.1% for Colorado as a whole.
With 726.4 square miles of land, that is 452 people per square mile (175 per km²), placing Boulder County #372 nationally for density and #7 in Colorado. The median county equivalent has 47 people per square mile; the Colorado median is 11. Put another way, there are about 1.4 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 330,982 | 327,325 | 327,914 | 327,945 | 329,485 | 328,560 |
| Change | — | −3,657 | +589 | +31 | +1,540 | −925 |
U.S. Census Bureau, Vintage 2025 population estimates. Estimates are revised each year and are not decennial census counts. Density divides the whole population by the whole county’s land area, so it describes an average, not how settlement is distributed within the county. Population sources and methods.
Cities and towns in Boulder County
10 incorporated places lie at least partly in Boulder County. Together they are home to 288,176 people (87.7%); the other 40,384 (12.3%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Boulder city, with 105,689 residents in the county. By land, Longmont city is the largest at 29.3 mi² across all its counties. Among places with at least 1,000 residents in 2020, Erie town grew fastest since then (+21.5%).
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Boulder cityCounty seat | 105,689 | −2.6% | 27 |
| Longmont cityAlso in 1 other county | 98,807 | +1.2% | 29.3 |
| Lafayette city | 30,116 | −0.8% | 9.257 |
| Louisville city | 20,581 | −3.1% | 7.978 |
| Erie townAlso in 1 other county | 15,395 | +21.5% | 21.5 |
| Superior townAlso in 1 other county | 13,637 | +4.1% | 3.933 |
| Lyons town | 2,119 | −4.1% | 1.373 |
| Nederland town | 1,463 | −0.3% | 1.48 |
| Jamestown town | 245 | −5.8% | 0.574 |
| Ward town | 124 | −5.3% | 0.536 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Boulder County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
Boulder County is a central county of the Boulder, CO Metropolitan Statistical Area. Boulder County is the area’s only county equivalent, so the metro area has the same 726.4 square miles of land. By combined land it ranks #314 of 393 metropolitan areas, and #163 by population. It belongs to the Denver-Aurora-Greeley, CO Combined Statistical Area.
ZIP codes
23 ZIP Code Tabulation Areas have land in Boulder County; 18 of them have most of their land inside the county, and the others mainly cover neighboring areas.
8002080025800268002780301803028030380304803058031080403804558046680471804818050180503805048051080513805168054080544
ZIP Code Tabulation Areas are the Census Bureau’s approximations of USPS ZIP Code service areas, from the 2020 Census relationship files. Lighter codes have less than half of their land in this county. Post-office-box and single-address ZIP Codes have no tabulation area.
Counties bordering Boulder County
Boulder County shares a land border with 6 county equivalents. It has more land than 2 of them. The largest neighbor is Weld County (3,985 mi²) and the smallest is Broomfield County (33 mi²). Together with its neighbors, Boulder County forms a block of 10,101.1 mi² of land, of which it holds 7.2%.
| Bordering county equivalent | Land mi² | Size vs. Boulder | Compare |
|---|---|---|---|
| Weld County | 3,985 | 5.5× larger | Compare |
| Larimer County | 2,595.7 | 3.6× larger | Compare |
| Grand County | 1,846.6 | 2.5× larger | Compare |
| Jefferson County | 764.4 | 1.1× larger | Compare |
| Gilpin County | 150 | 4.8× smaller | Compare |
| Broomfield County | 33 | 22× smaller | Compare |
How big is Boulder County compared with U.S. states?
Boulder County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.4 times as much. Colorado’s county land could hold about 143 areas the size of Boulder County.
Another way to picture it: the same land area would fill a square about 27 miles (43.4 km) on each side. Real county shapes are rarely square, so this is a scale reference rather than a width or driving distance.
State figures are the sums of each state’s county-equivalent land measurements in the same Census release, so states and counties are compared on the same basis. Published state totals can differ slightly because of rounding. Compare all states · Counties bigger than entire states.
What stands out about Boulder County?
Boulder County has more land than 2,000 of the other 3,221 county equivalents in the dataset.
A median is the middle county-equivalent measurement, not the average of the largest and smallest. All three bars use the same linear scale. Very small areas may appear as a thin sliver; the labels retain their values.
Does including water change its rank?
Using land plus water places Boulder County at #1,301 nationally by total area, 79 places farther from the top than its land rank. Other county equivalents gain more area when their water measurements are included.
Its water share is 1.9%, compared with a national county-equivalent median of 1.2% — 0.7 percentage points higher. This describes the statistical geography’s land/water mix, not flood exposure or developable land.
Closest positions in the national land ranking
These entries sit immediately beside Boulder County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1221 | Kershaw County, South Carolina | 726.6 | Compare |
| #1222 | Boulder County, Colorado | 726.4 | This county |
| #1223 | Rice County, Kansas | 726.2 | Compare |
Calculated from the same Census release as the area table. Total-area order uses original ALAND + AWATER; water shares and chart labels use published square miles. Calculation details.
Representative coordinates
The Census internal point is 40.094974, -105.397706. It represents a location inside the geography; it is not necessarily the centroid or the address of a courthouse.
Source and precision
U.S. Census Bureau, 2026 Gazetteer Files, Counties. Original ALAND: 1,881,409,474 m²; AWATER: 36,403,416 m². Rankings use ALAND; exact ties use ascending GEOID. Tables preserve Census-published square miles to three decimals, with conversions based on those values.
Common questions
How big is Boulder County?
Boulder County, Colorado has 726.4 square miles (1,881.4 km²) of land and 14.1 square miles of water, for a total area of 740.5 square miles. It ranks #1222 of 3,222 U.S. county equivalents by land area.
Is Boulder County the largest county in Colorado?
No. Boulder County ranks #52 of 64 in Colorado by land area. The largest is Las Animas County, with 4,772.9 square miles of land.
What counties border Boulder County?
Boulder County shares a land border with Larimer County, Grand County, Weld County, Gilpin County, Jefferson County, and Broomfield County.
What is the population of Boulder County?
The Census Bureau estimates Boulder County had 328,560 residents on July 1, 2025, ranking #230 of 3,222 county equivalents. That is 452 people per square mile of land.
What is the county seat of Boulder County?
The county seat of Boulder County is Boulder.
What cities and towns are in Boulder County?
Boulder County has 10 incorporated places. The most populous five are Boulder city, Longmont city, Lafayette city, Louisville city, and Erie town; 40,384 residents live outside any incorporated place.
Is Boulder County part of a metropolitan area?
Yes. It is a central county of the Boulder, CO Metropolitan Statistical Area.
How many ZIP codes are in Boulder County?
23 ZIP Code Tabulation Areas have land in Boulder County, and 18 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Boulder County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.4 times as much land.
How many acres of land are in Boulder County?
The land measurement converts to approximately 464,906 acres. Including water, total area is approximately 473,901 acres. One square mile equals 640 acres.
Why might another source show a different area?
Check whether it reports land or total area, its Census vintage, and its rounding. Our guide explains how to reconcile county area figures.
Can this establish a property boundary?
No. Statistical measurements and generalized maps do not establish property lines. Consult the relevant local surveying or land-records authority.