County outline
Where Mercer County is located
Area breakdown
Land accounts for 97.7% of Mercer County’s measured area and water for 2.3%. Its combined area converts to 1,226.2 square kilometers or approximately 302,992 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 462.447 | 1,197.732 | 295,966.1 |
| Water | 10.978 | 28.433 | 7,025.9 |
| Total | 473.425 | 1,226.165 | 302,992 |
How Mercer County compares
By original Census land area, Mercer County ranks #36 of 88 in Ohio and #2221 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.061 times the Ohio county-equivalent median of 436 square miles. It accounts for 1.1% of the combined county land area in the state.
See the complete Ohio ranking · What counts as a county equivalent?
Population and density of Mercer County
The Census Bureau estimates that 42,737 people lived in Mercer County on July 1, 2025. That ranks #1,159 of 3,222 county equivalents nationally and #57 of 88 in Ohio. Since the April 1, 2020 estimates base of 42,529, the population has grown by 208 (+0.5%), compared with +0.9% for Ohio as a whole.
With 462.4 square miles of land, that is 92 people per square mile (36 per km²), placing Mercer County #1,048 nationally for density and #59 in Ohio. The median county equivalent has 47 people per square mile; the Ohio median is 120. Put another way, there are about 6.9 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 42,534 | 42,366 | 42,343 | 42,360 | 42,520 | 42,737 |
| Change | — | −168 | −23 | +17 | +160 | +217 |
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 Mercer County
9 incorporated places lie at least partly in Mercer County. Together they are home to 22,106 people (51.7%); the other 20,631 (48.3%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Celina city, with 10,936 residents in the county. Among places with at least 1,000 residents in 2020, St. Henry village grew fastest since then (+0.8%).
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Celina cityCounty seat | 10,936 | −0% | 5.043 |
| Coldwater village | 4,716 | −1.1% | 1.988 |
| St. Henry village | 2,624 | +0.8% | 1.728 |
| Fort Recovery village | 1,476 | −1.5% | 1.235 |
| Rockford village | 1,039 | −1.4% | 0.865 |
| Mendon village | 613 | −2.7% | 0.545 |
| Chickasaw village | 351 | −1.4% | 0.241 |
| Burkettsville villageAlso in 1 other county | 202 | −1.9% | 0.173 |
| Montezuma village | 149 | −3.2% | 0.102 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Mercer County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
Mercer County is a central county of the Celina, OH Micropolitan Statistical Area. Mercer County is the area’s only county equivalent, so the micro area has the same 462.4 square miles of land. By combined land it ranks #453 of 542 micropolitan areas, and #294 by population. It belongs to the Lima-Van Wert-Celina, OH Combined Statistical Area.
ZIP codes
18 ZIP Code Tabulation Areas have land in Mercer County; 10 of them have most of their land inside the county, and the others mainly cover neighboring areas.
453104538845822458264582845845458464586045862458654586645869458824588345885458874589445898
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 Mercer County
Mercer County shares a land border with 5 county equivalents, including 2 in Indiana. It has more land than 4 of them. The largest neighbor is Darke County (598.1 mi²) and the smallest is Adams County (338.9 mi²). Together with its neighbors, Mercer County forms a block of 2,593.9 mi² of land, of which it holds 17.8%.
| Bordering county equivalent | Land mi² | Size vs. Mercer | Compare |
|---|---|---|---|
| Darke County | 598.1 | 1.3× larger | Compare |
| Van Wert County | 409.2 | 1.1× smaller | Compare |
| Auglaize County | 401.4 | 1.2× smaller | Compare |
| Jay CountyIndiana | 383.9 | 1.2× smaller | Compare |
| Adams CountyIndiana | 338.9 | 1.4× smaller | Compare |
How big is Mercer County compared with U.S. states?
Mercer County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 2.2 times as much. Ohio’s county land could hold about 88 areas the size of Mercer County.
Another way to picture it: the same land area would fill a square about 21.5 miles (34.6 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 Mercer County?
Mercer County has more land than 1,001 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 Mercer County at #2,268 nationally by total area, 47 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 2.3%, compared with a national county-equivalent median of 1.2% — 1.1 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 Mercer County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #2220 | Holt County, Missouri | 462.7 | Compare |
| #2221 | Mercer County, Ohio | 462.4 | This county |
| #2222 | Adams County, Mississippi | 462.3 | 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 40.535331, -84.632056. 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: 1,197,732,213 m²; AWATER: 28,431,996 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 Mercer County?
Mercer County, Ohio has 462.4 square miles (1,197.7 km²) of land and 11 square miles of water, for a total area of 473.4 square miles. It ranks #2221 of 3,222 U.S. county equivalents by land area.
Is Mercer County the largest county in Ohio?
No. Mercer County ranks #36 of 88 in Ohio by land area. The largest is Ashtabula County, with 702.4 square miles of land.
What counties border Mercer County?
Mercer County shares a land border with Auglaize County, Van Wert County, Darke County, Jay County, Indiana, and Adams County, Indiana.
What is the population of Mercer County?
The Census Bureau estimates Mercer County had 42,737 residents on July 1, 2025, ranking #1,159 of 3,222 county equivalents. That is 92 people per square mile of land.
What is the county seat of Mercer County?
The county seat of Mercer County is Celina.
What cities and towns are in Mercer County?
Mercer County has 9 incorporated places. The most populous five are Celina city, Coldwater village, St. Henry village, Fort Recovery village, and Rockford village; 20,631 residents live outside any incorporated place.
Is Mercer County part of a metropolitan area?
Yes. It is a central county of the Celina, OH Micropolitan Statistical Area.
How many ZIP codes are in Mercer County?
18 ZIP Code Tabulation Areas have land in Mercer County, and 10 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Mercer County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 2.2 times as much land.
How many acres of land are in Mercer County?
The land measurement converts to approximately 295,966 acres. Including water, total area is approximately 302,992 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.