County outline
Where Seneca County is located
Area breakdown
Land accounts for 99.7% of Seneca County’s measured area and water for 0.3%. Its combined area converts to 1,431.7 square kilometers or approximately 353,788 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 551.027 | 1,427.153 | 352,657.3 |
| Water | 1.767 | 4.577 | 1,130.9 |
| Total | 552.794 | 1,431.73 | 353,788.2 |
How Seneca County compares
By original Census land area, Seneca County ranks #16 of 88 in Ohio and #1897 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.264 times the Ohio county-equivalent median of 436 square miles. It accounts for 1.3% of the combined county land area in the state.
See the complete Ohio ranking · What counts as a county equivalent?
Population and density of Seneca County
The Census Bureau estimates that 54,522 people lived in Seneca County on July 1, 2025. That ranks #965 of 3,222 county equivalents nationally and #47 of 88 in Ohio. Since the April 1, 2020 estimates base of 55,080, the population has declined by 558 (−1%), compared with +0.9% for Ohio as a whole.
With 551 square miles of land, that is 99 people per square mile (38 per km²), placing Seneca County #1,006 nationally for density and #56 in Ohio. The median county equivalent has 47 people per square mile; the Ohio median is 120. Put another way, there are about 6.5 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 55,061 | 54,975 | 54,650 | 54,719 | 54,694 | 54,522 |
| Change | — | −86 | −325 | +69 | −25 | −172 |
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 Seneca County
8 incorporated places lie at least partly in Seneca County. Together they are home to 30,525 people (56%); the other 23,997 (44%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Tiffin city, with 17,803 residents in the county. By land, Fostoria city is the largest at 7.529 mi² across all its counties.
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Tiffin cityCounty seat | 17,803 | −1.2% | 7.426 |
| Fostoria cityAlso in 2 other counties | 8,976 | −0.7% | 7.529 |
| Attica village | 863 | −0.9% | 0.682 |
| Bloomville village | 845 | −2.8% | 0.598 |
| Green Springs villageAlso in 1 other county | 637 | −1.7% | 1.16 |
| Bettsville village | 584 | −1.4% | 0.542 |
| Republic village | 538 | −2.5% | 0.9 |
| New Riegel village | 279 | −2.4% | 0.23 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Seneca County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
Seneca County is a central county of the Tiffin, OH Micropolitan Statistical Area. Seneca County is the area’s only county equivalent, so the micro area has the same 551 square miles of land. By combined land it ranks #403 of 542 micropolitan areas, and #194 by population. It belongs to the Findlay-Tiffin, OH Combined Statistical Area.
ZIP codes
20 ZIP Code Tabulation Areas have land in Seneca County; 14 of them have most of their land inside the county, and the others mainly cover neighboring areas.
4331643410434574480244807448094481144815448184482844830448364484144844448534485444861448674488244883
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 Seneca County
Seneca County shares a land border with 6 county equivalents. It has more land than 5 of them. The largest neighbor is Wood County (617.2 mi²) and the smallest is Crawford County (401.8 mi²). Together with its neighbors, Seneca County forms a block of 3,408.7 mi² of land, of which it holds 16.2%.
| Bordering county equivalent | Land mi² | Size vs. Seneca | Compare |
|---|---|---|---|
| Wood County | 617.2 | 1.1× larger | Compare |
| Hancock County | 531.3 | about the same | Compare |
| Huron County | 492.2 | 1.1× smaller | Compare |
| Sandusky County | 408.2 | 1.3× smaller | Compare |
| Wyandot County | 406.9 | 1.4× smaller | Compare |
| Crawford County | 401.8 | 1.4× smaller | Compare |
How big is Seneca County compared with U.S. states?
Seneca County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.9 times as much. Ohio’s county land could hold about 74 areas the size of Seneca County.
Another way to picture it: the same land area would fill a square about 23.5 miles (37.8 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 Seneca County?
Seneca County has more land than 1,325 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 Seneca County at #1,997 nationally by total area, 100 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 0.3%, compared with a national county-equivalent median of 1.2% — 0.8 percentage points lower. 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 Seneca County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1896 | Perry County, Arkansas | 551.3 | Compare |
| #1897 | Seneca County, Ohio | 551 | This county |
| #1898 | Iron County, Missouri | 550.3 | 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 41.119992, -83.127547. 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: 1,427,153,359 m²; AWATER: 4,576,126 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 Seneca County?
Seneca County, Ohio has 551 square miles (1,427.2 km²) of land and 1.767 square miles of water, for a total area of 552.8 square miles. It ranks #1897 of 3,222 U.S. county equivalents by land area.
Is Seneca County the largest county in Ohio?
No. Seneca County ranks #16 of 88 in Ohio by land area. The largest is Ashtabula County, with 702.4 square miles of land.
What counties border Seneca County?
Seneca County shares a land border with Sandusky County, Huron County, Wyandot County, Crawford County, Hancock County, and Wood County.
What is the population of Seneca County?
The Census Bureau estimates Seneca County had 54,522 residents on July 1, 2025, ranking #965 of 3,222 county equivalents. That is 99 people per square mile of land.
What is the county seat of Seneca County?
The county seat of Seneca County is Tiffin.
What cities and towns are in Seneca County?
Seneca County has 8 incorporated places. The most populous five are Tiffin city, Fostoria city, Attica village, Bloomville village, and Green Springs village; 23,997 residents live outside any incorporated place.
Is Seneca County part of a metropolitan area?
Yes. It is a central county of the Tiffin, OH Micropolitan Statistical Area.
How many ZIP codes are in Seneca County?
20 ZIP Code Tabulation Areas have land in Seneca County, and 14 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Seneca County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.9 times as much land.
How many acres of land are in Seneca County?
The land measurement converts to approximately 352,657 acres. Including water, total area is approximately 353,788 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.