69 in Hamilton County, plus two that most directories miss
Most ZIP code directories, including several with active Cincinnati listings, file all 71 of the city's ZIP codes under a single county: Hamilton, where Cincinnati has served as county seat since Arthur St. Clair organized the county in 1790. This site's source data, built from GeoNames' postal code registry, agrees on the total (71, matching independent counts from zip-codes.com and ZipCodesToGo), but splits two of those codes into neighboring counties instead of Hamilton.
45245 belongs to Clermont County, covering Withamsville, Mount Carmel, and Summerside across Union and Pierce townships, an active suburban area on Cincinnati's east side with roughly 20,000 residents. This site's data attributes 45275 to Brown County, the only Cincinnati-addressed ZIP code that gets that treatment (more on a real disagreement over that county line below), and unlike 45245, it isn't a residential community. GeoNames and other postal trackers describe it as a single-route PO box facility, 18 boxes along Clay Drive, with no population recorded.
Why some directories put 45275 in Hamilton County instead
This isn't a case of one source being careless. This site's ZIP-to-county data comes from GeoNames' postal code registry, and it assigns 45275 to Brown County. But two of the most widely cited USPS-based ZIP directories disagree: zip-codes.com's own page for 45275, listed among this article's sources, tags the ZIP Hamilton County in its site navigation, and ZipCodesToGo's Cincinnati-area listing does the same. All three sources agree on everything else: that USPS assigns 45275 to a Cincinnati mailing address, that it's an 18-box PO-only facility on Clay Drive, and that no residential population is recorded there.
The likely cause is a boundary-conversion mismatch, not a factual error on any one source's part. Postal ZIP codes are drawn for mail routing, not county lines, so different providers convert a five-digit ZIP into a county using different geographic methods (ZIP centroid, primary carrier route, or ZCTA overlap), and a tiny, low-volume ZIP like 45275 is exactly the kind of edge case where those methods land on different counties. Anyone who needs 45275's county for a legal, tax, or voter-registration purpose should confirm directly with Hamilton County's or Brown County's own records rather than relying on any ZIP-code directory, including this one.
All 71 ZIP codes with "Cincinnati, OH" as the primary USPS place name
Sixty-nine of the 71 sit in Hamilton County. The two exceptions are marked below.
All 71 ZIP codes with "Cincinnati, OH" as the primary USPS place name, by county (per GeoNames.org, checked 2026-08-17)
| ZIP | County |
|---|---|
| 45201 | Hamilton |
| 45202 | Hamilton |
| 45203 | Hamilton |
| 45204 | Hamilton |
| 45205 | Hamilton |
| 45206 | Hamilton |
| 45207 | Hamilton |
| 45208 | Hamilton |
| 45209 | Hamilton |
| 45211 | Hamilton |
| 45212 | Hamilton |
| 45213 | Hamilton |
| 45214 | Hamilton |
| 45215 | Hamilton |
| 45216 | Hamilton |
| 45217 | Hamilton |
| 45218 | Hamilton |
| 45219 | Hamilton |
| 45220 | Hamilton |
| 45221 | Hamilton |
| 45222 | Hamilton |
| 45223 | Hamilton |
| 45224 | Hamilton |
| 45225 | Hamilton |
| 45226 | Hamilton |
| 45227 | Hamilton |
| 45229 | Hamilton |
| 45230 | Hamilton |
| 45231 | Hamilton |
| 45232 | Hamilton |
| 45233 | Hamilton |
| 45234 | Hamilton |
| 45235 | Hamilton |
| 45236 | Hamilton |
| 45237 | Hamilton |
| 45238 | Hamilton |
| 45239 | Hamilton |
| 45240 | Hamilton |
| 45241 | Hamilton |
| 45242 | Hamilton |
| 45243 | Hamilton |
| 45244 | Hamilton |
| 45245 | Clermont |
| 45246 | Hamilton |
| 45247 | Hamilton |
| 45248 | Hamilton |
| 45249 | Hamilton |
| 45250 | Hamilton |
| 45251 | Hamilton |
| 45252 | Hamilton |
| 45253 | Hamilton |
| 45254 | Hamilton |
| 45255 | Hamilton |
| 45258 | Hamilton |
| 45262 | Hamilton |
| 45263 | Hamilton |
| 45264 | Hamilton |
| 45267 | Hamilton |
| 45268 | Hamilton |
| 45269 | Hamilton |
| 45270 | Hamilton |
| 45271 | Hamilton |
| 45273 | Hamilton |
| 45274 | Hamilton |
| 45275 | Brown |
| 45277 | Hamilton |
| 45280 | Hamilton |
| 45296 | Hamilton |
| 45298 | Hamilton |
| 45299 | Hamilton |
| 45999 | Hamilton |
45999 isn't a ZIP code for anyone who lives there
One entry on the list, 45999, doesn't correspond to a neighborhood or a residential mail route: it's the ZIP code the IRS uses at its Cincinnati service center to process specific categories of business and international tax correspondence. IRS notices mailed from Cincinnati, including forms like the CP132 balance-due notice, carry a return address of "Internal Revenue Service, Cincinnati, OH 45999," with a numeric stop code appended for internal routing. Which specific filings route to Cincinnati rather than another IRS service center varies by form type (different notices and forms list different coverage areas), which is part of why the center got its own dedicated ZIP code, not one shared with any Cincinnati neighborhood.
Cincinnati isn't the only city on this site with an IRS-only ZIP in its list: Atlanta's does too, assigned to a separate service center entirely.
513 split once, in 1996, then got a second layer in 2023
Cincinnati's area code, 513, dates to 1947, one of the original area codes assigned nationwide. It covered the whole region alone for almost fifty years, until September 28, 1996, when regulators split off a new code, 937, for Dayton, Springfield, and the rest of southwestern Ohio outside the immediate Cincinnati metro, the same kind of geographic split this site has covered for Chicago and other major metro area codes.
513 itself was never split again after 1996. Instead, on April 28, 2023, regulators added an overlay code, 283, covering the identical territory as 513 rather than dividing it further. New phone numbers in the Cincinnati region can now be assigned either 513 or 283, while every number already carrying a 513 area code keeps working exactly as before.