Colorado Gazetteer
The 303 area code is used by SOME cities and towns in these Counties:
| Adams County | Cities & Towns | ZIP Codes | Features |
| Arapahoe County | Cities & Towns | ZIP Codes | Features |
| Boulder County | Cities & Towns | ZIP Codes | Features |
| Broomfield County | Cities & Towns | ZIP Codes | Features |
| Clear Creek County | Cities & Towns | ZIP Codes | Features |
| Denver County | Cities & Towns | ZIP Codes | Features |
| Douglas County | Cities & Towns | ZIP Codes | Features |
| Eagle County | Cities & Towns | ZIP Codes | Features |
| El Paso County | Cities & Towns | ZIP Codes | Features |
| Elbert County | Cities & Towns | ZIP Codes | Features |
| Gilpin County | Cities & Towns | ZIP Codes | Features |
| Jefferson County | Cities & Towns | ZIP Codes | Features |
| Larimer County | Cities & Towns | ZIP Codes | Features |
| Park County | Cities & Towns | ZIP Codes | Features |
| Sedgwick County | Cities & Towns | ZIP Codes | Features |
| Weld County | Cities & Towns | ZIP Codes | Features |
