As of 2015, ZIP codes can be looked up at USPS.com, UnitedStatesZIPCodes.org and ZIPcode.org. All three websites offer search tools to find ZIP codes for a given location.
To find a ZIP code on the USPS website, USPS.com, click on Look Up a ZIP Code. Enter the street address and city, and select the state. The nine-digit ZIP code is then displayed for the given address. Its website also includes a reverse lookup tool that lists all the cities in a given ZIP code.
To find a ZIP code on UnitedStatesZIPCodes.org, type the street, city and state and click Search. Its website features a regional map of the ZIP code's area and a reverse lookup tool. To find a ZIP code on ZIPCode.org, select ZIP Code Search on its homepage.