1 definition found plainwell - U.S. Gazetteer (1990) : Plainwell, MI (city, FIPS 64740) Location: 42.44514 N, 85.64408 W Population (1990): 4057 (1541 housing units) Area: 5.4 sq km (land), 0.2 sq km (water) Zip code(s): 49080