Castanea is located at 41°7'27" North, 77°25'48" West (41.124444, -77.430000).
According to the United States Census Bureau, the township has a total area of 1.37 mi2 (3.55 km2). 1.34 mi2 (3.47 km2) of it is land and 0.03 mi2 (0.08 km2) of it is covered by water. The total area is 2.34% water.
Castanea is located in the Eastern Standard Time (EST) timezone, GMT-05:00. It observes Daylight Saving Time (DST).