Arnold is located at 39°1'54" North, 76°30'10" West (39.031667, -76.502778).
According to the United States Census Bureau, the city has a total area of 13.41 mi2 (34.73 km2). 10.80 mi2 (27.97 km2) of it is land and 2.61 mi2 (6.76 km2) of it is covered by water. The total area is 19.45% water.
Arnold is located in the Eastern Standard Time (EST) timezone, GMT-05:00. It observes Daylight Saving Time (DST).