Complex sentence: The BMI measurement assumes the body is a two-dimensional square sheet and it measures weight per square unit of area, where as the Rohrer's Index assumes the body is a three-dimensional cube and measures weight per cubic unit of volume.
Split into simpler sentences: The BMI measurement assumes the body is a two-dimensional square sheet and it measures weight per square unit of area. Alternatively, the Rohrer's Index assumes the body is a three-dimensional cube and measures weight per cubic unit of volume.