Power drills come in 3 sizes; 1/4, 3/8, and 1/2-in. Size refers to the largest capacity of bit that will fit in the drill's chuck. 3/8-in. drills are the most versatile. They're powerful, yet light enough to easily hold and manage. Larger bits can drill bigger holes, so 1/2-in. drills are best for big, heavy-duty projects. Drills with higher RPM ratings (1200-rpm or more) are better for boring smaller holes into wood. Lower speed models are better at making larger holes into metal.
Air conditioners are rated by the number of British Thermal Units (Btu) of heat they can remove per hour. Another common rating term for air conditioning size is the “ton,” which is 12,000 Btu per hour.