Most drains perform one function: to drain off water from the sink. In order to accomplish this and to make changing drains easier, sink drain sizes are standardized. Different types of drains, in different areas of the home, have different sizes. In virtually all cases, the drain size suits the function of the drain.
Kitchen Drain Sizes
A standard kitchen drain is 3 1/2 inches in diameter. This is the measurement across the center of the drain, from one side to another. The drain strainer, the bowl-shaped piece inside the drain mouth, matches this dimension. It is designed to sit inside the drain mount and seal it closed when flush to the drain bowl. All kitchen sinks have a drain this size to make installations easier. The drain connects to a pipe with a 1 1/2-inch diameter.
Bathroom Sink Drain Sizes
A bathroom sink has a much smaller drain size. Since it is only used to wash hands, shave and brush teeth, there is no need for a larger drain hole. Bathroom sink drains are 1 3/4 inches in diameter -- half as large as the kitchen sink drain. After installing new bathroom faucets, the drain is usually replaced. So, it makes sense to standardize the drain size. That way bathroom faucet manufacturers can provide the right-sized drain with their faucet. The drains have a 1 1/4-inch tail pipe.
Bar Sink Drain Openings
A bar sink drain is slightly larger than a bathroom drain, but smaller than a kitchen drain. It handles more heavy-duty usage than bathroom sinks but not as much use as the kitchen sink. Bar drains are 2 inches in diameter. They connect to either a 1 1/4-inch or 1-1/2 inch tail pipe. These drains, like bathroom and kitchen drains, use a P-trap, which is a J-shaped pipe that connects to the main drain line.
Tub and Shower Drains
Shower drains handle large volumes of water and measure 3 1/4 inches in diameter. The drains hook up to a 2-inch drain pipe. Tub drains are slightly smaller, at 1 1/2 inches in diameter with the same sized drain pipe. This is for a tub-only design; tub and shower combos use the 3 1/4-inch drain.