Sink drains carry water from the sink basin into your home's main drainage lines. These drains come in various sizes, depending on what type of sink it is and how much volume of water the drain needs to carry. The drain size for your kitchen sink and bathroom sink are not the same, for instance. Each one handles different-sized faucets and water flow rates.
A kitchen drain measures 3 1/2 inches in diameter, the total overall size of the drain opening in the bottom of the sink. This measurement is not the same size as the drainpipe, however. The sink drain includes a basket strainer, a round object with a grate that screens out food and debris before it enters the drainpipe. The sink drain acts like a funnel toward the drainpipe and P-trap, which are 1 1/2 inches in diameter.
Bathroom Sink Drains
A bathroom sink drain is much smaller than a kitchen drain because it doesn't receive as much use. A bathroom sink drain is 1 1/4 inches wide, less than half the size of the kitchen drain. The drain opening in the sink basin is a little larger, commonly 1.375 inches wide. This width is larger so the drain plug, which is a separate, cylindrical-shaped part, fits inside the drain hole. Bathroom sinks use 1 1/4-inch drainpipe that connects to a 1 1/4- or 1 1/2-inch P-trap.
Showers and Tubs
A shower drain is 3 1/4 inches in diameter, just like a kitchen drain. This drain also has a screened strainer that fits over the opening. As the shower continually drains water as you shower, it is larger in diameter and fits onto a P-trap and drainpipe, both of which are 2 inches in diameter. A tub drain is 1 1/2 inches in diameter and connects to a 1 1/2-inch P-trap and drainpipe. The tub drain doesn't need to continually drain off water like the shower, so it is smaller.
Other Drain Sizes
Bar sinks typically use the standard 3 1/4-inch drain size like a kitchen sink. They also use 1 1/2-inch drainpipes and P-traps. A laundry sink drain is 1 1/2 inches in diameter and uses a 1 1/2-inch drainpipe and P-trap. A whirlpool tub uses a 1 1/2-inch drain with the same size drainpipe and P-trap. Other drains follow similar dimensions. Drain sizes are usually chosen based on how much volume of drainage is needed.