Every sink has a drain line plumbed to the main drainage line in the home. This drain line is made up of pipes, each with a certain size. Since different sinks are used for a variety of things and have different flow rates, the drainpipe sizes are adjusted to accommodate them.
The kitchen sink is the most frequently used sink in the house, so the drainpipe is slightly larger than the bathroom sink. The kitchen sink drainpipe is 1 1/2 inches in diameter. It is larger to handle food wastes and water from dishwashing and cooking. This pipe runs from the sink bottom to the P-trap and then on to the home's main drainpipe. Kitchen drainpipes are not standardised, and some kitchens with smaller sinks and lower water flow use 1 1/4-inch pipes.
The bathroom sink drain is smaller in size because the bathroom sink is only used for washing and shaving. Bathroom drainpipes are 1 1/4 inches in diameter. These pipes connect to the sink's drain hole and house the drain plug. Bathroom drains also use a P-trap that connects to the home's main drain line. A reducing bushing, a pipe fitting with one end a slightly smaller diameter than the other, connects the drainpipe to the home's main drain line.
Shower drainpipes handle a larger volume of water; therefore, they measure in at 2 inches in diameter. Since shower head flow rates are lower today, some shower drainpipes measure only 1 1/2 inches. Shower heads with reduced flow rates of 2.5 gallons per minute drain sufficiently into smaller drainpipes. The size of the pipe does affect drainage and, since showers drain constantly, a larger pipe ensures consistent and effective draining.
Tub drainpipes measure 1 1/2 inches in diameter, which is smaller than a shower drain. Since showers must drain steadily during use, they require larger drainpipes. A tub drain only requires sufficient drainage to empty the tub after each use. The tub drain is also fit to a P-trap which feeds into the home's main drain line. The same reducing bushings make the connections between different sized pipes if necessary.