Water lines under your sink lead into your faucets and fixtures. These pipe sizes vary based on the type of faucet installed, and they usually connect to shutoff valves near the sink. Standardized sizes for each fixture are available, and if you plan to install your own water lines, you should familiarize yourself with them.
A kitchen faucet water line is 1/2 inch in diameter. This 1/2-inch pipe supplies the water for the faucet from the home's main water line. Two water lines are present under your sink at the kitchen faucet -- one is for the hot water, the other for cold water. Both lines connect to flexible supply tubing that runs up to the base of the faucet. The kitchen drain pipe running from the sink basin to the home's drainage line is 1.5 inches in most cases.
A bathroom faucet has the same water-line pipe size as a kitchen faucet -- 1/2 inch. Usually, this pipe is the exact same type of pipe running to the kitchen. Both bathrooms and kitchens use copper pipes -- all of the home's water-supply pipes are typically copper. Bathroom faucets have smaller drains, however. The bathroom faucet simply does not need as large a drain pipe as the kitchen, and is usually 1.25 inches wide.
The water-line size for these two faucets allows for standardized fittings on supply-line tubing that connects the pipes to the faucets. It also allows manufacturers to design faucets for specific water-flow rates. With standardized pipe sizes feeding the faucets, it is easier to judge the amount of water flowing into the faucet. Therefore, it is easier to produce a faucet with a set flow rate. Federal regulations mandate limited flow rates on fixtures such as showerheads to conserve water.
Showers and Tubs
For comparison purposes, a shower faucet uses 1/2-inch copper pipe water lines. These pipes feed the shower head and tub, if it is a shower/tub combination unit. In a tub-only faucet, only a larger water line is present, usually 3/4 inch in diameter. They are larger because tubs demand a higher flow rate. The shower may have a larger drain pipe than the bathroom or kitchen because showers have a higher flow rate than kitchen or bathroom faucets. The drain in the shower is usually 1.5 or 2 inches in diameter.