To find the size of drain pipe your washing machine requires, locate the drain hose and measure the interior diameter of its pipe fitting. The interior diameter of the fitting is the minimum size of drain pipe the washing machine requires. Most washing machines use 2-inch drain pipes.
The most common size for a washing machine drain hose is 1.5 inches. Many washing machine owners use 1.5-inch drain pipes for this reason. While an 1.5-inch drain pipe is adequate for use with a 1.5-inch washing machine drain hose, using the same size pipe presents a few problems.
When the drain hose and the drain pipe have the same diameter, air has a difficult time moving throughout the pipe, causing slow water flow. Slow water flow in the drain hose and pipe frequently leads to washing machine overflows, potentially causing serious water damage. A 1.5-inch pipe is also prone to clogs from lint, soap scum, sand and dirt that wash off in the machine.
Avoid clogs and backups by using a pipe with a larger diameter than the drain hose. Using a reducing fitting, increase the size of the drain pipe to at least 2 inches. Using a larger diameter drain pipe decreases problems by increasing the speed of water flow and reducing clogs.