Drain Sizes for Do-It-Yourself Plumbing

Obtain a code book that contains the plumbing codes for your area from your local building department.

Look up each specific fixture in the fixture unit chart, also called a fixture unit determination chart or a DFU chart, within the code book.

Add up the total number of fixture units listed for that fixture in the chart. This gives you the fixture unit rating.

Find the corresponding pipe size for the fixture unit rating in a drainpipe sizing chart. Use this pipe size as the diameter size for the fixture's drainpipes. Look up the pipe size listed in the chart for the fixture rating of both a horizontal drain and a vertical drain, because the two sizes can be different.

Look up the size of the fixture's drain in the chart outlining the critical distance for drainpipes, and find the corresponding distance for this drainpipe. The critical distance is the maximum distance allowed for drainpipe to run from the fixture's curved trap to the vent pipe. The vent pipe releases fumes from the drainpipes and stabilizes the pressure within them.