Plumbing fixtures, fixture fittings, and fixture unit calculations
Fixture units and fittings
Fixture unit calculations convert different fixtures into a common demand measure. Drainage fixture units size drainage branches and stacks; supply fixture units size water piping. The adopted table supplies the values and any diversity rules. Fixture fittings include faucets, flush valves, shower valves, tempering valves, stops, and air gaps. Installation must meet clearance, accessibility, and manufacturer requirements. Water closets need the correct rough-in and carrier; lavatories need an accessible trap; showers need a pressure-balancing or thermostatic valve where required.
Counting rules
Count each fixture, including floor drains, laundry trays, and indirect waste receptors. Do not mix drainage and supply fixture units. Apply the code's rules for private versus public use and for continuous or intermittent demand. Add fixture-unit values before sizing the branch, then check the stack and building drain. Fixture fittings must be protected against backflow by an air gap or listed device.
Worked example
Using typical table values, a bathroom group has a water closet at 3 drainage fixture units, a lavatory at 1, and a shower at 2, totaling 6. Three identical groups total 18 fixture units. The branch and stack must be sized from the table and the allowable loading, not from pipe diameter alone. If a floor drain at 1 fixture unit is added, the total becomes 19, and the venting and trap primer for that drain must also be shown.
Traps
Mixing supply and drainage units; omitting floor drains or laundry; assuming larger pipe always solves load; ignoring public-use multipliers; forgetting accessible trap and cleanout requirements.