How to calculate installation units by plumbing code

Installing pipes in a remodel or a new home requires some math skills, welding ability, some knee pads, and a little knowledge of the plumbing code. Most pipe systems consist of what is known as DWV, which is drainage, water and ventilation. In order to configure the system, you must decide what kind of accessories you want to install. Each accessory has a unit value assigned for supply and for drainage.


  1. Take note of all the plumbing fixtures you wish to install on the left side of a notebook page. Make two vertical columns to the right of your accessories. Titles the first column Water Supply Units (WSFU) and the second column Drainage Accessories Vertical Units (DFU).
  2. Open the Uniform Code of Plumbing (UPC) and go to the chapter entitled “Water Supply and Distribution”. If you do not have a UPC volume, search the Internet and you will usually find the information you need for free. If the Internet option is also a problem, go to your local building department and ask for a copy of the WSFU and DFU book lists.
  3. Using the WSFU table, look for the type of accessory you are going to install and note the associated value. Enter the number that most closely resembles the type of sanitary installation you are installing. If there are two values ​​and you are not sure which one to choose, it is generally recommended to select the higher number. After completing the WSFU table, repeat the process in the second vertical column for the DFU.
  4. Add the values ​​of both columns and now you have your source and the totals of accessory of the drainage unit. Use these values ​​to now determine the pipe diameter and the length you will need to buy. Use the calculation drain fitting to determine the size of your ventilation system with the information provided in the chapter “Vents” (Ventilation) of UPC.