DWV Fixture Unit Calculator

About the DWV Fixture Unit Calculator

Totals the Drainage Fixture Units (DFU) for a set of plumbing fixtures to help size drain, waste and vent piping.

How to use this calculator

1. List every plumbing fixture you want to include, such as toilets, sinks, showers, and urinals.
2. Find the drainage fixture unit value for each fixture from the plumbing code or the calculator’s fixture list.
3. Add all of the fixture unit values together to get the total drainage fixture units.
4. Use the total FUs to help determine the proper drain or pipe size.

The formula explained

The calculator computes the total drainage fixture units by adding the FU value of each fixture. In symbols, the total is T = f_1 + f_2 + dots + f_n, where each term is one fixture’s assigned value.

• T = total drainage fixture units
• \(f_1\) = drainage fixture unit value of the first fixture
• \(f_n\) = drainage fixture unit value of the last fixture
• n = number of fixtures included

Step by step method

1. Write down each fixture in the system and its drainage fixture unit value.
2. Add all of the values together carefully.
3. Check that you used the correct code values for each fixture type and branch of the system.

Worked example

Problem. A restroom has 2 water closets at 4 FUs each, 2 lavatories at 1 FU each, and 1 shower at 2 FUs. Find the total drainage fixture units.

1. Compute the water closets: 2 d7 4 = 8.
2. Compute the lavatories and shower: 2 d7 1 = 2 and 1 d7 2 = 2.
3. Add them: 8 + 2 + 2 = 12, so the total is 12 FUs.

Tips and common mistakes

• Make sure you use drainage fixture units, not water supply fixture units, because they are not the same.
• If a fixture is unusual or commercial, verify its assigned value from the correct plumbing code before adding it in.

Frequently asked questions