How to Calculate Plumbing Vent Sizing


All plumbing requires an air-admittance valve, otherwise known as a vent or vent stack. Vents allow air into pipes that drain water or waste. Without this air, material in the pipe would not drain properly because of the negative air pressure inside the pipe. While plumbing codes vary, most municipalities require the size of the vent be at least half that of the largest pipe on the drain. This means identifying the size of the largest drain pipe in your system.

Things You'll Need

  • Flashlight
  • Pen and paper
  • Identify the fixtures you need to vent. This includes toilets, sinks and showers.

  • Access the pipes below or attached to the drain on each fixture. You may need to enter a crawlspace or basement to see these pipes.

  • Look over the pipes for the printing on the side. The printing contains the diameter of the pipes. Diameter is listed in terms of inches. You may need to use a flashlight to see the wording.

  • Write down the sizes of all the drain pipes. Divide the largest of the sizes by 2 to get half of the pipe diameter. This is the size that must be used for the vent stack. For example, if the largest pipe on the drain system is 3 inches in diameter, the vent stack should be at least 1.5 inches in diameter.

Tips & Warnings

  • Consult local plumbing codes for specific rules on installing vent stacks in your area.

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