HVAC Ductwork, Air Handling & Mechanical Products worked example

Damper Sizing with duct opening width of 12 in: a worked example

Here is what the math looks like when conditions slip. We hold every other input steady and drop duct opening width to 12 in, then walk the calculation through step by step. Estimate the face area and sizing check for a volume control, fire, or smoke damper based on duct dimensions, design airflow, and maximum allowable face velocity. Use for damper selection in rectangular ductwork and air handling unit connections.

The inputs for this scenario

  • Duct opening width: 12 in (the input this scenario stresses; the baseline uses 24)
  • Duct opening height: 16 in (held at the documented default)
  • Design airflow: 1,800 CFM (held at the documented default)
  • Maximum allowable face velocity: 2,000 FPM (held at the documented default)

Working through the calculation

  • The calculation starts from the formula this tool documents: Damper face area = (width × height) ÷ 144 (sq ft).
  • Result works out to 691,200,000 units at these inputs, and this is the headline figure for the scenario.
  • Base product works out to 345,600 value at these inputs.
  • Multiplier works out to 2,000 x at these inputs.
  • Factor A x B works out to 192 value at these inputs.

How this compares with the baseline

  • Against the tool's baseline example, where duct opening width sits at 24 in and the headline result is 1,382,400,000 units, this scenario comes in 50% below the baseline at 691,200,000 units.
  • The practical read: the gap between this scenario and the baseline is entirely attributable to duct opening width, so recovering it is worth quantifying in dollars before considering equipment or staffing changes. It gives a clean face velocity only; it does not model blade blockage, free-area derating, or the actual pressure drop and torque, which depend on the specific damper model.

Results at a glance

  • Result: 691,200,000 units (headline result)
  • Base product: 345,600 value
  • Multiplier: 2,000 x
  • Factor A x B: 192 value

Run it with your numbers

  • To rerun this with your own numbers, open the live Damper Sizing calculator, set duct opening width to your actual value, and adjust the remaining inputs to match your operation.

Last reviewed 2026-05-12.