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Calculation Machine

Duct Cross-Section Area Calculator

Cross-sectional area of round or rectangular duct.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Cross-section area
78.5 sq in (0.545 sq ft)
Resulting velocity
917 FPM

Quick answer

Duct cross-section area determines velocity for a given CFM: round ducts use pi/4 x diameter squared, rectangular ducts use width x height.

The formula

Area (round) = pi/4 x D^2; Area (rect) = W x H

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Duct shapeRound
  • Diameter or width10 in
  • Height (rectangular only)8 in
  • Airflow500 CFM

Result

  • Cross-section area78.5 sq in (0.545 sq ft)
  • Resulting velocity917 FPM

How to use the Duct Cross-Section Area Calculator

  1. 1Choose round or rectangular duct shape.
  2. 2Enter the diameter, or width and height for rectangular.
  3. 3Enter airflow to see the resulting velocity through that cross-section.

Frequently asked questions

Why does velocity matter here?

The same duct area can be too fast or too slow depending on airflow, so always check velocity against comfort and noise targets.

Which is more efficient, round or rectangular?

Round duct has less surface area per unit of airflow, so it is generally more efficient and cheaper per CFM carried.

About this calculator