Duct geometry

Duct Cross-Sectional Area Calculator

Calculate round or rectangular duct face area from actual inside dimensions in US customary or SI units.

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Round duct area

Use the clear inside diameter available to airflow.

Square feetft²
Square inchesin²
Square metresm²
Square millimetresmm²

Rectangular duct area

Use the clear inside width and height.

Square feetft²
Square inchesin²
Square metresm²
Square millimetresmm²

How to use

  1. Choose the dimension unit and enter the actual clear inside diameter or width and height.
  2. Read the same cross-sectional area in ft², in², m², and mm² without converting the dimensions by hand.
  3. Use the area with an identified airflow in the Duct Velocity Calculator when face velocity is the next question.

Formulas

Round area: A = πD² ÷ 4
Rectangular area: A = W × H
Velocity from known airflow: V = Q ÷ A

Keep every dimension in the same length unit before applying a formula. The calculator converts the resulting area with exact international inch and foot relationships.

Worked examples

A 12 in round duct has 113.10 in², or 0.7854 ft², of cross-sectional area. A 100 mm round duct has 7,853.98 mm², or 0.007854 m². An 18 in × 8 in rectangular duct has 144 in², or 1.00 ft².

Result interpretation

Cross-sectional area is the clear geometric opening normal to the airflow. It is not exterior sheet-metal surface area, grille free area, or an equivalent-round diameter. Equal face area also does not mean equal friction loss: shape, wetted perimeter, roughness, fittings, length, and air conditions still matter.

Continue the workflow

Use a known CFM and this area in the Duct Velocity Calculator. Use the Duct Size Calculator when airflow and a documented velocity basis are known, then check resistance with the Duct Friction Loss Calculator.

Frequently asked questions

What is the cross-sectional area of a 100 mm round duct?

Using A = πD² ÷ 4 with D = 100 mm gives 7,853.98 mm², which is 0.007854 m² before rounding.

Can I use outside duct dimensions?

Use clear inside dimensions for airflow area whenever available. Liner, insulation inside the shell, dampers, or other obstructions can reduce the usable opening.

Does equal area mean equal friction loss?

No. Equal area is only a geometry comparison. Round and rectangular ducts with the same area can have different wetted perimeter and resistance.

Is this the square footage of sheet metal?

No. This calculator finds the cross-sectional opening. Sheet-metal surface area also requires duct length and perimeter and answers a different question.

Related resources

Sources and scope

The shape formulas are standard plane geometry. NIST conversion factors are used for the simultaneous US customary and SI area results. The measurement source confirms that duct volume flow uses the cross-sectional area normal to airflow.