Square to Round Duct Calculator

Last updated: 30 Sep 2025 | Author: Brij | Review By: Irshad

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Use our Square to Round Duct Calculator to find the equivalent round diameter and area differences of square or rectangular ducts for efficient HVAC design.

Square Duct Width:

Square Duct Height:

Round Duct Diameter:

Square-to-Round Duct Calculation Results

Equivalent Diameter of Square Duct:

Difference in Area:

Cross-Sectional Area of Square Duct:

in²

Cross-Sectional Area of Round Duct:

in²

Calculation Details

Square Duct Dimensions: 0 × 0 in

Round Duct Diameter: 0 in

Hydraulic Diameter Formula: D_h = 4A/P

Aspect Ratio: 1.00

Perimeter (Square): 0 in

Area Comparison: Equal

When designing HVAC systems, engineers often need to convert square or rectangular duct dimensions into an equivalent round duct. A round duct can reduce resistance, improve airflow, and simplify installation. This calculator quickly determines the equivalent round diameter and the cross-sectional areas for your square/rectangular duct.

Formulas

1. Cross-Sectional Area of Square (Rectangular) Duct

$A_\text{square} = W \times H$

2. Cross-Sectional Area of Round Duct

$A_\text{round} = \pi \times \left( \frac{D}{2} \right)^2$

3. Equivalent Round Diameter for a Square Duct

A widely used formula is:

$D_\text{equiv} = 1.30 \times \left( \frac{W \times H}{W + H} \right)$

Where W and H are the width and height of the square/rectangular duct.

4. Difference in Area (%)

$\%\ \text{Difference} = \frac{A_\text{round} - A_\text{square}}{A_\text{square}} \times 100$

Example Calculation

Suppose:

Square Duct Width $W=20$ in
Square Duct Height $H=20$ in
Round Duct Diameter $D = 22$ in

Step 1: Square Duct Area

$A_\text{square} = 20 \times 20 = 400 \text{ in}^2$

Step 2: Round Duct Area

$A_\text{round} = \pi \left(\frac{22}{2}\right)^2 = \pi \times 11^2 = 380.13 \text{ in}^2$

Step 3: Equivalent Round Diameter

$D_\text{equiv} = 1.30 \times \frac{20 \times 20}{20 + 20} = 1.30 \times \frac{400}{40} = 13 \text{ in}$

Step 4: Area Difference

$\%\ \text{Difference} = \frac{380.13 - 400}{400} \times 100 = -4.97\%$

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