Square to Round Duct Calculator

Square to Round Duct Equivalent Calculator

Calculate the equivalent round diameter for rectangular or square ductwork based on the Huebscher formula.

Results:

Equivalent Round Diameter: 0 inches

Total Cross-Sectional Area: 0 sq inches

Aspect Ratio: 0:1

*Calculation based on Huebscher's formula for equal friction and capacity.

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Understanding Square to Round Duct Conversion

In HVAC (Heating, Ventilation, and Air Conditioning) design, it is often necessary to transition from rectangular ductwork to round ductwork due to space constraints or structural requirements. However, you cannot simply match the cross-sectional area. Because air moving through a rectangular duct experiences more friction (due to the larger surface area per volume) than air in a round duct, we use the Equivalent Diameter.

The Huebscher Formula

The industry standard for calculating the circular equivalent of a rectangular duct is the Huebscher formula. This formula ensures that the round duct will have the same friction loss and capacity as the rectangular duct at the same air flow rate (CFM).

De = 1.30 * ((a * b)^0.625 / (a + b)^0.25)

• De: Equivalent Round Diameter
• a: Duct Width
• b: Duct Height

Why the Aspect Ratio Matters

The aspect ratio is the ratio between the width and height of your rectangular duct. For maximum efficiency, HVAC engineers recommend keeping the aspect ratio as close to 1:1 as possible. As the duct becomes flatter and wider (e.g., a 4:1 ratio), the internal friction increases significantly, requiring more fan power to move the same amount of air.

Practical Example

If you have a rectangular duct that is 12 inches wide and 8 inches high:

1. The total area is 96 square inches.
2. Using the formula, the equivalent round diameter is approximately 10.7 inches.
3. In practice, an installer would likely use an 11-inch or 12-inch round duct to ensure proper airflow and account for standard manufacturing sizes.

Frequently Asked Questions

Is equivalent diameter the same as equal area?
No. A round duct with the same surface area as a rectangular duct will actually have less friction loss. Therefore, the "equivalent diameter" calculation results in a round duct that performs the same, rather than just looking the same on paper.

What happens if I use a round duct that is too small?
Using a smaller round duct increases static pressure. This forces your blower motor to work harder, increases noise levels, and can significantly reduce the lifespan of your HVAC equipment.

function calculateDuctSize() { var width = parseFloat(document.getElementById('ductWidth').value); var height = parseFloat(document.getElementById('ductHeight').value); var resultDiv = document.getElementById('ductResult'); if (isNaN(width) || isNaN(height) || width <= 0 || height = height) ? (width / height) : (height / width); // Output results document.getElementById('roundDiameter').innerText = equivalentDiameter.toFixed(2); document.getElementById('ductArea').innerText = area.toFixed(2); document.getElementById('aspectRatio').innerText = ratio.toFixed(2); // Show result container resultDiv.style.display = "block"; // Smooth scroll to result resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }