Mortgages Calculator

HVAC Duct Size Calculator

Calculate the required round duct diameter and rectangular dimensions based on CFM and friction loss.

Cubic Feet per Minute required for the space.

Standard design is usually 0.1 in. w.g./100ft.

Optional: Provide one side to calculate the corresponding width.

Calculation Results

Required Round Diameter: inches

Velocity: FPM (Feet Per Minute)

Equivalent Rectangular Size: " (Width) x " (Height)

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Understanding Proper HVAC Duct Sizing

Proper duct sizing is critical for ensuring efficient HVAC operation, maintaining indoor comfort, and minimizing noise. If ducts are too small, the system works harder (increasing energy costs) and produces excessive noise. If they are too large, the air velocity drops, preventing air from mixing properly within the room.

The Factors of Duct Design

• CFM (Cubic Feet per Minute): This is the volume of air required to heat or cool a specific room. This value is typically determined through a Manual J load calculation.
• Friction Rate: This represents the pressure drop the air encounters as it moves through the ductwork. Most residential systems are designed using a friction rate of 0.1 inches of water gauge per 100 feet of duct.
• Velocity (FPM): This is the speed of the air. High velocity (over 900-1000 FPM in residential supply branches) often leads to "whistling" or "rushing" sounds.

How the Calculation Works

This calculator uses the Equal Friction Method. The mathematical relationship between CFM, Friction, and Diameter is derived from the Darcy-Weisbach equation. To find the equivalent rectangular size from a round diameter, we use the Huebscher formula:

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

Where 'D' is the round diameter, and 'a' and 'b' are the sides of the rectangular duct.

Practical Example

Imagine you have a master bedroom requiring 200 CFM. Using a standard friction rate of 0.1, the calculator determines you need an 8-inch round duct. If you are constrained by ceiling space and can only fit a 6-inch high rectangular duct, the calculator will tell you that the duct must be approximately 9 inches wide to maintain the same airflow capacity.

function calculateDuctSize() { var cfm = parseFloat(document.getElementById('hvac_cfm').value); var friction = parseFloat(document.getElementById('hvac_friction').value); var height = parseFloat(document.getElementById('hvac_height').value); var warningDiv = document.getElementById('hvac_warning'); warningDiv.style.display = 'none'; warningDiv.innerHTML = "; if (isNaN(cfm) || cfm <= 0 || isNaN(friction) || friction 0) { // Iterative search for width for (var i = 0; i = diameter) { break; } width += 0.1; } } else { width = 0; } // Display Results document.getElementById('res_diameter').innerText = diameter.toFixed(2); document.getElementById('res_velocity').innerText = Math.round(velocity); document.getElementById('res_rect_width').innerText = width > 0 ? width.toFixed(1) : "N/A"; document.getElementById('res_rect_height').innerText = !isNaN(height) ? height : "N/A"; document.getElementById('hvac_results').style.display = 'block'; // SEO/Expert Warnings if (velocity > 900) { warningDiv.innerHTML = "Warning: Velocity exceeds 900 FPM. This duct may be noisy in residential applications."; warningDiv.style.display = 'block'; } else if (velocity < 400) { warningDiv.innerHTML = "Note: Velocity is below 400 FPM. Air throw may be insufficient to mix the room air effectively."; warningDiv.style.display = 'block'; } }