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HVAC Duct Sizing Calculator

Equal Friction Method Sizing

Standard residential value is 0.1

function calculateHvacDuct() { var cfm = parseFloat(document.getElementById('cfm_input').value); var friction = parseFloat(document.getElementById('friction_input').value); var resultBox = document.getElementById('hvac_result_box'); if (isNaN(cfm) || cfm <= 0 || isNaN(friction) || friction <= 0) { alert("Please enter valid positive numbers for CFM and Friction Rate."); return; } // Equal Friction Method Formula for Round Duct Diameter (d) in inches // d = 1.63 * (Q^0.375 / ΔP^0.188) // Q = CFM, ΔP = Friction Rate var diameter = 1.63 * (Math.pow(cfm, 0.375) / Math.pow(friction, 0.188)); // Circular Area A = π * r^2 var roundArea = Math.PI * Math.pow((diameter / 2), 2); // Calculate Rectangular Equivalents (Huebscher Formula approximation for Area) // Using common heights: 8, 10, and 12 inches var width8 = roundArea / 8; var width10 = roundArea / 10; var width12 = roundArea / 12; document.getElementById('round_diameter_text').innerHTML = "Round Duct Diameter: " + diameter.toFixed(1) + " inches"; document.getElementById('rect_8_text').innerHTML = "Rectangular Option (8\" Height): " + Math.ceil(width8) + "\" x 8\""; document.getElementById('rect_10_text').innerHTML = "Rectangular Option (10\" Height): " + Math.ceil(width10) + "\" x 10\""; document.getElementById('rect_12_text').innerHTML = "Rectangular Option (12\" Height): " + Math.ceil(width12) + "\" x 12\""; resultBox.style.display = 'block'; }

Properly sizing HVAC ductwork is the most critical step in ensuring home comfort, energy efficiency, and equipment longevity. This HVAC Duct Sizing Calculator uses the Equal Friction Method, the industry standard for sizing residential and light commercial supply and return ducts.

Why Proper Duct Sizing Matters

If ducts are sized incorrectly, several issues can occur:

• Ducts too small: Causes high static pressure, noisy registers, and premature blower motor failure.
• Ducts too large: Air velocity drops too low, preventing air from reaching the furthest rooms in the house.
• Inefficiency: Your AC or furnace must work harder to push air through restricted passages, increasing utility bills.

Pro Tip: Most residential systems are designed based on a friction loss of 0.10 inches of water column (w.g.) per 100 feet of duct. However, if you have a high-static application or very long duct runs, you might use 0.08 or 0.06.

How to Use This Calculator

To get an accurate measurement, follow these three steps:

1. Determine CFM: Calculate the Cubic Feet per Minute (CFM) required for the room. A general rule of thumb is 400 CFM per ton of cooling.
2. Set Friction Rate: Consult your Manual D calculation. If unavailable, 0.1 is the common default for residential flex duct and rigid metal.
3. Choose Shape: The calculator provides the round diameter and rectangular equivalents. Use the rectangular dimensions if you are constrained by ceiling joists or wall cavities.

Example Calculation

Suppose you are installing a 3-ton unit. At 400 CFM per ton, the total airflow is 1,200 CFM. If you use a standard friction rate of 0.1:

• CFM: 1200
• Friction: 0.1
• Result: Approx. 14.5″ Round Duct or a 20″ x 8″ Rectangular Duct.

CFM Estimation Table

Tonnage	Average CFM	Typical Main Trunk Size (Round)
1.5 Tons	600	10″ – 12″
2.0 Tons	800	12″ – 14″
3.0 Tons	1200	14″ – 16″
5.0 Tons	2000	18″ – 20″

Frequently Asked Questions

Is flex duct sized differently than metal duct?
Yes. Flex duct has higher internal friction. While the math is the same, you should often use a lower friction rate (like 0.05) or size up by one standard size to compensate for the "ribbed" interior of flexible ducts.

What is the maximum velocity for residential ducts?
To keep noise to a minimum, supply ducts should generally keep air velocity under 700-900 feet per minute (FPM), while return ducts should stay under 600 FPM.