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HVAC Duct Sizing & CFM Calculator

Determine the required airflow (CFM) and optimal duct diameter for your room.

4 (Low – Bedrooms) 6 (Standard Residential) 8 (High – Kitchens/Labs) 10 (Heavy Ventilation)
Residential avg: 600-900 FPM

Calculation Results

Required Airflow:

0 CFM

Round Duct Diameter:

0 Inches

Rectangular Equivalent Suggestions:

Understanding HVAC Duct Sizing and CFM

Proper duct sizing is the most critical factor in maintaining home comfort and system efficiency. If your ducts are too small, your HVAC system will work harder, leading to higher energy bills, excessive noise, and premature component failure. If they are too large, the air velocity will be too low to effectively throw air into the room.

How Airflow (CFM) is Calculated

CFM stands for Cubic Feet per Minute. It measures the volume of air moving through your HVAC system. To calculate the required CFM for a specific room, we use the Air Changes per Hour (ACH) method:

  • Step 1: Calculate Room Volume (Square Footage × Ceiling Height).
  • Step 2: Multiply Volume by the desired ACH.
  • Step 3: Divide the result by 60 to get CFM.

Choosing the Right Duct Velocity

Velocity is the speed at which air travels through the duct, measured in Feet Per Minute (FPM). For residential settings, a velocity of 600 to 900 FPM is standard. Higher velocities (above 1,000 FPM) are typically reserved for commercial buildings or trunk lines because they create significant "whooshing" sounds in smaller residential ducts.

Practical Example

Suppose you have a 150 sq. ft. bedroom with 10-foot ceilings. You want standard residential airflow (6 ACH) and a quiet system (600 FPM):

  1. Volume: 150 × 10 = 1,500 Cubic Feet.
  2. Total Air per Hour: 1,500 × 6 = 9,000 Cubic Feet/Hr.
  3. CFM: 9,000 / 60 = 150 CFM.
  4. Using the duct formula, this would require approximately an 8-inch round duct.

Round vs. Rectangular Ducts

While round ducts are the most efficient for airflow due to less friction, rectangular ducts are often used in joist spaces or walls where height is restricted. Use the calculator above to find the equivalent rectangular dimensions if you cannot fit a standard round pipe.

function calculateDucting() { var area = parseFloat(document.getElementById('roomArea').value); var height = parseFloat(document.getElementById('ceilingHeight').value); var ach = parseFloat(document.getElementById('achValue').value); var velocity = parseFloat(document.getElementById('velocity').value); if (isNaN(area) || isNaN(height) || isNaN(velocity) || area <= 0 || height <= 0 || velocity <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Calculate CFM: (Volume * ACH) / 60 var volume = area * height; var cfm = (volume * ach) / 60; // Calculate Duct Area in Sq Ft: Area = CFM / Velocity var ductAreaSqFt = cfm / velocity; // Convert to Sq Inches var ductAreaSqIn = ductAreaSqFt * 144; // Round Duct Diameter: sqrt((4 * Area) / PI) var diameter = Math.sqrt((4 * ductAreaSqIn) / Math.PI); // Display results document.getElementById('resCFM').innerText = Math.round(cfm); document.getElementById('resDiameter').innerText = diameter.toFixed(1); // Generate rectangular suggestions var rectList = document.getElementById('rectSuggestions'); rectList.innerHTML = ""; // Calculate a few common rectangular heights var heights = [4, 6, 8, 10]; for (var i = 0; i 2) { var li = document.createElement("li"); li.style.marginBottom = "5px"; li.innerHTML = "• " + h + "\" high x " + Math.round(w) + "\" wide"; rectList.appendChild(li); } } document.getElementById('hvacResults').style.display = "block"; // Smooth scroll to result document.getElementById('hvacResults').scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }

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