Calculate CFM requirements and recommended round duct diameter
Excellent (Modern Insulation)
Average (Standard Walls/Windows)
Poor (Older Home/Large Windows)
Residential standard: 600-900 FPM
Calculation Results
Estimated Heat Load:–
Required Airflow:–
Recommended Round Duct Diameter:–
Minimum Rectangular Area:–
Complete Guide to HVAC Duct Sizing
Properly sizing your HVAC ducts is critical for maintaining home comfort, energy efficiency, and equipment longevity. If ducts are too small, your system will be noisy and work harder than necessary. If they are too large, the air velocity will be too low to effectively circulate air through the room.
How the Calculation Works
This calculator utilizes several industry-standard variables to determine your ductwork needs:
BTU Load Estimation: We estimate the British Thermal Units (BTUs) required based on the volume of the room (Sq. Ft. × Height) and an insulation multiplier.
CFM (Cubic Feet per Minute): Most residential systems require approximately 400 CFM per ton (12,000 BTU) of cooling capacity. This calculator derives the CFM based on your specific heat load.
Friction Loss and Velocity: Velocity is measured in Feet Per Minute (FPM). For residential supply branches, 600-700 FPM is standard to maintain a quiet environment.
The Formula
To find the cross-sectional area of the duct, we use the following equation:
Area (Sq. Inches) = (CFM × 144) / Velocity (FPM)
Once the area is known, the round diameter is calculated using the area of a circle formula.
Practical Example
Imagine a 400 sq. ft. master bedroom with 10 ft. ceilings. The volume is 4,000 cubic feet. Using standard factors, this might require roughly 300 CFM. If we target a velocity of 600 FPM:
Area = (300 × 144) / 600 = 72 sq. inches.
A 72 sq. inch area corresponds to approximately a 10-inch round duct.
Common Duct Size Chart (Standard Round)
Duct Diameter
Approx. CFM (@ 600 FPM)
6 Inch
~110 CFM
8 Inch
~210 CFM
10 Inch
~325 CFM
12 Inch
~470 CFM
function calculateDuctSizing() {
var sqft = parseFloat(document.getElementById('hvac_sqft').value);
var height = parseFloat(document.getElementById('hvac_height').value);
var insulation = parseFloat(document.getElementById('hvac_insulation').value);
var velocity = parseFloat(document.getElementById('hvac_velocity').value);
if (!sqft || !height || !velocity || sqft <= 0 || height <= 0 || velocity <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// Estimate BTU: Base factor of 20 BTU per cubic foot adjusted for insulation
// Calculation: Volume * 5 (standard rough multiplier) * insulation factor
var volume = sqft * height;
var estimatedBTU = volume * 5 * insulation;
// Standard HVAC rule: 400 CFM per 12,000 BTU (1 Ton)
var cfm = (estimatedBTU / 12000) * 400;
// Duct Area Calculation: (CFM * 144) / Velocity
var areaSqIn = (cfm * 144) / velocity;
// Diameter Calculation: sqrt((4 * Area) / Pi)
var diameter = Math.sqrt((4 * areaSqIn) / Math.PI);
// Display Results
document.getElementById('res_btu').innerHTML = Math.round(estimatedBTU).toLocaleString() + " BTU/hr";
document.getElementById('res_cfm').innerHTML = Math.round(cfm) + " CFM";
document.getElementById('res_diameter').innerHTML = diameter.toFixed(1) + " Inches";
document.getElementById('res_rect').innerHTML = Math.round(areaSqIn) + " Sq. Inches";
document.getElementById('hvac_results').style.display = 'block';
// Smooth scroll to results
document.getElementById('hvac_results').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}