Duct Calculator
Use our free Duct Calculator to quickly estimate duct size and airflow requirements, based on key inputs like CFM, velocity, and friction loss. Perfect for HVAC contractors and homeowners.
Get StartedCalculation Inputs
Supply: 700-1000 FPM, Return: 500-700 FPM
Calculation Results
Area (sq in) = CFM ÷ FPM × 144
Round Diameter = √(4 × Area ÷ π)
Rect Eq. Round = 1.3 × (W × H)^0.625 ÷ (W + H)^0.25
Recommended Duct Size
0" diameter
Air Velocity
0 FPM
Friction Loss
0.00" / 100 ft
All calculations are estimates and should be verified by a professional.
Duct Sizing Formula
Proper duct sizing ensures balanced airflow, system efficiency, and comfort. Undersized ducts create noise and pressure drops; oversized ducts waste material and cost. Skip manual math—use our free Duct Calculator for instant results.
How this duct calculator works
This calculator estimates duct size using airflow (CFM), velocity, and friction loss inputs. It applies standard HVAC design formulas to ensure accurate duct sizing for residential or commercial projects.
When to use this duct calculator
Designing HVAC ductwork for new installations
Checking airflow balance during renovations
Comparing duct sizes for different HVAC system loads
Troubleshooting airflow or comfort issues
Planning material costs and labor for ductwork projects
Want to make your HVAC duct projects smarter?
Use ServiceAgent.ai to automate estimates, manage proposals, and track profitability—all from one dashboard.
Book a Free DemoAverage Profit Margins by Industry
Understand how HVAC ductwork compares to other service industries. Below are typical profit margin ranges across common service businesses:
HVAC Contractors
10–20%
Ductwork Fabricators
15–25%
Plumbing Contractors
20–35%
Electrical Contractors
15–25%
Roofing Contractors
20–40%
Construction/Contracting
15–30%
These benchmarks help HVAC professionals evaluate whether duct installation costs are competitive and profitable.
Frequently Asked Questions
How accurate is this duct calculator?
It uses industry-standard airflow and velocity formulas to provide reliable estimates. Final designs should follow ASHRAE or SMACNA guidelines.
What factors affect duct size the most?
Key drivers include airflow (CFM), air velocity, static pressure, and friction loss.
Can I use this for both round and rectangular ducts?
Yes—this calculator provides area and diameter. Rectangular equivalents can be sized from the same values.
Is this calculator useful for contractors?
Absolutely. It provides quick estimates for proposals and installations.
Can I adjust assumptions like velocity limits?
Yes—enter velocity values that match your design standards.
Does the estimate include installation or material costs?
No—it only calculates duct size. Add labor and material costs separately.
Can I use this for both residential and commercial systems?
Yes—just input the appropriate airflow and velocity for the project type.
How does this integrate with my workflow?
Pair it with ServiceAgent.ai to streamline duct sizing, quotes, and project management.