Duct Size Calculator
Plan your hvac project with our free duct size calculator. Get precise measurements, material lists, and budgets. Enter your values for instant results.
Formula
Area = CFM / Velocity | Diameter = โ(4 ร Area / ฯ)
Duct area = airflow / velocity. ASHRAE recommends 600-900 ft/min for main ducts, 400-600 ft/min for branch ducts to minimize noise.
Frequently Asked Questions
How do I size an HVAC system for a building?
HVAC sizing uses Manual J calculations considering square footage, insulation, window area, climate zone, and occupancy. A rough estimate is 1 ton of cooling per 400-600 square feet. Oversized systems short-cycle and waste energy; undersized systems cannot maintain comfort.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Does Duct Size Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.
How accurate are the results from Duct Size Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.