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.
Reviewed by Abdullah, Technical Content Specialist
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.
Worked Examples
Example 1: Bedroom supply
Problem:400 CFM at 900 FPM
Solution:Area = 400/900 = 0.444 ftยฒ = 64 inยฒ. ร = โ(4ร64/ฯ) = 9.0\"
Result:10\" round duct
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.
Reviewed by Abdullah, Technical Content Specialist ยท Editorial policy