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🌀 Duct Size Calculator
HVAC Systems Air Flow & Duct Dimension Tool
Calculation Summary
Required Cross-Area:
0 sq.ft
Round Duct Diameter:
0 inches
Rectangular Duct (Width × Height):
0" × 0"
Key Rules of HVAC Duct Sizing
The primary objective of duct design is to deliver conditioned or fresh air from the AC/ventilation unit to every room with minimal pressure drop and zero objectionable airflow noise.
- CFM (Cubic Feet Per Minute): Total air volume determined based on room heat load and air exchange requirements. Learn more on our JCI Classes Main Portal.
- Velocity (FPM): Speed of air moving inside the ductwork. Recommended design velocity follows standard guidelines by ASHRAE Standards.
- Aspect Ratio: The ratio of duct width to duct height should ideally remain between 1:1 and 3:1 to minimize friction loss.
Frequently Asked Questions (FAQ)
Q1. What formula is used to calculate duct cross-sectional area?
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Duct area is calculated using the continuity formula: Area (sq. ft.) = CFM ÷ Velocity (FPM). Based on this area, equivalent round and rectangular duct sizes are derived.
Q2. What happens if air velocity inside the duct is too high?
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Higher air velocity allows smaller duct sizes, but significantly increases static pressure drop, energy consumption, and high airflow noise.
Q3. How much CFM is typically required per Ton of air conditioning?
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As a standard HVAC rule of thumb, 1 Ton of cooling capacity requires approximately 400 CFM of airflow.