Duct Sizing Calculator: engineer's guide
Ducts are sized to deliver the required airflow at acceptable velocity and pressure drop. Equal-friction is the most common method.
Open the Duct Sizing calculator →
What this calculator does
The calculator solves for duct cross-section given CFM and one constraint (velocity or friction rate) using the Darcy-Weisbach form of the airflow equation.
Formulas used
- Duct area:
A = CFM / V (ft² = CFM / fpm) - Equivalent round diameter:
Deq = 1.30 × (a × b)0.625 / (a + b)0.25 - Pressure drop:
ΔP = f × (L / D) × (ρ V² / 2)
Variables
- V = velocity, fpm
- a, b = rectangular duct sides, inches
- f = friction factor, ρ = air density
Worked example
Scenario: 3000 CFM main supply duct, target friction 0.08 in wg/100 ft, standard air.
- From ductulator: 3000 CFM at 0.08 in/100 ft → round D ≈ 20 in
- Rectangular equivalent 22 × 16 in gives similar friction
- Velocity = 3000 / (22 × 16 / 144) ≈ 1230 fpm — acceptable for main supply
Common mistakes to avoid
- Using aspect ratio above 4:1 (excessive friction and material)
- Same friction rate for main and branch (branches should be quieter)
- Missing dynamic loss (elbows, tees) — add via equivalent length
- Ignoring return-air duct sizing (often as critical as supply)
Frequently asked questions
Equal friction or static regain?
Equal friction for supply and return in typical commercial. Static regain for high-velocity systems and long runs.
Typical velocities?
Main supply 1200 to 1800 fpm, branches 800 to 1200, VAV runouts 500 to 800.
Standards and references
SMACNA HVAC Duct Design, ASHRAE Fundamentals Ch. 21
Related tools on this site
Open the calculator that pairs with this guide: Duct Sizing Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.