Duct Size Calculator — HVAC Ductulator | SMACNA

ℹ️ About This Calculator

Correct duct sizing ensures the right amount of conditioned air reaches each zone while keeping fan energy, noise levels, and pressure drop within acceptable limits. Undersized ducts create excessive velocity (noise, high pressure drop, energy waste); oversized ducts waste sheet metal cost and ceiling space. The two standard methods are the equal friction method (maintaining constant pressure drop per metre) and the velocity method (setting duct velocity based on application).

SMACNA (Sheet Metal and Air Conditioning Contractors National Association) duct construction standards are the globally accepted reference for HVAC ductwork. The equal friction method — typically 0.8 Pa/m for commercial systems — balances the duct network automatically and simplifies balancing damper adjustment. For high-velocity systems in critical spaces (hospitals, clean rooms), the velocity method is preferred to control noise and ensure LAM (laminar air movement).

📐 Duct Sizing Formulas (SMACNA)

SMACNA / ASHRAE

Velocity Method:
  A = Q / v
  D = √(4A/π)   [circular duct]
  For rectangular: W × H = A (maintaining aspect ratio ≤ 4:1)

Equal Friction Method (Darcy-Weisbach):
  ΔP/L = f × (ρv²) / (2 × Dh)
  Dh = 4A / P   [hydraulic diameter]

Recommended Velocities (SMACNA):
  Main ducts:        5–8 m/s
  Branch ducts:      3–5 m/s
  Diffuser necks:    2–3 m/s
  Return air grille: 1.5–2.5 m/s

❓ Frequently Asked Questions

What is the recommended duct velocity for a commercial office HVAC system? +

For commercial office buildings, SMACNA recommends 5–8 m/s for main supply ducts, 3–5 m/s for branch ducts, and 2–3 m/s at diffuser connection necks. Higher velocities increase noise and pressure drop; lower velocities require oversized ducts and increase material cost.

What is the maximum aspect ratio for rectangular ducts? +

SMACNA recommends a maximum aspect ratio of 4:1 (width to height) for rectangular ducts. Higher aspect ratios significantly increase the perimeter-to-area ratio, which increases friction and requires more sheet metal per unit of airflow. Ideally keep the ratio at 2:1 or less for efficiency.

Should I use round or rectangular ducts? +

Round ducts are more efficient — they have the minimum perimeter for a given cross-section, minimizing friction and material. However, rectangular ducts fit better in low-ceiling plenum spaces and around structural elements. Oval (flat oval) ducts offer a compromise. Use round wherever the ceiling void permits.

What friction rate should I use for duct design? +

For commercial HVAC systems, the equal friction rate of 0.8–1.0 Pa/m (0.1 in.wg/100ft) is standard. For low-pressure residential systems, 0.5 Pa/m. For high-velocity systems: 2.0–4.0 Pa/m. The design friction rate determines duct sizes throughout the system.

How do I convert CFM to m³/h for duct sizing? +

1 CFM = 1.699 m³/h. So 1000 CFM = 1699 m³/h ≈ 1700 m³/h. Alternatively, 1 m³/s = 3600 m³/h = 2119 CFM. Most Indian HVAC projects use m³/h or L/s; equipment from US suppliers is often rated in CFM.

HVAC Duct Size Calculator (Ductulator)

ℹ️ About This Calculator

📐 Duct Sizing Formulas (SMACNA)

SMACNA / ASHRAE

❓ Frequently Asked Questions

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