Duct Size Calculator — Compare Available Sizes
Compare your round and rectangular duct sizes against airflow, velocity, straight friction and available space. Work in US or SI units and save your comparison.
Enter the airflow you need to carry. Compare available duct sizes against your own velocity, friction and space limits.
Fit the available space
Edit available sizes
Example sizes, not a universal stock list. Use clear inside dimensions; rectangular sizes keep the orientation entered.
Air properties and surface
Illustrative air properties. Enter values for your conditions. The roughness example is galvanized spiral duct from the manufacturer source below.
Inspect size 3
Smallest listed area meeting your targets: size 3. Select any size below to inspect.
- Airflow
- 1,000 CFM · 1,699.01 m³/h
- Actual air velocity
- 750 FPM
- Straight friction
- 0.071 in.wg / 100 ft
- Loss for entered length
- 8.9 Pa
- Equivalent round diameter
- 14.61 in
- Reynolds · equivalent round
- 107,934
This size meets all entered targets.
Available sizes
This checks straight turbulent airflow only. Rectangular friction uses an equivalent-round approximation; air velocity uses the real cross-section. This is not a complete duct network, fan selection or acoustic assessment.
Full comparison
Duct size comparison · straight turbulent model
Airflow: 1,000 CFM = 1,699.01 m³/h = 471.95 L/s
Straight length: 15.24 m
Density 1.2 kg/m³; kinematic viscosity 0.000015 m²/s; roughness 0.09144 mm
Velocity limit: 900 FPM
Straight friction limit: 0.1 in.wg / 100 ft
Size 1: Round ⌀ 14 in; 935.44 FPM; 0.088 in.wg / 100 ft; straight loss 10.98 Pa; Above velocity target
Size 2: Round ⌀ 16 in; 716.2 FPM; 0.046 in.wg / 100 ft; straight loss 5.71 Pa; Meets entered targets
Size 3: Rectangular 24 in × 8 in; 750 FPM; 0.071 in.wg / 100 ft; straight loss 8.9 Pa; Meets entered targets
Size 4: Rectangular 20 in × 10 in; 720 FPM; 0.058 in.wg / 100 ft; straight loss 7.26 Pa; Meets entered targets
This checks straight turbulent airflow only. Rectangular friction uses an equivalent-round approximation; air velocity uses the real cross-section. This is not a complete duct network, fan selection or acoustic assessment.
Model, limitations and sources
Velocity = volume flow ÷ actual area. For a rectangular section, equivalent round diameter = 1.30 × (width × height)0.625 ÷ (width + height)0.25. Straight friction = Darcy factor × density × equivalent-round velocity² ÷ (2 × equivalent diameter). The Darcy factor solves the Colebrook equation; rectangular equivalent velocity is distinct from actual velocity.
Only Reynolds numbers from 4000 to 10⁹, speeds up to 50 m/s and relative roughness up to 0.05 are supported. These are model limits, not equipment approval. Flexible duct, fittings, filters, leakage, balancing and compressible flow are excluded. A size that meets your targets does not establish safe or code-compliant system performance. Air properties and targets are your inputs.
Sources: Eastern Sheet Metal equivalent duct guide; NIST flow conversion table. One CFM is 0.4719474432 L/s or 1.69901079552 m³/h.
