🌬️ Fan Duct Size Calculator
Professional duct sizing based on ASHRAE / SMACNA standards | Worldwide units (Imperial / Metric)
⚙️ Design parameters
Real-time results
CFM
in wg/100ft
📘 How to use this tool
- 1. Enter airflow (CFM or m³/h) – actual fan flow rate.
- 2. Set target friction loss (typical 0.05–0.15 in wg/100ft for residential/commercial).
- 3. Choose duct shape + aspect ratio (if rectangular).
- 4. The calculator instantly shows required duct size, velocity, and reference friction curve.
- 5. Use the graph to visualize effect of diameter on friction loss.
📐 Recommended duct size
✅ ROUND DUCT DIAMETER
—
inches
💨 Air velocity
—
fpm
⚡ Actual friction loss (at design D)
—
in wg/100ft
🎯 Cross-section area
—
sq.in
🔍 US standards reference: ASHRAE Handbook – recommended friction rate 0.08–0.12 in wg/100ft for efficient systems. Velocity kept under 1500 fpm for low noise (residential & commercial). Worldwide equivalent: 0.65–1.0 Pa/m.
📈 Advanced friction curve analysis
For current airflow & friction target◉ Curve shows friction loss vs round duct diameter. Diamond marker = selected design diameter @ target friction.
🌎 Global design notes
✅ Metric (EU/Asia/ANZ): 1 m³/h ≈ 0.5886 CFM, friction 1 Pa/m ≈ 0.122 in wg/100ft. Our calculator auto-converts.
✅ ASHRAE / CIBSE standards: circular ducts preferred for low friction, rectangular used for space constraints.
✅ Recommended velocities: Residential: 600–900 fpm (3–4.5 m/s), Commercial: 900–1300 fpm (4.5–6.5 m/s).
✅ ASHRAE / CIBSE standards: circular ducts preferred for low friction, rectangular used for space constraints.
✅ Recommended velocities: Residential: 600–900 fpm (3–4.5 m/s), Commercial: 900–1300 fpm (4.5–6.5 m/s).
🧠 Pro expert tips
✔️ Oversizing duct by 10–15% reduces noise and future expansion margin.
✔️ For long duct runs (>50ft), lower friction target (0.05–0.08 in wg/100ft) to reduce static pressure.
✔️ Smooth interior (galvanized steel, spiral) decreases friction compared to flex duct (15-30% penalty).
✔️ Always verify velocity: exceeding 2000 fpm can cause audible turbulence and erosion.
✔️ Use duct sealants & insulation in unconditioned spaces (energy efficiency).
✔️ For long duct runs (>50ft), lower friction target (0.05–0.08 in wg/100ft) to reduce static pressure.
✔️ Smooth interior (galvanized steel, spiral) decreases friction compared to flex duct (15-30% penalty).
✔️ Always verify velocity: exceeding 2000 fpm can cause audible turbulence and erosion.
✔️ Use duct sealants & insulation in unconditioned spaces (energy efficiency).
⚙️ Why model year matters
Modern fan systems (2024+) often feature EC motors with higher static capabilities, allowing slightly higher friction design (0.12–0.15). Older systems (pre-2010) benefit from conservative duct sizing. Use this field for project reference.