HVAC Duct Size Calculator

HVAC Professional Duct Size Calculator

© 2025 Md. Sofiqur Rahman — All Rights Reserved. Engineered for precise airflow & duct dimensioning.

🌬️ Airflow (CFM)

Typical range: 100 – 2000 CFM

⚡ Friction Loss (per 100ft)

Recommended: 0.06 – 0.15 in wg/100ft (residential/commercial)

📐 Duct Shape

🔴 Round

🟧 Rectangular

📏 Round Duct Diameter (inches) — Suggested

Optimal diameter based on friction & airflow

↔️ Rectangular Width (in)

↕️ Rectangular Height (in)

Enter dimensions to verify velocity or get equivalent round

📊 Velocity Constraint (optional FPM)

Velocity check for noise & pressure drop

📌 DESIGN PARAMETERS

400 CFM

Friction: 0.08 in wg/100ft

🌀 ROUND DUCT (Optimal)

— inch diameter

Velocity: — FPM

📦 RECTANGULAR DUCT (Specified / Equivalent)

— x — inch

Velocity: — FPM

✅ VELOCITY & CODE CHECK

✅ Within recommended range

📐 DUCT AREA

— sq.in

Equivalent round diameter (if rect): — in

💡 HVAC Engineering Note: Calculation based on Darcy-Weisbach / Altshul-Tsal friction approximation and standard duct sizing principles. Round sizes are optimized for minimal friction loss at given CFM. For rectangular ducts, equivalent diameter is computed from actual dimensions (aspect ratio correction). Velocity should ideally stay between 500–1000 FPM for low noise. © Md. Sofiqur Rahman – professional tool.

🔧 HVAC Duct Sizing Calculator — Developed by Md. Sofiqur Rahman | Trusted method: ASHRAE fundamentals | For estimation use only, always verify local codes.