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Duct Size Calculator

Calculate round & rectangular HVAC duct dimensions using the Equal Friction Method — the ASHRAE industry standard. Instant results, zero registration.

✓ ASHRAE compliant ✓ Imperial & Metric ✓ Round & Rectangular

Input Parameters

Imperial (CFM · in.) Metric (L/s · mm)

Air Flow Rate * The volume of air the duct must carry

CFM

Design Method * Equal friction is the industry standard; velocity is used for low-noise design

Friction Loss Target Velocity

Friction Loss Typical: 0.08 – 0.12 in.wg/100ft (0.65 – 1.0 Pa/m). Lower = larger duct.

in.wg/100ft

Application Preset friction / velocity limits

Max Aspect Ratio (Rectangular) SMACNA recommends ≤ 4:1. Higher = harder to fabricate.

Method: Equal Friction (ASHRAE, SMACNA) · Standard air (0.075 lb/ft³, 20°C)

Ready to size your duct?

Enter airflow (CFM) and press Calculate — you'll get both round & rectangular options.

How does the Equal Friction Method work?

The Equal Friction Method sizes all ducts in a system to have the same friction loss per unit length (typically 0.08–0.10 in.wg per 100 ft). This ensures a self-balancing system with predictable pressure drops.

The friction loss for round duct is calculated using the ASHRAE simplified equation:

Δp = 0.109136 × Q^1.9 / D^5.02

Where Δp = friction (in.wg/100 ft), Q = airflow (CFM), D = diameter (in).

Recommended Velocity Ranges (Supply Air)

Application	Main Duct	Branch Duct
Residential	700–900 fpm	600 fpm
Commercial (offices)	1200–1500 fpm	800–1000 fpm
Public / Retail	1500–1800 fpm	1000–1300 fpm
Industrial	1800–2500 fpm	1500–2000 fpm
Hospital / Studio	800–1200 fpm	600–800 fpm

Round vs Rectangular — Which to choose?

Round ducts are more efficient — up to 30% less friction loss for the same airflow. They're preferred where space allows.

Rectangular ducts fit better in tight ceiling spaces and are easier to seal at joints. Aspect ratios above 4:1 are discouraged because they lose efficiency and are harder to fabricate rigidly.

Castle Engineering fabricates both to SMACNA and DW144 standards.

Disclaimer

This tool provides estimates based on standard air properties (0.075 lb/ft³ at 20°C, sea level) and clean galvanized steel ductwork. For non-standard air (high temperature, altitude, humidity), duct roughness other than galvanized steel, or complex systems requiring detailed calculation, please consult a licensed HVAC engineer.