Duct Sizer

Airflow (CFM)

Sizing Method

Friction Rate (in. w.g. / 100 ft)

Rectangular Constraint (optional)

Max Width (in)

Max Height (in)

Set one — the other calculates automatically

Industry Velocity & Pressure Guide

ApplicationVelocity (FPM)Pressure

Low Pressure Mains800–1200½" – 1" w.g.

Low Pressure Branches600–1000½" – 1" w.g.

Branch to Diffuser/Grille400–600½" w.g.

Medium Pressure Mains1500–25002" – 3" w.g.

High Pressure Mains2500–35003" – 6" w.g.

VAV Upstream of Box1500–25002" w.g. min

Kitchen Exhaust (Grease)1500–25001" – 2" w.g.

Toilet / General Exhaust1000–1500½" – 1" w.g.

Based on industry practice & SMACNA HVAC Duct Construction Standards

Pressure ClassOperating Pressure

½" w.g.Up to ½" w.g.

1" w.g.Over ½" up to 1"

2" w.g.Over 1" up to 2"

3" w.g.Over 2" up to 3"

4" w.g.Over 3" up to 4"

6" w.g.Over 4" up to 6"

10" w.g.Over 6" up to 10"

Source: SMACNA HVAC Duct Construction Standards, 4th Edition