Duct Size Calculator

Inputs

Sizing method

Airflow (Q)

Volumetric airflow through the duct.

m³/h

Target velocity (v)

Design velocity used for velocity sizing.

m/s

Rectangular aspect ratio (W/H)

Rectangular width divided by height. 2 means a duct twice as wide as it is high.

Duct length for pressure drop (L)

Straight duct length used to convert friction rate into pressure drop.

Duct roughness (ε)

Equivalent absolute roughness. Galvanized steel duct is often screened near 0.09 mm.

Air density (ρ)

Air density at the duct condition. Room-temperature sea-level air is about 1.2 kg/m³.

kg/m³

Default result

Round duct diameter(D)

252.3mm

Pass

area sized to 5.00 m/s

Also computed

Rectangular width(W)316.2mm

Rectangular height(H)158.1mm

Rect. equivalent round diameter(D_eq)240.9mm

Round duct velocity(v_r)5m/s

typical comfort-duct screening range

Rectangular duct velocity(v_rect)5m/s

typical comfort-duct screening range

Round friction rate(Δp/L)1.207Pa/m

moderate friction; check fan static pressure

Round-duct friction rate from Darcy-Weisbach. Smaller ducts raise both velocity and static-pressure loss quickly.

Method notes 3 notes

velocity sizing: round and rectangular options are calculated from the same airflow and roughness assumptions.
Pressure loss uses Darcy-Weisbach with hydraulic diameter, room-temperature air viscosity and a Swamee-Jain turbulent friction factor.
Final HVAC duct design should also check fittings, transitions, diffusers, balancing dampers, acoustics, leakage, insulation, code requirements and fan static pressure.