Free Tool

Duct Velocity Calculator

Calculate the duct velocity and friction loss for your fume cupboard extraction system. Enter either the air volume directly or calculate it from sash dimensions and face velocity.

Fume Cupboard Duct Velocity Calculator

Select Input Type:

Air Volume (m³/s):

Duct Diameter (mm):

Duct Velocity: —

Friction Loss: —

Loss per 90° Bend: —

Loss per 45° Bend: —

How the calculations work

Duct Velocity

Velocity is calculated from the continuity equation — the air volume flow rate divided by the duct cross-sectional area:

v = \frac{Q}{A}

$v$ = duct velocity (m/s)
$Q$ = volume flow rate (m³/s)
$A$ = duct cross-sectional area (m²), where $A = π {(\frac{d}{2})}^{2}$

For sash or area inputs, $Q$ is first derived as: $Q = A_{face} \times v_{face}$

BS EN 14175 recommends a duct velocity of 3 – 7.5 m/s for laboratory fume cupboard extract systems.

Duct Friction Loss

Friction loss per metre of straight duct is calculated using the Darcy–Weisbach equation:

\frac{Δ P}{L} = \frac{f \cdot ρ \cdot v^{2}}{2 \cdot d}

$Δ P / L$ = friction pressure loss per metre (Pa/m)
$f$ = Darcy friction factor — 0.02 (smooth PVC duct)
$ρ$ = air density — 1.2 kg/m³ (at ~20°C)
$v$ = duct velocity (m/s)
$d$ = duct internal diameter (m)

Bend Losses (Smooth PVC)

Pressure loss through each bend is calculated using the loss coefficient method applied to the dynamic pressure:

{Δ P}_{bend} = ζ \cdot \frac{ρ \cdot v^{2}}{2}

$ζ$ = loss coefficient — 0.11 for 90°, 0.06 for 45° (smooth PVC, $R / d \approx 1.5$ , per CIBSE Guide C)
$\frac{ρ \cdot v^{2}}{2}$ = dynamic pressure (Pa)

Multiply the per-bend result by the number of bends of each type in your system to find total bend losses.

Values assume standard air at 20°C and 1 atm. Friction factor applies to smooth-bore PVC ductwork. For other materials or temperatures, adjust accordingly.

Recommended Duct Velocities

According to BS EN 14175, the recommended duct velocity range for fume cupboard extraction systems is:

3 - 7.5 m/s

Optimal internal duct velocity

This ensures sufficient transport of contaminants whilst maintaining manageable noise levels and pressure losses.

How to Use

1 Select your input type - either direct air volume or sash dimensions
2 Enter your values (air volume in m³/s, or sash width/height in mm)
3 Enter the duct diameter in mm
4 Click Calculate to see the duct velocity and friction loss

Need Help?

Our team can help you design the optimal extraction system for your fume cupboard installation.

Contact our experts

Contact

We'd Love to Hear From You

Have questions about our fume cupboards, LEV systems, or need a service quote? Our team in North Yorkshire is here to assist you.

Email Address

sales@fumecupboards-direct.com

Phone No.

01423 781101

Office Address

Unit 2 (The Ivy)
New York Industrial Estate
Summerbridge, Harrogate
HG3 4LA

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