What air velocity should I design for in domestic ductwork?

For domestic ventilation ductwork, design velocities of 3-5 m/s are recommended. Below 3 m/s, ducts become oversized. Above 5 m/s, noise becomes unacceptable in living spaces. For main runs through non-habitable spaces (lofts, voids), up to 5 m/s is acceptable. For branches serving bedrooms and living rooms, 3-4 m/s keeps noise levels low. MVHR manufacturers typically specify maximum duct velocities for their systems.

Should I use rigid or flexible duct?

Rigid duct (galvanised steel or plastic) has much lower pressure drop than flexible duct — typically 3-5 times lower. Rigid duct should be used for main runs wherever possible. Flexible duct is acceptable for short final connections (under 1.5m) to terminals and diffusers, but should be pulled taut with no kinks or tight bends. Crushed or kinked flexible duct can reduce airflow by 50% or more. For MVHR systems, semi-rigid plastic duct (75mm or 90mm) is increasingly popular.

What size duct do I need for a bathroom fan?

A typical bathroom extract fan at 15 l/s requires a minimum 100mm (4 inch) diameter duct. A 150mm (6 inch) duct gives lower pressure drop and quieter operation for the same flow rate. For long duct runs (over 3m), in-line fans with 150mm duct are preferable. Never reduce the duct size below the fan spigot diameter — this increases back pressure and reduces airflow. Keep runs as short and straight as possible.

How do I convert between round and rectangular duct sizes?

Equivalent rectangular duct sizes carry the same flow at the same pressure drop as the round duct. For example, a 100mm round duct (7,854 mm² area) is approximately equivalent to a 110 x 54mm flat duct. Flat channel duct is useful in confined spaces like floor voids and ceiling cavities. Use the manufacturer's equivalent area data rather than simple area calculation, as the shape affects friction differently.

Duct Sizing Tables — Domestic Ventilation

Duct sizes for domestic ventilation at recommended velocities. Round and rectangular equivalents, pressure drop per metre, flexible vs rigid duct comparison.

Round Duct — Flow Rates by Velocity

Flow rate Q = Area × Velocity. Area = pi × (d/2)².

Duct Diameter	Area (mm²)	At 3 m/s (l/s)	At 4 m/s (l/s)	At 5 m/s (l/s)
75mm	4,418	13.3	17.7	22.1
100mm (4")	7,854	23.6	31.4	39.3
125mm (5")	12,272	36.8	49.1	61.4
150mm (6")	17,671	53.0	70.7	88.4
200mm (8")	31,416	94.2	125.7	157.1

Round to Rectangular Duct Equivalents

Common flat channel duct equivalents for domestic installations.

Round Diameter	Equivalent Flat Channel	Flat Channel Area	Typical Use
100mm	110 x 54mm	5,940 mm²	Bathroom extract
125mm	204 x 60mm	12,240 mm²	Kitchen extract
150mm	220 x 90mm	19,800 mm²	Cooker hood, main runs

Pressure Drop Comparison

Approximate pressure drop per metre of straight duct at typical domestic velocities.

Duct Type	100mm at 4 m/s	150mm at 4 m/s	Notes
Galvanised steel (rigid)	~1.0 Pa/m	~0.5 Pa/m	Best performance
Rigid plastic (PVC)	~0.8 Pa/m	~0.4 Pa/m	Smooth bore, good performance
Semi-rigid plastic	~1.5 Pa/m	~0.7 Pa/m	Common for MVHR branches
Flexible aluminium duct	~3-5 Pa/m	~2-3 Pa/m	3-5x higher than rigid — minimise use

Fitting Equivalent Lengths

Add these equivalent lengths to the straight duct run for total effective length.

Fitting	Equivalent Length (m)
90° bend (rigid)	1.0 – 1.5
90° bend (flexible)	2.0 – 3.0
45° bend	0.5 – 0.7
Tee junction	2.0 – 3.0
Grille/louvre terminal	2.0 – 4.0
In-line backdraught shutter	1.0 – 2.0

Note: Values are approximate guidance. Always refer to specific duct and fan manufacturer data for system design. MVHR systems require balanced supply and extract flows — commissioning with measurement at every terminal is essential.

See the Ventilation Rates by Room, the Part F Ventilation Rates, and the Flow Rate Conversions.