What Duct Size Do I Need?

Duct Size by Application

• 100mm (4"): Bathroom extract fans, en-suite, WC. Up to 15 l/s
• 125mm (5"): Larger bathrooms, utility rooms. Up to 25 l/s
• 150mm (6"): Kitchen extract, cooker hoods. Up to 40 l/s
• 200mm (8"): Commercial kitchens, large cooker hoods
• Flat channel (110×54mm): Equivalent to 100mm round, fits in floor/ceiling voids

Round vs Flat Ducting

• Round (rigid): Best airflow, lowest resistance. Use where space allows
• Flat channel: Fits in shallow ceiling voids and between joists. Higher resistance
• Flexible (concertina): Only for short connections. High resistance — avoid long runs

Maximum Duct Run Lengths

Fan performance drops with longer duct runs. As a general guide:

• 100mm rigid: Up to 6m with 2 bends
• 100mm flexible: Maximum 1.5m (connection only)
• 150mm rigid: Up to 10m with 2 bends
• Each 90° bend adds approximately 1.5m equivalent length
• Each 45° bend adds approximately 0.75m equivalent length

Building Regulations (Part F)

Approved Document F specifies minimum extract rates for domestic ventilation. The duct must be sized to deliver these rates with the chosen fan. Never reduce duct size below the fan spigot — this increases back-pressure and reduces airflow significantly.

Tips

Use rigid ducting wherever possible. Insulate ducting in cold spaces (lofts) to prevent condensation. Ensure external termination has a gravity flap or louvre to prevent draughts. Avoid running ducting downhill as condensation will pool.