How to Choose a Bathroom Exhaust Fan: Size, Airflow & Ducting
Last updated June 2026. How to size and choose a bathroom or laundry exhaust fan in Australia — airflow, fan type and ducting. Installation must be carried out by a licensed electrician.
How do I size an exhaust fan?
Work out the room volume, then multiply by the air changes per hour (ACH) you want:
Airflow (m³/h) = Length × Width × Height (m) × Air changes per hour
Bathrooms with a shower need plenty of extraction — aim for 15+ ACH. For a typical 2.5 m × 3 m bathroom with a 2.4 m ceiling: 2.5 × 3 × 2.4 = 18 m³, × 15 = ~270 m³/h of airflow. Whatever the maths says, don't go below the National Construction Code minimum of 25 L/s (about 90 m³/h) for a bathroom.
| Room | Approx. volume | Target airflow (15 ACH) |
|---|---|---|
| Small powder room (2 × 2 × 2.4) | ~10 m³ | ~150 m³/h (min 90) |
| Typical bathroom (2.5 × 3 × 2.4) | ~18 m³ | ~270 m³/h |
| Large ensuite/bathroom (3 × 4 × 2.4) | ~29 m³ | ~435 m³/h |
Tip: choose the next size up to cover duct losses — every metre of duct cuts effective airflow by roughly 5%, and runs over 3 m can need 20–30% more capacity.
Ceiling, inline or wall fan — which type?
| Type | Best for | Notes |
|---|---|---|
| Ceiling (axial) | Standard bathrooms with short ducts | Easiest to fit; light/heat combos available |
| Inline (in-duct) | Long runs, multiple grilles, quiet rooms | Motor sits in the roof — quietest at the grille, strongest airflow |
| Wall / window | Rooms with no roof access | Vents straight through an external wall |
Provent RIL-150 Inline Duct Fan
Strong, quiet 150 mm inline fan — ideal for long ducts or a near-silent bathroom.
$199.50 View product →
Fantech DOM-150C Ceiling Exhaust Fan (150mm duct)
Flush square ceiling fan for a standard bathroom with a short duct run.
$91.80 View product →Why ducting makes or breaks it
- Always vent to outside — a roof cowl, eave or wall vent. Dumping moist air into the roof void causes condensation, mould and rusted fixings.
- Keep runs short and straight — bends and length kill airflow. Each metre costs ~5%; over 3 m, upsize the fan 20–30%.
- Match the duct to the fan — a 150 mm fan wants 150 mm duct, not a reducer.
- Use the right wall/roof termination — a proper wall tube or cowl with a backdraft flap.
Don't forget
If the fan sits over a shower it falls in a bathroom zone and needs a suitable IP rating (see our IP ratings guide). Consider a light/heat combo for small bathrooms, and a timer or humidity switch so the fan runs on after a shower to clear moisture properly.
Browse exhaust fans at ARCK
We stock ceiling, inline and wall fans plus ducting and accessories at trade prices — see the full exhaust fan range. Not sure on size? Call the trade counter with your room dimensions and we'll work out the airflow.
Frequently asked questions
What size exhaust fan do I need for a bathroom?
Multiply the room's length × width × height to get its volume in m³, then multiply by 15 air changes per hour. A typical 18 m³ bathroom needs around 270 m³/h, and never less than the NCC minimum of 25 L/s (90 m³/h).
Can I vent an exhaust fan into the roof space?
No. Venting into the roof dumps moist air where it causes condensation, mould and rust. Ducting must run to outside through a wall vent, eave or roof cowl.
Is an inline fan better than a ceiling fan?
For long duct runs, multiple grilles or a quiet room, yes — an inline fan sits in the roof so it's quieter at the grille and moves more air. For a standard bathroom with a short duct, a ceiling fan is simpler and cheaper.
Does a bathroom fan have to vent outside?
Yes. To control moisture and meet the NCC, the fan must discharge to outside air, not into the roof cavity or wall space.
Need a hand sizing it? Call the trade counter on (02) 9890 9693 or drop in to 589 Church St, North Parramatta. Order online for free Click & Collect, with metro delivery from $19.90 calculated at checkout.