What Size Exhaust Fan Do I Need? Bathroom Ventilation Sizing Guide
An undersized exhaust fan does not fail loudly. The room simply stays damp a bit longer than it should, the mirror takes an extra few minutes to clear, and eighteen months later there is black spotting in the grout and along the ceiling cornice. By then it reads like a cleaning problem rather than a ventilation one.
This guide gives you the sizing calculation, the figures the National Construction Code actually requires, and the three installation details that decide whether you get the airflow you paid for.
The calculation
Air extraction is rated in cubic metres per hour (m³/hr). You want the fan to be capable of replacing the air in the room about ten times an hour — ten air changes per hour, or ACH.
Length (m) × width (m) × height (m) × 10 = minimum m³/hr
A bathroom 2.5 m long, 2.0 m wide with a 2.4 m ceiling is 12 m³, so 120 m³/hr is your floor. In practice, buy well above the floor. Fans are rated in free air; your fan will be connected to several metres of flexible duct and a grille, and the difference between the rated figure and what actually leaves the building is substantial.
Tip: If you have a separate toilet cubicle, a spa bath, or a shower with a door that traps steam, treat the room as one size larger than the tape measure says. Trapped steam takes longer to reach the fan.
Sizing by room
| Room | Typical volume | NCC minimum | What we'd fit |
|---|---|---|---|
| Powder room / WC | 4–6 m³ | 90 m³/hr | 180–250 m³/hr |
| Small ensuite | 6–9 m³ | 90 m³/hr | 250–320 m³/hr |
| Standard family bathroom | 10–14 m³ | 120–140 m³/hr | 320–450 m³/hr |
| Large bathroom | 15–22 m³ | 150–220 m³/hr | 440–600 m³/hr |
| Laundry | 8–15 m³ | — | 250–450 m³/hr |
| Bathroom + separate WC on one fan | 18–26 m³ | 180–260 m³/hr | 500–600 m³/hr |
Two rooms on one fan is a compromise worth avoiding where you can. A single fan ducted to serve both a bathroom and a separate toilet will always favour whichever room it sits closest to, and the further room gets a fraction of the rated flow.
What the National Construction Code actually requires
Australian requirements for a mechanically ventilated sanitary compartment, bathroom or laundry are set out in the NCC, which references AS 1668.2. The headline numbers most people care about:
- Bathroom or sanitary compartment: 25 L/s — that is 90 m³/hr
- Laundry: 20 L/s — that is 72 m³/hr
- Kitchen (range hood, domestic): 40 L/s where not directly exhausted over the cooktop
To convert between the two units you will see quoted: 1 L/s = 3.6 m³/hr. Manufacturers of ducted domestic fans almost always quote m³/hr; mechanical services drawings almost always quote L/s.
Note: These are minimum compliance figures for a new build or a certified renovation, and they assume the exhaust discharges to outside air. They are not performance targets. A 90 m³/hr fan will technically satisfy the code in a 12 m³ bathroom and still leave that bathroom damp.
The other requirement people miss: the exhaust must discharge outside the building, not into the roof space. Dumping a bathroom's worth of moisture into the roof cavity every morning is how you end up with damp insulation, corroded fixings and mould on the underside of the sarking.
Why your fan won't hit its rated figure
The m³/hr on the box is measured with the fan running free. Once it is connected to a duct system, every metre and every bend adds static pressure the motor has to work against. Rules of thumb from installation practice:
- Flexible duct costs you significantly more airflow than rigid duct of the same diameter — the corrugations create turbulence along the whole run.
- Every 90° bend is worth roughly several metres of straight duct in pressure loss.
- A duct left slack and sagging between joists behaves like a much longer duct, and collects condensate in the low points.
- Reducing the duct diameter below the fan's outlet — a 150 mm outlet squeezed into a 100 mm duct — throttles the fan badly.
Practical version: run the shortest, straightest, largest-diameter duct you can, pull it taut, and buy a fan rated 1.5–2× your calculated minimum so the losses come out of the margin rather than out of your ventilation.
Mounting position
Mount the fan between the shower and the door, not directly above the shower rose. Directly overhead, the fan captures the plume before it circulates and leaves the rest of the room untouched — and in a cold snap the fan body itself becomes the coldest surface in the room and drips. Give makeup air a path in, too: a 10–20 mm undercut on the door is usually all it takes. A perfectly sealed room starves the fan.
Noise, and why dB matters more than you think
A bathroom is a small, hard, tiled box — every surface reflects. A fan that measures a reasonable 50 dB in a lab feels considerably louder in a 12 m³ room with no soft furnishings. That is why so many well-specified fans get switched off after two minutes, which defeats the entire point.
- ≤40 dB — quiet enough that you will leave it running. Worth paying for.
- ≤45 dB — noticeable but unobtrusive; the sweet spot for most of this range.
- ≤50 dB — present. Fine in a laundry, less so in an ensuite off a bedroom.
DC motors are the main reason the quieter units are quieter. They also let the fan run at a genuine low speed for continuous background extraction, rather than only having "on".
Choosing from the range
If you need a fan only
The Saturn series is the straightforward ceiling exhaust: a turbo cylinder blade, a ball-bearing motor, a fitted back-draft stopper and an IPX4 rating, supplied with a 900 mm flex and plug.
- Saturn 240 Round and Saturn 240 Square — 320 m³/hr, 25W, ≤45 dB, 240 mm cut-out, 100 mm duct — $58
- Saturn 295 Round and Saturn 295 Square — 443 m³/hr, 28W, ≤45 dB, 295 mm cut-out, 150 mm duct — $68
The jump from the 240 to the 295 is $10 for an extra 123 m³/hr. There are very few rooms where the 240 is the right answer once you know that.
If you want a fan and a light in one
- Saturn 240 Round with LED light — 180 m³/hr with a 17W switchable-CCT LED up to 1,200 lm, ≤45 dB, IPX4 — $97
- Saturn 295 Round with tri-colour LED — 320 m³/hr, 28W motor, 17W LED at 1,200–1,400 lm, 150 mm side duct — $130
Watch out: Adding a light costs airflow. The Saturn 240 drops from 320 m³/hr to 180 m³/hr once the LED panel is in the same body. If extraction is your priority, a separate fan and a separate light will always out-perform a combined unit of the same diameter.
If you want the low-profile square or round look
The Flow series uses a turbo fan drum blade with a swivel quick-mount base, a polypropylene housing with ABS fascia, side duct access and a draft stopper — designed for apartments and tight ceiling spaces.
- Flow Round 250 mm — $113 · with light — $135.99
- Flow Round 300 mm — $125 · with light — $149.50
- Flow Square 250 mm — $96.10 · with light — $137.49
- Flow Square 300 mm — $140 · with light — $168.99
If you also want heating
A 3-in-1 combines all three functions in one cut-out and one set of switches. Extraction across that range runs from 240 m³/hr to 600 m³/hr — see our guide to the best 3-in-1 bathroom heaters for the model-by-model breakdown, or browse bathroom heaters directly.
In Australia, hard-wired ceiling fittings, heaters and fans must be installed by a licensed electrician. Nothing on this page is a substitute for the installation instructions supplied with the product or for AS/NZS 3000.
Frequently asked questions
How many m³/hr do I need for a standard bathroom?
For a typical 12 m³ family bathroom (2.5 × 2.0 × 2.4 m), 120 m³/hr is the calculated minimum and 320–450 m³/hr is what will actually keep it dry once ducting losses are accounted for.
What does the NCC require for bathroom ventilation?
25 litres per second — 90 m³/hr — for a bathroom or sanitary compartment, and 20 L/s (72 m³/hr) for a laundry, discharged to outside air. These are minimum compliance figures, not performance targets.
How do I convert L/s to m³/hr?
Multiply litres per second by 3.6. So 25 L/s = 90 m³/hr, and 100 m³/hr ≈ 27.8 L/s.
Can I vent an exhaust fan into the roof space?
No. It has to discharge outside the building. Venting into the roof cavity relocates the moisture into your insulation, framing and sarking, and it is not compliant.
Is a bigger exhaust fan noisier?
Not necessarily — motor type matters more than capacity. A DC-motor unit moving 600 m³/hr can run at ≤40 dB, while a smaller AC unit moving 300 m³/hr may sit at ≤50 dB. Check the dB rating rather than assuming from the airflow figure.
Do I need a separate fan if I already have a 3-in-1?
Usually not, provided the 3-in-1's extraction matches your room volume. In a large bathroom with a separate WC, a second dedicated fan in the toilet is often better than trying to serve both from one unit.
Keep reading
Browse the full exhaust & bathroom fans range, or the bathroom heaters range if you want heat in the same fitting.