Steamy mirror twenty minutes after the shower? Paint starting to lift, or mould creeping along the ceiling? It usually comes down to one of two things: the fan up there is too small for the room, or it's fighting the ducting it's connected to. Either way, you can work out what you actually need with a tape measure and about a minute of maths.

Short version: measure the room, multiply it out, then look for a fan with enough airflow for the space, the steam you actually make and the ducting it has to push through. IXL's Australian rule of thumb is a fan that can change the room's air 15 to 20 times an hour. Where people come unstuck is assuming the number on the box is the number they'll get once it's connected to real ducting, bends and a grille on an outside wall. We'll get to that too.

Bathroom exhaust fan airflow formula

Exhaust fan airflow is measured in cubic metres per hour, or m³/h. Two lines of maths and you'll have your number:

Bathroom volume (m³) = length × width × ceiling height

Suggested fan capacity (m³/h) = bathroom volume × 15 to 20 air changes per hour

The 15 to 20 air changes per hour figure comes from IXL's Australian ventilation checklist. Treat the answer as a guide for choosing a product. It isn't a legal minimum, and it won't tell you what the fan delivers once it's in the ceiling. Your layout and your ducting decide that part.

How to measure your bathroom

Grab a tape measure and write down three numbers in metres:

• Length: from one wall to the opposite wall.
• Width: across the other side of the room.
• Ceiling height: from the finished floor to the ceiling.

Multiply all three for the room volume. If the bathroom opens straight into another space, like a walk-in robe or a bedroom, count that space in as well. IXL says to include any room that flows directly into the bathroom when you work out the extraction needed.

Worked example

Say your bathroom is 2.4m long, 2.0m wide and 2.4m high, with an enclosed shower and no doorway straight into another room.

Step 1: calculate the room volume

2.4 × 2.0 × 2.4 = 11.52m³

Step 2: calculate the airflow range

11.52 × 15 = 172.8m³/h

11.52 × 20 = 230.4m³/h

So you're after roughly 173 to 230m³/h. Don't round a fan rating down to make it fit. A published rating of 230m³/h or more covers the calculation, and what you get in the room also comes down to the duct route, the outside outlet and where the replacement air enters. That's the next section.

Quick bathroom airflow guide

Same 15 to 20 air changes per hour method, run over some common bathroom sizes. Use them to sense-check your own numbers, not to skip the tape measure.

Got a high ceiling? Use the real number. A 2.7m ceiling gives the fan noticeably more air to shift than a 2.4m ceiling over the same floor area.

Why the number on the box is only the starting point

Enclosed and open showers

Steam escapes into the room far more freely from an open shower than an enclosed one, so IXL recommends extra ventilation, including a second fan above the shower area with the right IP rating. Not every bathroom fan is rated to sit above a shower, and both the IP rating and the exact position are your licensed electrician's call.

Duct length and bends

The fan has to push air along the whole duct route. Long runs, tight bends, crushed flexible duct and sagging sections all add resistance. We can't hand you a percentage to subtract per bend or per metre, because the loss comes from the whole system, not the individual parts.

Keep the route as short and straight as the building allows. Where a fan has a published performance curve, that's the honest way to see how it holds up against resistance.

Duct diameter

The duct has to suit both the fan outlet and the air you're moving. Squeezing a 150mm outlet into a smaller existing duct adds resistance, and it's a common reason a perfectly good fan underperforms. Check the full route, not just the join beside the fan.

Exterior grilles and vents

The outside outlet matters just as much. A restrictive grille, a stuck flap or a blocked vent all work against the fan. It needs to suit the airflow and be fitted so the damp air genuinely gets outdoors.

Bathroom exhaust has to discharge outdoors, either directly or through a shaft or duct. That's clause 10.8.2 under the NCC 2022 Housing Provisions Deemed-to-Satisfy pathway. Which NCC edition, building class and compliance pathway apply to your job depends on the building and where it is.

Air needs a way into the room

A fan pulls air out, so the same amount of air has to find its way back in. Usually that's an open window or the gap under the door, which is what IXL suggests. Seal a room up tight and even a powerful fan struggles, so it's worth knowing where the replacement air comes from before you blame the fan.

Comparing noise, honestly

Putting the fan in an ensuite off your bedroom? Then noise probably matters to you more than airflow does. Do check the published sound figure, just don't treat a dB or dB(A) number as a straight comparison between brands. A fair comparison needs the same test method, the same distance and the same conditions, and they don't all test the same way. Duct design, vibration and how the fan is mounted all change what you actually hear at 6am. If noise is top of your list, ask us and we'll chase up comparable figures.

Which Alstra fan matches your number?

Once you have a target number, look for a published airflow rating that meets or beats it. Here's where our current range sits. Specifications checked 10 September 2026, from Atom's current Australian product information. Specs and stock do move, so have a quick look at the product page before you order.

A note on the numbers. These are manufacturer ratings, not our own side-by-side testing, and they're free-air figures, measured with nothing attached to the outlet. You won't see the same number at the end of an installed duct run. If your calculation lands right on a product's limit, or the route is long or awkward, step up a size rather than cutting it fine.

You can also browse our full range of bathroom exhaust fans, exhaust fans with lights and bathroom heaters.

Your before-you-order checklist

Replacing an existing fan? A fascia that looks about the same size doesn't make the new one a straight swap. Run through this before you buy:

• Ceiling opening: measure the actual cutout behind the fascia, not the grille you can see. Check the shape as well as the size.
• Ceiling space: check the clearances, any insulation restrictions, and whether someone can get up there to install and service it.
• Ducting: confirm the diameter, the route, the bends, and where it discharges outside.
• Controls: settle the switching, any required light interlock and the run-on timer before you buy.
• Replacement air: work out how air gets back into the room while the fan pulls it out. A sealed bathroom starves the fan.
• Electrical work: a licensed electrician does the wiring, always. A plug on the end of the cord doesn't make it a DIY job.

Frequently asked questions

What size exhaust fan do I need for my bathroom?

Multiply length by width by ceiling height, then multiply that by 15 to 20 air changes per hour. That's your starting range. From there, factor in the shower type, the duct route, the outlet and where replacement air comes in. Fascia size on its own tells you nothing about airflow.

Is a bigger exhaust fan always better?

No. More airflow helps in a bigger or steamier bathroom, but the fan still has to suit the duct, the ceiling opening and the room. Bigger often means a different cut-out, a larger duct and more noise.

Do open showers need more airflow?

Usually, yes. Moisture spreads more freely through the room, so IXL recommends extra ventilation for an open shower, including a suitable IP-rated fan above the shower area.

How much airflow do I lose through ducting?

There's no honest single percentage. It depends on the duct type, diameter, length and bends, how tightly the flexible duct is pulled, and the exterior grille. Where a manufacturer publishes a performance curve, that's the reliable way to read it.

Why is my bathroom still steamy with the fan on?

An undersized fan is the obvious suspect, and often not the guilty one. Look for crushed or blocked ducting, a restrictive outside vent, a grille thick with dust, or nowhere for replacement air to come in. Then check it's actually running during the shower and for a while afterwards, not just while someone's in the room.

Does a bathroom exhaust fan have to vent outside?

Yes. Vent it outdoors, not into the ceiling cavity, where damp air just moves the problem somewhere more expensive to fix. NCC 2022 Housing Provisions clause 10.8.2 requires outdoor discharge under its Deemed-to-Satisfy pathway, though the edition and pathway that apply depend on your project.

How long should I run the exhaust fan after a shower?

Run it during the shower and keep it going until the steam has cleared. There's no perfect number of minutes for every bathroom. One rule is worth knowing: where NCC 2022 Housing Provisions 10.8.2(4) applies, a non-continuous fan in a bathroom without the specified natural ventilation has to start with the room light and run for 10 minutes after that light goes off. A timer or a run-on model does that for you.

Not sure which one? Talk to us.

Give our Thornton team a call with your room measurements, your current model number, the ceiling cutout size and whatever you know about the duct route. We'll help you narrow down the options before you book the installation.