An undersized bathroom exhaust fan is one of the most common, least understood causes of recurring mould in the Illawarra. The fan runs, it sounds like it’s working, and the ceiling still grows mould every winter regardless. The reason is usually straightforward: the fan simply isn’t rated to move enough air for the size of the room, measured in litres per second (L/s), and no amount of running time fixes a fan that was undersized from the start. This guide explains the standard sizing calculation, why it matters more than most people realise, and how to check whether your own fan is actually up to the job.
Why Litres Per Second, Not Just “Has a Fan”
Exhaust fans are rated by airflow, how much air they move per second, expressed in litres per second (L/s) in Australian product specifications, though some fans (especially older or imported units) are still labelled in cubic feet per minute (CFM) or cubic metres per hour (m³/h). A fan’s airflow rating tells you what it’s capable of moving under ideal conditions; whether that’s actually enough for your bathroom depends entirely on the size of the room it’s clearing.
A fan rated at 15 L/s might be perfectly adequate for a small ensuite and hopelessly undersized for a larger family bathroom with a bath, a shower and a window that rarely opens. This is why “does the bathroom have an exhaust fan” is the wrong question. The right question is whether the fan installed is rated for the volume of that specific room, and that’s what a sizing calculation answers.
The Standard Calculation Method
The building industry standard approach, used in Australian wet-area ventilation design, calculates the minimum required airflow based on the volume of the room and a target number of air changes per hour, then converts that figure into litres per second.
The calculation runs in three steps:
- Work out the room volume. Multiply length x width x ceiling height, in metres, to get the room volume in cubic metres (m³).
- Apply the target air change rate. Wet areas are generally designed around a minimum of roughly 10 to 15 air changes per hour, meaning the entire volume of air in the room should be replaced that many times every hour.
- Convert to litres per second. Multiply the room volume by the air change rate to get cubic metres per hour, then divide by 3.6 to convert to litres per second (since there are 3,600 seconds in an hour and 1 m³ = 1,000 litres).
As a worked example: a bathroom measuring 2.5m x 2m with a 2.4m ceiling has a volume of 12 m³ (2.5 x 2 x 2.4). At 10 air changes per hour, that’s 120 m³ of air to move every hour, which converts to roughly 33 L/s (120,000 litres ÷ 3,600 seconds). A fan rated below that figure is, on paper, undersized for that room, regardless of what the packaging calls it.
Why This Matters More Than People Expect
The gap between an undersized and correctly sized fan doesn’t just mean the room clears a bit slower. Below the required air change rate, humid air lingers in the room long enough to condense repeatedly on cold surfaces, mainly the ceiling and any external wall, night after night through winter. That repeated condensation cycle is exactly what establishes mould on plasterboard and paint film over a season, even in a bathroom that’s cleaned regularly. Running time doesn’t fully compensate for an airflow shortfall: a small fan left on for an hour still moves far less total air than a correctly sized fan run for fifteen minutes.
This is a genuinely common finding on our inspections: a functioning fan, correctly wired, making the expected noise, that’s simply rated below what the room needs. It’s one of the quieter causes of “the fan runs but the mould keeps coming back,” alongside a fan that’s ducted into the roof cavity instead of outside, which is a separate problem covered in our guide on ventilation upgrades to prevent mould.
How Do I Check My Own Fan Without a Calculator?
A rough practical version of the calculation, without doing the full arithmetic yourself:
- Measure your bathroom’s length, width and ceiling height in metres and multiply them together for the room volume in m³.
- Multiply that volume by roughly 10 to 15 (the target air changes per hour) to get an hourly airflow figure in m³/h.
- Divide by 3.6 to get the required L/s figure.
- Compare that number to the rating printed on your existing fan (usually on a sticker inside the fan housing, or in the original product documentation), converting from CFM if that’s what’s listed: 1 L/s is roughly equal to 2.12 CFM.
If your fan’s rating comes in well below the figure you calculated, particularly if it’s less than half, that’s a strong sign the fan is undersized for the room, independent of any ducting or installation issues.
What If the Fan Is Correctly Sized but Mould Still Returns?
A correctly sized fan is necessary but not always sufficient. Mould can keep returning even with adequate L/s capacity if the fan is ducted into the roof cavity rather than to the outside, if it’s switched off before the room has actually cleared (running time matters as much as capacity), or if the underlying moisture problem isn’t really about the bathroom at all but a subfloor, roof leak or a cold, poorly ventilated wall elsewhere in the home. Where general humidity across the house rather than one specific room is the driver, a dehumidifier can help manage the balance, though it’s a different tool solving a different part of the problem to correct exhaust sizing.
What Does Upgrading an Undersized Fan Involve and Cost?
Where an inspection or your own sizing check confirms the fan is undersized, upgrading it is electrical work and needs a licensed electrician, particularly if the new fan draws more power or needs a larger duct run to the outside. Indicative pricing for a straightforward like-for-like fan upgrade sits around $250-$800 installed, more if the ducting itself needs replacing or rerouting to terminate outside rather than into the roof cavity.
Where mould has already established on the ceiling or walls from years of an undersized fan, treating that existing growth is a separate step from the fan upgrade itself. Our mould fogging and antimicrobial treatment is one of the tools used to reach awkward geometry like cornices and extractor housings as part of that treatment, alongside surface cleaning and, where needed, plasterboard replacement.
| Job type | Indicative price range* |
|---|---|
| Fan sizing check as part of an inspection | Included in standard inspection, $300-$800 |
| Like-for-like fan upgrade, correctly sized and ducted outside (licensed electrician) | $250-$800 |
| Bathroom ceiling treatment for existing condensation mould | $500-$1,500 |
*Indicative and region-general only. Every job is confirmed after inspection or photo assessment with a formal quote. See our mould removal cost guide for pricing across every job type.
If your bathroom fan runs but mould keeps coming back, get a free quote with your bathroom’s rough dimensions and a photo of the ceiling, and we’ll give you an honest read on whether sizing, ducting, or something else entirely is the actual cause.
FAQs
What’s the minimum litres-per-second rating for a standard bathroom?
For a typical Australian family bathroom of around 10 to 15 m³, the calculation generally lands somewhere in the 25 to 40 L/s range at a 10 to 15 air change rate, though the exact figure depends on the specific room’s dimensions. Working it out for your own bathroom, using its actual volume, gives a far more reliable answer than relying on a generic minimum.
Is a higher L/s rating always better?
Not necessarily. Oversizing rarely causes a mould problem, but it does mean unnecessary noise and, on some models, higher running cost for little practical benefit beyond what the room’s volume actually needs. Matching the fan to the calculated requirement, rather than buying the largest available unit, is the more sensible approach.
My fan is rated in CFM, not L/s. How do I compare it?
Multiply the CFM figure by roughly 0.47 to convert to L/s (or divide L/s by roughly 2.12 to get CFM). Once both figures are in the same unit, you can compare the fan’s rating directly against your calculated requirement.
Does the ceiling height matter for the calculation?
Yes, significantly. Room volume is length x width x height, so a bathroom with a raked or higher-than-standard ceiling has a larger volume than the floor area alone would suggest, and needs a higher-rated fan to achieve the same air change rate as an equivalent room with a standard 2.4m ceiling.
Can I just upgrade the fan myself?
No. Installing or replacing an exhaust fan involves electrical wiring and is legally work for a licensed electrician in NSW. Ducting changes, particularly rerouting a duct to terminate outside rather than into the roof cavity, often need coordination with a licensed roofer as well.