Bathroom Exhaust Fan Sizing Guide: What Size Do You Actually Need?

A standard bathroom without a window needs an exhaust fan rated at least 25 L/s, the floor set by the NCC through AS 1668.2. Bathrooms with a shower, bath, or limited natural airflow generally need more, because steam and humidity build up faster than a code-minimum fan can clear it. The right number depends on the room's volume and how it's used, not just its floor area.

• A small ensuite or powder room with no shower usually gets by on the 25 L/s minimum.
• A bathroom with a shower recess at a standard ceiling height performs better with something closer to 40 to 50 L/s.
• A larger bathroom over 10m², or one with a spa bath or steam shower, generally needs 50 L/s or more, worked out from the room's volume (see below).

Trade tip:any 240V hardwired exhaust fan needs a licensed electrician for the wiring, even though the fan unit itself is bought separately. Confirm the model's L/s rating on the box or spec sheet before installation, because upsizing the duct later costs more than specifying it correctly the first time.

The NCC and AS 1668.2 set Australian bathroom ventilation requirements in litres per second, while CFM (cubic feet per minute) is the imperial unit used in US building codes. Most bathroom exhaust fan sizing guides that turn up in a search are written for the American market, so they quote CFM figures that don't line up with an Australian product listing. If a fan's spec sheet only lists CFM, converting it is straightforward.

• 25 L/s (the NCC bathroom minimum) is roughly 53 CFM.
• 50 L/s is roughly 106 CFM.
• 100 L/s is roughly 212 CFM.
• To convert either way, divide CFM by 2.12 to get L/s, or multiply L/s by 2.12 to get CFM.

Some Australian fans list m³/hr on the box instead of L/s. Divide that figure by 3.6 to get L/s for a like-for-like comparison against the NCC minimum.

Airflow for a bathroom comes from the room's volume in cubic metres, multiplied by the air changes per hour appropriate to how the room is used, then converted to L/s. This gives a room-specific figure rather than a generic size chart, because a 3m x 2.5m ensuite doesn't need the same fan as a 2m x 1.5m powder room. The calculation takes four steps.

• Measure the room's volume by multiplying length, width, and height in metres (a 3m x 2.5m bathroom with a 2.4m ceiling comes to 18m³).
• Choose an air changes per hour (ACH) figure, around 15 ACH for a bathroom without a shower, or 20 to 25 ACH for one with a shower or bath.
• Multiply volume by ACH to get cubic metres per hour (18m³ x 15 ACH = 270m³/hr).
• Divide by 3.6 to convert to L/s (270 ÷ 3.6 = 75 L/s).

Compare that figure against the NCC's 25 L/s floor and choose whichever is higher for the room in question. In the example above, 75 L/s clears steam from daily showers comfortably, well above the regulatory minimum.

An undersized exhaust fan leaves visible condensation on mirrors, tiles, and ceilings well after a shower has finished, and that trapped moisture is what leads to mould in grout lines and around ceiling fixtures over time. Beyond the mould risk, a fan rated below the NCC's 25 L/s minimum can also fail a compliance check during a renovation inspection or building certification. Both problems are cheaper to prevent at the sizing stage than to fix after installation.

• Condensation still visible on the mirror more than 10 minutes after a shower is a sign the fan is undersized or the duct run is restricting airflow.
• A musty smell or dark spotting in grout lines and around the exhaust vent itself points to moisture the fan isn't clearing fast enough.
• Paint bubbling or peeling on the bathroom ceiling, especially directly above the shower, is a longer-term sign of chronic humidity the fan hasn't kept up with.