An exhaust fan only works if it moves enough air for the room. Here’s a rough airflow guide by room, what affects sizing, and why an undersized fan leaves moisture and mould behind.
7 min read FreshDuct Melbourne Melbourne, Victoria
An exhaust fan only works if it moves enough air for the room. Here’s a rough airflow guide by room, what affects sizing, and why an undersized fan leaves moisture and mould behind.
Match the RoomAirflow to volume & load
Too Weak FailsWon’t clear moisture
Vent OutsideSizing only works if vented out
Exhaust fans should move enough air for the room and its moisture load — indicatively from around 25 L/s for a bathroom and 40 L/s for a kitchen, more for larger rooms. Correct sizing (and venting outside) is what actually keeps a room dry.
Why Sizing Matters
An exhaust fan has one job — move moist air out of the room — and it can only do that if it moves enough air for the space. A fan that’s too small for the room runs away merrily while the room stays humid, because it simply can’t shift the moisture fast enough. This is why a room can have a fan and still grow mould. Correct sizing — matching airflow to the room’s volume and moisture load — is the foundation of effective ventilation (see the guide above).
A Rough Sizing Guide
Exhaust fan capacity is measured by airflow, usually litres per second (L/s). As indicative starting points: a bathroom fan should move from around 25 L/s, a larger bathroom more; a kitchen needs more again, from around 40 L/s, because cooking produces more moisture and odour; and a laundry should be sized to its volume and how much drying happens there. These are starting points — the room’s specifics refine the right number.
What Affects Sizing
Beyond floor area, sizing accounts for the ceiling height (more volume to clear), the room’s moisture load (a busy family bathroom or a laundry where clothes dry indoors needs more), and the layout. A larger or higher room, or one producing more moisture, needs a fan with more airflow. Matching the fan to these factors — rather than grabbing the cheapest or smallest — is what keeps the room dry.
The Undersized-Fan Problem
The most common ventilation mistake is an undersized fan. It looks like the room is ventilated — there’s a fan, it runs — but it can’t move enough air to clear the moisture, so humidity lingers and mould grows anyway. People then blame the room or repeatedly clean the mould, when the real fix is a properly sized fan. If you have a fan but persistent moisture or mould, undersizing (or venting into the roof) is the likely culprit. See our mould guide.
Getting It Sized Right
We size exhaust fans to the room — its volume, moisture load and use — so the fan actually clears the moisture, and we vent it outside and can add timer or humidity control so it runs long enough. Correct sizing, proper venting and adequate run time together keep a room dry. Call 0431 918 137 or request a quote. See our installation cost guide.
Frequently Asked Questions
What size exhaust fan do I need for my bathroom?
It depends on the room’s size and how much moisture it produces, but as an indicative starting point a bathroom fan should move from around 25 litres per second, with larger bathrooms needing more. The key is matching the fan’s airflow to the room’s volume so it can clear the shower moisture in a reasonable time. An undersized fan, however quiet or cheap, won’t keep the room dry. We can size it properly for your bathroom.
How is exhaust fan size measured?
By airflow — how much air it moves, usually in litres per second (L/s) or cubic metres per hour. A fan’s rated airflow tells you whether it can clear a given room’s moisture. Matching that airflow to the room’s volume and moisture load is what proper sizing means. A higher number isn’t automatically better — it should suit the room — but an airflow that’s too low for the space is the common mistake.
What happens if my exhaust fan is too small?
An undersized fan can’t move enough air to clear the room’s moisture, so humidity lingers after showers or cooking and feeds mould and damp — the fan runs but the room stays wet. This is a common cause of persistent bathroom mould despite having a fan. The fix is a correctly sized fan (and venting it outside). Sizing up to the room’s actual need is what makes the difference. See our mould guide.
Does kitchen exhaust need to be bigger than bathroom?
Generally yes — kitchens produce more moisture, grease and odour from cooking, so kitchen extraction (a rangehood or kitchen exhaust) needs higher airflow than a bathroom fan, indicatively from around 40 L/s and often more for serious cooking. The right size depends on the kitchen and cooking type. See our kitchen rangehood guide.
Besides airflow, what else affects how well an exhaust fan works?
Two big things: the run time (the fan must run long enough to clear the moisture — a timer or humidity sensor helps), and venting outside (a correctly sized fan that vents into the roof just moves moisture into the roof space). So effective ventilation is the right airflow, run for long enough, vented outside. Get all three right and the room stays dry. See our timer & sensor guide.
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