How to Work Out What Size Exhaust Fan You Need

Exhaust fans are rated by how much air they move, in cubic metres per hour (m³/hr). To size one correctly you only need two numbers: your room's volume, and how many times per hour the air in that room should be completely replaced, known as the air changes per hour (ACH). Our calculator handles both, but here is the logic so you can sanity check the result.

1. Work out your room volume

Multiply length × width × ceiling height, in metres. A 4m × 3m bathroom with a 2.5m ceiling has a volume of 30 m³.

2. Multiply by the air changes per hour

Steamy or moist rooms need their air replaced more often than dry ones. A bathroom with a shower needs 15 to 25 air changes per hour, so for the room above: 30 m³ × 15 to 25 = a fan rated between 450 and 750 m³/hr. When in doubt, size up. An oversized fan clears steam faster and can always run on a lower speed, but an undersized fan can never catch up.

Some fans list airflow in litres per second (L/s) instead. Divide m³/hr by 3.6 to convert: 450 m³/hr is 125 L/s.

Recommended Air Changes Per Hour by Room

The damper the room, the more air changes it needs. Use this table as your guide:

Location	Air changes per hour
Bathroom (toilet only)	6 – 15
Bathroom with a shower	15 – 25
Spa bathroom	15 – 25
Bedroom	5 – 8
Domestic kitchen	15 – 25
Commercial kitchen	20 – 30
Laundry (no dryer)	6 – 15
Laundry (with dryer)	15 – 25
Commercial laundry	11 – 20
Garage	6 – 8
Office	4 – 6
Computer room	6 – 10
Factory / workshop	6 – 10
Cafe	15 – 25
Roof space	8 – 10
Sub floor	6 – 10

What Else to Consider When Choosing an Exhaust Fan

Noise

All fans make some noise, but the range is huge: from almost silent to intrusive. Fan noise is often listed in sones. A refrigerator is about 1 sone, and a good bathroom fan should be 1 sone or less (the quietest get down to around 0.3). In-line fans mounted in the ceiling cavity are typically quieter than fans mounted directly on the wall or ceiling.

IP rating

Fans in wet or dusty areas should carry an appropriate IP rating, a measure of protection against water and dust. For bathrooms and showers look for IPX4 or higher. Our IP rating guide explains what the numbers mean.

2-in-1 and 3-in-1 units

For bathrooms, consider a 2-in-1 (light + fan) or 3-in-1 (light + fan + heat) unit. They save space, run efficiently and are ideal for smaller rooms.

Ceiling clearance and ducting

Check your ceiling cavity depth before buying, as some fans need significant clearance. Wherever possible, duct the fan to outside air rather than venting into the roof space, so moisture actually leaves the building.

Run-on timers and humidity sensors

A run-on timer keeps the fan going for a few minutes after you switch off to clear lingering steam. Setting it for 3 to 5 minutes is usually enough. Humidity-sensing models switch themselves on automatically when moisture builds up.

Installation

In Australia, any 240V hardwired exhaust fan must be installed by a licensed electrician. Plan the position, wiring and duct run before cutting any holes, and check the new fan's requirements against your existing wiring if you are replacing an old unit.

Exhaust Fan Calculator FAQs

What size exhaust fan do I need for my bathroom?

Multiply your bathroom's volume (length × width × height) by 15 to 25 air changes per hour. A typical 3m × 2.5m bathroom with a 2.4m ceiling is 18 m³, so it needs a fan rated between about 270 and 450 m³/hr.

How is exhaust fan airflow measured?

In Australia most exhaust fans list extraction in cubic metres per hour (m³/hr); some list litres per second (L/s). Divide m³/hr by 3.6 to convert to L/s.

What happens if my exhaust fan is too small?

It cannot clear steam and moisture fast enough, which leads to condensation, mould and peeling paint. Sizing up slightly is always safer than under-sizing.

Do bathroom exhaust fans need an IP rating?

Fans installed in wet areas should carry at least IPX4 water protection, and higher near showers. Check the product specifications before you buy.

Can I install an exhaust fan myself?

Any 240V hardwired exhaust fan must be installed by a licensed electrician in Australia. Ductwork planning you can do yourself, but leave the electrical connection to a professional.