As a rule of thumb 150mm models will suit bathrooms and kitchens. 100mm fans are usually only big enough to ventilate a toilet or laundry; and 120mm fans suit shower cubicles or smaller bathrooms. Really large kitchens may need two fans, or a combination of rangehood and fan.

Your fan should be big enough to work, but not so big that it causes draughts. In a large kitchen, it would have to be very big to be draughty. If you think your shower or cooking style creates more than the usual amount of steam, make sure you go for a large capacity fan.

Working it out

There are four steps to working out the size fan you need.

• Step one: measure the room. Say it's 2.0m long by 2.5m wide and 2.4m high - multiply all three dimensions to get the room volume: 12m³.
  
  
• Step two: decide how many air changes you want. Eight changes per hour is a good benchmark. That will give two complete changes of air in 15 minutes - long enough for a shower, plus a few minutes after, to remove remaining steam. Multiply the room volume by the air changes: 12 x 8 = 96m³/hr.
  
  
• Step three: Convert to litres per second, in case that's how the fan maker specifies its product. You divide the 96m³/hr by 3.6, which gives 27L/s for our example room.
  
  
• Step four: Buy a fan with a claimed hourly rate (in m³/hr) that's at least 20 percent above what you've calculated. Makers often quote the "free air movement" for the fan but don't allow for losses from grilles, passing down long ducts, and so on. Ask if there are figures with ducts and grilles. If you have no option but to exhaust through a gravity louvre, allow at least another 30 percent more. So for the sample room you may need around 32 to 36L/s (115 to 130m³/hr).