How to work out what size exhaust fan (extractor) you need for your grow area:

exhaust fans

If you are running with hot HID grow lights you will need to change the air within the grow room once a minute for good optimum results.


  1. take a tape and measure (in meters) the length, width and height of your grow tent – this will give you the cubic area of the tent
  2. then multiply this figure by 60 (the number of minutes in an hour) which is how many times you want the area exchanged in a hour

here’s an example:

my grow tent is 1.2 M x 1.2 M x 2M

so 1.2 x 1.2 x2= 2.8 cubic meters in total

2.8 X 60 =172.8 cubic meters an hour needed

and here’s a quick approximate guide to the size of fan for what size area you are growing in


4″/100mm – 175m3/hour
5″/125mm – 225m3/hour
6″/150mm – 420m3/hour
8″/200mm – 750m3/hour
10″/250mm – 1000m3/hour
12″/315mm – 1300m3/hour



However!  any 90º bend in the ducting means that you will need to add a 30% power increase due to resistance –

So for me with my Homebox GL120 which has a 90º bend

172m3 X 1.3=223.6m3

so in effect a 5″ extractor is ideal and funny that as its what I run with :)

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