[The Art of Ventilation] How to Size Up Fans (Part 2 of 3)

[The Art of Ventilation] How to Size Up Fans (Part 2 of 3)

14 Jul 2013

If you want plants to thrive, you need to extract stale, hot CO2 depleted air and replace it with fresh, CO2 rich air. Better pick a fan that's up to the job.

This is part 2 of a 3 part series into ventilation:

Here you'll find out how to size up fans!

Extractor Fan Size

You need to know how many m3 of air you need your fan to extract every hour.

You can do this in 3 simple steps.

Step 1:

First of all, work out how big your ‘active growing area’ is. That's the space that's covered by your grow lights (more on lamp coverage here).

Multiply your active area's L x W x D to get the volume. If in doubt, use your tent's L x W x D. 

Example (BAY6 XL Tent)
  • Step 1: 1.2m x 1.2m x 2m = 2.88m3
Calculation Calculation

Step 2:

Next, you need to factor in carbon filters and ducting. They tend to reduce your flow rate. 

The exact reduction varies (e.g the longer the ducting, the bigger the reduction). But for most, it'll be about 25%. To factor this in, multiply your first flow rate by 1.33.

Example (BAY6 XL Tent)
  • Step 1: 1.2m x 1.2m x 2m = 2.88m3
  • Step 2: 2.88m3 x 1.33 = 230m3

If your ducting is very long or goes around multiple bends, add another 20% (multiply by 1.2)

Calculation Calculation

Step 3:

Think about things that might affect your grow room temperature and adjust your flow rate: 

  • Add 20% for a warm attic (multiply by 1.2)
  • Minus 15% in a basement (muliply by 0.85)
  • Minus 25 - 30% for air cooled lights (multiply by 0.75 - 0.7)

Don't forget that south facing rooms are also warmer than north facing ones.

Example (BAY6 XL Tent):
  • Step 1: 1.2m x 1.2m x 2m = 2.88m3
  • Step 2: 2.88m3 x 1.33 = 230m3
  • Step 3: 230m3 x 1.2 = 276m3
master the art of ventilation: How to Size Fans master the art of ventilation: How to Size Fans


Your final figure is the maximum amount of air you should extract every hour. If you extract more than this, you could remove too much heat and humidity.


So, now you're extracting air, you need to make sure fresh air can enter.

If you're not extracting much air, a few air vents will do the job. Air will enter through them passively.

Otherwise, you'll need another fan - make sure you maintain negative pressure. This is where you extract more air than you draw in.

If you don't, your tent can overfill and unfiltered air can start seeping out of vents, ports and zips. If you're unlucky enough, your tent might even start to inflate. You don't want that.

Intake Fan Size

Pick one that's 15 - 20% less powerful than your extractor fan. Don't forget to factor in the 25% flow rate reduction from filters and ducting (step 2 above).


(extraction fan flow rate - 25% filter & ducting reduction) x 0.85 - 0.80 = intake flow rate

Example (BAY6 XL Tent):

(276m3 extraction rate x 0.75 reduction) x 0.8 = 176m3 per hour

Calculation Calculation

Intake Vent Size

If you’ve got a smaller grow room with only one grow light (up to 600W), you should be OK with air vents.

For every 1000m3/hr of air extracted, 0.09m2 of fresh air must be allowed in.

0.09m2 (900cm2) = 3 x 200mm circular holes  

If you've got a different size vent, or if it has grilles, you need to calculate how much air can be replaced through vents every hour.

Vent Vent

Step 1

First, you calculate your vent's area.  

This one's simple enough:

grill W x H = area   


200mm x 200mm = 0.04m2

Step 2

If it has grilles, around 60% of the area is free air space 

So - grill area x 0.6 = amount of free air space


0.04m2 area x 0.6 = 0.024m2 free air space

Step 3

Then, calculate how much air can be replaced through your vent every hour:

(Free air space / 0.09) x 1000 = 267m3/hour  


 (0.024/ 0.09) x 1000 = 267m3/hour  


Here you can see that it'd take 4 x 200cm vents (with grilles) to allow every 1000m3 of extracter air to be replaced.

About the author

Dan's the man behind our marketing. He makes the newsletters, emails, catalogue and shop displays happen! 

Head Office

Order Online:
0333 003 22 96