Calculating By Room Volume
You will find many calculations on the web for sizing a fan for ventilating indoor gardens; however, what many of these calculations fail to take into consideration is the friction loss on carbon filters and increased temperatures from HID lights. So here’s my calculation method which you can use as a guide for sizing an exhaust fan for a growing area (keep in mind that this calculation will give you the lowest required CFM (Cubic feet of air per minute) required to ventilate the indoor garden.)
Step 1: Room Volume
First the volume of the room needs to be calculated. To calculate multiply length x width x height of growing area e.g. A room that is 8’ x 8’ x 8’ will have a volume of 512 cubic feet.
Step 2: CFM Required
Your extraction fan should be able to adequately exchange the air in an indoor garden once every three minutes. Therefore, 512 cubic feet / 3 minutes = 171 CFM. This will be the absolute minimum CFM for exchanging the air in an indoor garden.
Step 3: Additional factors
Unfortunately, the minimum CFM needed to ventilate a indoor garden is never quite that simple. Once the grower has calculated the minimum CFM required for their indoor garden the following additional factors need to be considered:
Number of HID lights � add 5% per air cooled light or 1015% per nonair cooled light.
CO2: add 5% for rooms with CO2 enrichment
Filters: if a carbon filter is to be used with the exhaust system then add 20%
Ambient temperature: for hot climates (such as Southern California) add 25%, for hot and humid climates (such as Florida) add up to 40%.
An Example
In our 8’ x 8’ room we have 2 x 1000w air cooled lights, and we plan to use a carbon filter. We also plan to use CO2 in this room. The ambient temperature is 90 �F (32�C), however, we will be using air from another room that is airconditioned. Here’s the minimum required CFM to ventilate room:

Calculate the CFM required for room (see above.)

Add 10% (for 2 air cooled lights.)

Add 5% of original CFM calculation (For CO2.)

Add 20% of original CFM calculation for Carbon Filter.

Air is coming from airconditioned room so no need to add any other percentages.

CFM = (171 CFM) + (171CFM x 10%) + (171 CFM x 5%) + (171CFM x 20%) + ( 0 )= 231 CFM.
This is the absolute minimum CFM required to ventilate your room.
The next step might seem to match the closest fan to this CFM. However, for this example I’d choose a six inch fan with a CFM of around 400 or more, and a 6 inch carbon filter to match. The extra CFMs may seem a bit excessive (calculations on most indoor gardening websites would recommend a 4" fan and a 4" carbon filter) but it’s always better to overspec since we need to compensate for air resistance in ducting too.
Also, as we are using a carbon filter we will need to match the fan with the filter so that the fan that will neatly fit onto the filter.
If all the variables are kept the same and we changed the room size from 8’ x 8’ to a 12’ x 12’ then the minimum required CFM would be 519 CFM.