I'm trying to figure out the same thing and ran across this info of how to calculate what you need taking all the factors into account: Comes from this website:
http://www.bghydro.com/BGH/static/articles/0606_vent.asp
How to size your exhaust fan
There are many calculations on the web for sizing a fan for ventilating grow rooms; however, what these calculations fail to take into consideration is the friction loss on carbon filters, increased temperatures from HID lights and CO2, etc. The following calculation can be used 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 required to ventilate the grow room):
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
The fan should be able to adequately exchange the air in a grow room once every three minutes. Therefore, 512 cubic feet/3 minutes = 171 cfm. This will be the absolute minimum cfm for exchanging the air in a grow room.
Step 3 – Additional factors
Unfortunately, the minimum cfm needed to ventilate a grow room is never quite that simple. Once the grower has calculated the minimum cfm required for their grow room the following additional factors need to be considered:
- Number of HID lights – add 5% per air cooled light or 10-15% per non-air 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%.
Example 1:
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, however, we will be using air from another room that is air-conditioned.
Minimum Required CFM to ventilate room:
(CFM required for room – step 2) + (CFM required for room – step 2 x 10% (2 air cooled lights)) + (CFM required for room – step 2 x 5% (CO2)) + (CFM required for room – step 2 x 20% (Carbon Filter)) + (Ambient Temperature 0 (Air coming from air-conditioned room)).
= (171cfm) + (171cfm x 10%) +
(171cfm x 5%) + (171cfm x 20%) + ( 0 )
= 231cfm – this is the absolute minimum cfm required to ventilate your room.