Question about exhaust fan size for tent


I’m new to the forum but must say I’m really enjoying it my question Is this I’m looking at buying a 3x3x6 tent to run my 400 watt hps my only question is what size exhaust fan would I need I’m leaning towards a 6 inch 240cfm fan but wanted to get others input before I ordered one


I run a 4 inch in my 3x3, and it works fine.

That being said, I run LED, so heat is not an issue for me. With HPS you may need the 6" one to help get some of the heat out of the tent. I haven’t used HPS, so unsure of the needs…

I am sure more will chime in with info.


I feel as if I posted in the wrong section admin please delete



I believe you are in the right section.

I think you would be fine with a 240 cfm 4in. But if you are concerned about heat you could go with a 6in 400cfm with a speed controller. That would let you slow it down if not needed- but could always speed it up if heat becomes an issue.

Here is the one I have for my 4x4 tent.


Hello and Welcome. I just completed my first ever tent grow. My tent is 39"x39"x72". I set it up with a 4" 190cfm exhaust and had four 300w leds and a couple of 125w cfl’s in there. It was not enough exhaust.

I have since installed a 6" intake and upgraded to a 442cfm 6" exhaust going forward.

I think you are right on the money @Hoss29


@Ray4x @Screwauger thanks for your input guys I’m going to go ahead get the 6inch 240cfm fan .


I use a 4" exhaust in a 36x36x72" , however using mh & HPs I also have a cool tube & haven’t had a problem with heat.


I run 2000 watts of leds in a 4x4 tent and a 4 inch handles it but I wish I had went ahead and got the 6 but at the time my tent was 3x3 .I suggest the few extra bucks now for a 6 just in case you need it later for heat or a larger tent ( which we all want after a grow or 2 ) you already have it . Get a variable speed controller and dont turn it up till you need it good luck


I agree with @Oldstoner - better to have it now and not need to upgrade later.


Always think I’ve got all I need for tent, then oh what if I do that? Or OMG, how will I ever handle humidity. Had planned to start up second tent ( only have shell) but going to try to tweak this one to max and then tackle new one.


Hey @Ray4x does that 6inch fan come with the plug already connected? I am thinking i might pick it up but I don’t want to wire stuff lol. I just got a 4x8 tent so I want a strong exhaust.



Yeah it’s already wired in.


I think @Laurap has a good point. If you plan on drawing through a filter, cooled hood, and depending on how much ductwork you have could impact your airflow quite a bit. It wouldn’t hurt to have a little more fan than you need and tune with speed control, like the others have suggested.


Thanks everyone for your input I appreciate it I just ordered my tent and exhaust fan . I can wait to get everything up and running again it’s been a few years since I’ve done a grow so I’m sure I’ll have tons of questions later on