What size light do I need for my tent?

I see people commonly say that you need 35-75 watts per sq ft. Is that per sq foot of canopy or total grow space?

I currently have a 600w hps in a 2.5x5 tent (12.5 sq ft) but can only run it at 400w because it gets too hot at the full 600w. My plan is to use this tent for veg and get a 5x5 for flower. I’ll have a canopy about 3x3 or 3.5x3.5 and the reason I want a bigger tent is so

  1. to make up for the sides of the tent sucking in from the exhaust fan
  2. I want room around the edges to place my fans/portable ac so I can use the full 600w
  3. have extra room to work with inside the tent so I’m not having to reach to my back plants as much

Is a 5x5 ok or should I go with a 4x4?

Here check this light build out I did. If that’s something that interest you I’m sure @dbrn32 will assist you in specs and details. Well whether you DIY or not he would help you lol.

QB 132 DIY build. In progress Join in follow along


It’s not 100% clear what you’re planning on putting where, but I’ll just throw out a few random comments:
Your 600 watt HID light is a little robust for 12.5 square feet, but if you put it in an air-cooled reflector, it will probably be fine.
Your 600 watt HID light will be fine for a 4x4 tent as well. But - if you decide to put it into a vegetation tent, switch your bulb to MH (Metal Halide) as it is better suited for that stage of growth. If you use it in your flowering tent, you should keep using the HPS bulb.
Of course, LED will really bring your temperatures down.

Edit: I just noticed your comment about the portable air conditioner… it sounds like you are planning on putting that inside a tent. You should condition the air outside the tent and then allow it to be pulled in.


I currently have been using the same tent for veg and flower. I use t5 for veg and hps for flower. I want to add a separate tent for flower so I can harvest every 8 weeks instead of 16. My main question was whether the 50 watts per sq ft meant canopy size or tent size. I’d like to get a tent that is bigger than my canopy size so I can have space for fans and to move stuff around.

It applies to tent size. 35-50 watts per square foot. And, while there is question to how accurate those numbers are, they’ve been in use for quite a while and are good numbers to shoot for.

BUT - if you at some point decide to go down the LED route, those numbers mean nothing at all.

1 Like

Thank you. I figured the 50 sq ft was more of an estimation as there are lots of contributing factors. I’d eventually like to get into led lights but as of now I dont really have funds for new equipment or time to learn about it. LED is next on the list after I experiment with ebb and flow systems


The problem is running a 3x3 canopy in a 5x5 tent will have a lot of light spill without the canopy being wrapped in reflective surface. Your 600 watt hps should do fine in there, but I would agree you’ll probably want an air cooled hood.

1 Like