Lights required per sq foot



New to the forum.

Have an inquiry about lighting as was tossed some new information and I am left cratching my head …

Example :

20 x 12 room
4x4 rule for spacing
15 plants
Lights required ?
Following my belief and practices, I would use 15 600 W HIDs, using 1 light per 4x4 area to achieve optimal lighting.

Received an article which was to explain proper watts and lighting required.

  • Length and width of room
    20x12 = 240 sq feet
  • Divide this number by 4 (saw feet of plant)
    Answer 60 sq feet Canopy
  • Multiply 60 with 65 (ideal wattage of 65w/sq.ft)
    Answer 3,900 Watts

From this, I would only require 7 600W Hid’s to cover entire Canopy with optimal lighting
*** or 4 1000W HID’s if that’s your preference.

Is this correct? Am to assume I will get the same end results (12-15 lbs) off half the lights ???

Thanks G


hello,and welcome to ilgm @HempNB
@dbrn32 @Aolelon @MAXHeadRoom can you guys help us here?
vote @BIGE may bud of the month!
i’m going to say 6-8 lights off the top of my head


Thanks for your input and advice. I appreciate your response. Energy wasted is something I would like to eliminate. Not to mention the heat factor :wink:

Cheers. I look forward to many chats in here.


yea,this is a great place to learn about growing mmj…
glad you are here!


Thanks for tag @BIGE

I think it really depends on if you plan on having 60 square feet of canopy or 240 square feet of canopy. Which really comes down to how you use the floor space. I don’t think you necessarily need 15 lights, but I’m not sure you’ll be anywhere near 15 #’s with seven lights either. You’re talking about almost a thousand grams per 600 right? Or something like 1800 grams per 1000.

It seems a gram per watt is the target with hid within reasonable canopy size. And usually take a decent yielding pheno and some experience to get that. You can use movers and probably do a little better. Or bust out with some de’s. Otherwise not really sure what to suggest.


thank you for the support @dbrn32


Have you accounted for space to move around and walk in ect. to do maintenance on your plants? You need to make sure to plug this into the equation as well. When they are small it may not be a problem, but once the plants begin to fill in the 4x4 area is where it might get a little tight in there.


Why are you dividing by 4 here


If I had a room that big this is what I would do.

In my experience you need 2 x 2 to grow a good plant. So if you divide this room into 10 sections leaving a 4’ aisle to walk and care for your plants in then you would have 10 4 x 4 areas which could hold 4 plants each, for a total of 40 plants. Then each 4 x 4 area would need a 1000w hid light. So 10 lights.

Hope this helps :sunglasses::us:


Depening on if you wanted to install light movers or not, you can reduce the amount of lights needed with a bit of work as well.


The space allotted for pruning and care was the outside perimeter of the canopy in the example they used. So lots of room to walk around. The middle row might be difficult. That was one of my concerns and discussion with my team.
Good point.



Agreed and have used light movers in the past for in larger rooms.



Cheers and thanks.

I prefer not to use 1000W bulbs for the liken per watt loss bs 600W and power consumption.
That’s just a personal choice. I would prefer 2 600W bs 1 1000W.
Unless you tell me otherwise :wink:



The 240 sq feet used by the plants

3 rows of 5 at 4x4.



I would say you would need at minimum 8 600w if they cover a 3x3 area. You could probably get away with them overlapping eachother. sticking one on each corner of the growing box like so.

Or you could do 12.


What software did you use to make that drawing?


But those middle plants are going to be hard as hell to get to imo. You could cut them into 3x3 boxes that would give you 1 foot walk way in between each row or so.


Just a floor plan designer haha pretty easy. fun to mess around with.


On single ended I would agree. Going something like gavita double ended almost double the par but only 10% more power draw. I’m led guy personally, but if I had to do a room that big on hid that’s the way I would go.


Yea with the DE’s you could get away with using far less. the Gavita DE 1000’s are suppose to cover a 1.2mx1.8m footprint but with them overlapping you get far better coverage. Along the edges is where I would worry