Watts per Square Foot


Staff member
This topic has been discussed here and there but there doesn't seem to be any "definitive" or at least "ballpark/guided" answer.

With HID, it was typically known to be 50-60w per sq ft for optimal lighting. LED, in theory, should be more efficient for "usable" light and therefore theoretically should require less watts per sq ft. However it seems after several discussions and articles from other sources, it's a bit more complicated than this since LED takes into effect PAR, LUX, and all of those other terms I'm not well versed on.

So is there a general guideline? Can we judge and help make decisions on how much LED to buy using watts per sq ft? Or is it needing an expensive PAR meter or "good enough" cheapo meter using some sort of calculation?

I've also heard that you should only need 60% of LED watt as you should 100% HID watt - again, not claiming valid, just another viewpoint.

Let's start with a simple one. HLG claims its HLG550 (600 w) replaces a double ended 1000 w HPS. Can this be a true statement? (Despite the QB boards seeming too close together for even coverage)

At this point I'm going with watts per sq ft and if it's too much I'll dial it down, run a wider spread of lights more "efficiently" and not at full blast. Better to have more than less and wishing you had it.

Would love to hear some thoughts on the matter.


Cannabis Chaotician
Staff member
I was always told 35w/ft² for LED but honestly the one time I flowered under just LEDs was just about there (270w in a 2x4) and I did not find it adequate. But beyond that Ive only used my LEDs in conjuction with the CMH so I never really found an optimal range. Right now I'm running 715w of mixed in my 4x4 (45w/ft²) and it seems just about right

Chip Green

In Bloom
Given the fact that in no way, are all LED fixtures created equal, having a definite wattage per square foot rule of thumb, can be misleading. There are a few different levels, so to speak of LED "grow lighting."
You've got you're targeted spectrum types, commonly known as "blurple" in the underground. Combinations of blue, and red diodes, with the idea of producing spectrums specific to photosynthesis. These types are generally about as efficient as traditional HID lighting. Lots of heat produced, in relation to the photon output.
Those types, can be found cheap, often marketed with extremely exaggerated wattage equivalencies.
They can, and will grow dope, but you're looking at 50+W per square foot of actual power draw, to get worthwhile results.

Then you get into the white based styles. Typical to find these in board/panel, or strip/bar designs. 3000-3500K spectrums, with much higher efficiency than the "blurples", this is the area you start getting into the 35W/sqft zone.

On top of that, one can start getting into the white based, with supplemental spectrum, extra reds, deep reds, far reds....or blues, UVs, depending on the application.

It really comes down to the level of power being supplied to the emitters, the quality of said emitters, thermal management, and form factor of the fixture.

The LED realm is a deep, intriguing rabbit hole.

The Horticulture Lighting Group are pioneers in the field, their reputation and the quality of their products is of the highest regard in the LED underground.


Staff member
I was always told 35w/ft² for LED but honestly the one time I flowered under just LEDs was just about there (270w in a 2x4) and I did not find it adequate. But beyond that Ive only used my LEDs in conjuction with the CMH so I never really found an optimal range. Right now I'm running 715w of mixed in my 4x4 (45w/ft²) and it seems just about right
I love that LED/CMH combo. I need to go full LED this run though due to environment. I'm thinking of starting w 50 w per sq ft (turned 50% during veg) then crank up to about 100% and dial it back down after observation.

Chip Green

In Bloom
You've got it completely under control.
What ends up happening to many making the "switch", is even with quality hardware, the conditions they are accustomed to maintaining will actually cause the issues.
Higher heat, like low 80s F, is where the magic happens.
Man I think we overthink sometimes, so many variables in our setups I'd go caveman and just go hard until you see light stress then back off a tad, going for a set standard could quite honestly be hurting us imo say you stay at 30 and nuke a light sensitive plant then you change cultivars and now your 30 isn't enough for a really light loving line I'm being loose with the numbers but you get the point we gotta get back to doing what the plants want and not just what is standard ✌✌
Using the stuff I diy ed with the 561 diodes in 3500k I'm at 35 watts per sq ft but according to the par meter I'm at about 800ppfd at 20 inches off canopy honestly I think that the ppfd is what really matters, watts per sq ft I think is kinda like grams per watt there's a few other things going on like efficiency and hanging distance etc

Capt's Farm

I'm looking for 50w a sq.. when I set up any led room......

I have ran 75% to 100% and trust me more is better...

Also I seen BH say something about yellow to white on looking at his light's

301 b will look More White
301 h will have a yellow tinted to your eye.....

Skunky Dunk Farms

Cannabinoid Receptor
No studies here, just observations.
I've ran qd's now for 15 months.
I started out with 4 - 288's side by side over a 2x6 bag.
It was too much!
I then purchased 2 - 266 qb's and run two 288's and one 266 end to end over a 2x6 each of 2 bags and it appears to be the sweet spot.
That works out to about 70w sq ft.
I start at about 30" at teansplant @70%.
After about a week i start lowering till i end up with a height of around 15".
And slowly bring power up to max of 95%.
I've got rock hard bud, great frost and great yeilds.
I do run 4 - 16" occilating fans and plants are always moving under the light. And they use ALOT of water.
Anywho, thats what is,working for me so far.

Up next,,,,,, strip lights.


In Bloom
In my long hours of research when buying lighting, it was under my impression that 30-50w/ft² was the rule of thumb for led lighting. I ran HPS my first grow and LED my 2nd and noticed no difference in the grows. If there was, it was so low it wasn't noticeable. Maybe on a larger scale it would make a difference. Even though I average just under 50w/ft², I run my drivers at around 60-75% so I don't max out the drivers power and potentially kill it faster. I have also read that any higher than 50w/ft² that you would then need to supplement CO² into the grow as well. I am not expert or scientist, just a potatoe that reads alot.
My general rule of thumb is 30W/sqft for quality Samsung LEDs and 50W/sqft for the cheaper LEDs you find in a lot of those Amazon/eBay lights. I was running 600W of cheap Amazon lights in my 4x4 and my yields were garbage compared to when I swapped those lights out for 480W of Samsung F-Series strips. I went from getting about 6oz - 8oz from each grow to over 20oz per grow.
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