Calculation for lighting

I read on here one time the rough calculation for light wattage needed? it was something like 4x4=16 ,16 divided by true wattage? Is this correct or do I got something backwards?

Rule of thumb is 50w sqft

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50 watts / square foot for blurple. Around that for hps/cmh.

I use 35w/sq foot for quantum boards/light strips, but in reality with new tech it’s closer to 30w/sq foot


What @Drinkslinger said. Perfect :+1::v::bear:


I know some sites have a calculator, you enter your measurements and it will tell you specifically what wattage to look for. Some even offer product suggestions. Good Luck!

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I used to know the calculation for roughly figuring it out but forgot it. Probably the short term memory thing lol

If you’re using efficient leds you can use:
Grow space sqft x 30 = total wattage needed.

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@BobbyDigital, @MattyBear, @HornHead, @Drinkslinger… When do you reach the point of having TOO MUCH light??

I had 2 plants that started to flower at the same time. They were in separate tents. I put the 2 plants into one 2’x4’x6’ tent, and added the 2 LED from each tent. One LED is 600w and the other is 300w for a total of 900w. I also have one of those full spectrum 800w COB LED’s as well. I was thinking about hanging THAT one in there too, for 1700w.
Over kill? Not a good idea? Any thoughts???

You don’t have 1700w of light, sorry. That 600 is probably 250 at most, the 300w is 130 tops and the 800, probably another 200 tops

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It would be over kill in my eyes. If I use the square foot x 30 = a

16 sq foot x 30 = 480 true watts needed and my 2 HLG 260xl rspec = 570 true watts.

So a little over kill. This is about my rig setup.

@HornHead. yes, I know that. I was giving the total of what I have on hand. But, I was asking about the TOO MUCH light issue also. I know you want to hit the plants in flower with as much light as you can, but, when is it too much??

@MrPeat… that’s what I was thinking too.

@RockClarke You can have to much light and it will cause a new set of problems. It may make your plants turn hermi due to the extra heat stress that the plants will go through.

I also think the soil will dry out faster due to the extra heat.

@MrPeat……… Yeah that COB produces a lot of heat.

@RockClarke Yeah I’ve read that is plenty of threads. I was using Viparspectra PAR600 lights and know how they put out heat compared to my HLG kits.

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DFI is the o Ly way to know if its overkill
I run a HLG 320xl in a 4x2 &
2x 240 R spec boards for a total of 480w in my 3x3.

So many variables to consider though…


What brand are those lights. Can you give me the Amazon link and I can tell you more

I had a Black Dog Phytomax 2 400 (420W actual draw) in a 2x2 (60cmx60cm) tent and it was going great, that was 105W per square foot,had a plant that filled the space right up to within 5cm of the light, then the cops got it :frowning:


HLG Quantum’s all the way. I have 1 right now next to my Vipar. Eventually I’m going full QBs and Vipar will be veg light.

They’re so bright and have little to no heat.

O.k. over kill is what I am going for
4x4=16 ×50 = 800 watts running 2 600w mh and hps lights for 1200watt
Plus added 2 288qb boards another 240 watts and my driver should be here today for my red and green color strip lights
My par meter is at 810 or something right now i will charge it up and post pictures of it im trying to get to around 921 ppf on my meter
Right now at 1440 true watts of light

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My setup right now