How Many Grow Lights Do Your Plants Need?

How Many Grow Lights Do Your Plants Need?

How much light is enough to provide excellent growth from my plants?

So you did the research and finished picking your perfect grow tent. Now you need to measure how many grow lights you need to get the biggest and best yield you can grow.

Related: Growing Marijuana Indoors Guide 2018

No matter if you’re new or you’ve been growing for a long time,  figuring out how much light you need can be tricky. There are so many factors to deal with that most people just go off of hearsay (heck, we did before we wised up).

One of the most popular ways to figure out how much light you need in your grow room is by using the chart to our left. When I first started growing around 6 years back this was the chart me and scores of growers used.

To an extent, it works but it’s not precise. I ended up with a below average yield and overall decent growth but my plants were never amazing. There was not enough light for them to absorb or was too intense for my plants.

I did tons of research and came across articles from indoor grow masters like Eljay from Advanced Nutrients. They teach lessons on how to grow weed and growing your own medical marijuana indoors, and I learned what I was doing wrong: I was measuring the grow room space, not the canopy; I didn’t take in to account the amount of room each plant needs for proper growth, and I really didn’t take in to account the watts I needed for good growth.

That’s when I learned the trick that saved my garden’s life, and it’s not even hard to pull off! To get there, just follow three simple steps:

1) Find the Area (Square Footage) of Your Canopy

Calculating your lights depends entirely on one section of your grow room– the Canopy. The canopy is the area of your grow room that your plants occupy. This is the only area of your grow room or grow tent your grow lights should be focused on (the old method calculates for the entire room, which wastes lots of light).

To do this, simply measure the length and the width of the area you’re going to have your plants in, and multiply them together. Yup, just like we learned when we were little. So why does that matter?

Well, the most reliable way to figure out how much light you need to light your canopy is by using the “Watts per Square Ft.” method. This method says that you should give your canopy 50 to 75 watts per sq. ft.

NOTE: The optimal spot is right in the middle of that which is basically 65w. This is considered the Sweet Spot. This is an educated point of reference, but your mileage may vary. Be aware that the upper end of this range (75w per sq ft.) may sometimes burn plants or cause white leaves. In general, this is the limit of what plants need before they go through heat and light stress.

When I learned this method they left out a very important note that I want you to know…

You have to also apply this method to the “4 square foot per plant” rule. This rule is exactly how it sounds. You should allow your plants a minimum amount of space to grow tall and wide without disrupting other plants in your garden.

This method aims to identify the area of your canopy in square ft. and then decide how much light per square foot to shine into the canopy.

NOTE: Of course this rule applies in regular growing applications and can be ignored if growing in a Sea of Green (SOG), Screen of Green (SCROG), or other forms of training methods.

2) Calculate the Sq.Ft. Per Plant Within Your Canopy

Now you’ll be putting all of these numbers together to find the total wattage you need to cover your canopy:

  • Take the length and width of your canopy and multiply them to find the area in sq.ft.
  • Then take the sq.ft. and divide it by 4. This will give you the square foot of an individual plant, which is vital in figuring out the right wattage you need over your plants

3) Calculate How Many Watts It Will Take to Cover Your Canopy and Get Some Lighting!

After you have those parts calculated, all you have to do is take the sq.ft. of your individual plant and multiply it by the wattage you want (remember, between 50-75w/sq.ft).

Sound confusing? It was to us at first, too, but it’s actually pretty simple. In fact, I’ll show you what I did step by step:

A) Find the Square Footage of your Canopy

You get the square footage of your canopy by multiplying the width of your canopy to the length of it. Once you have that, divide the square footage by the 4 sq.ft. each plant will need (remember the 4 square foot per plant rule). In our example, the length was 20ft and the width was 12 ft, which gave us 240 sq.ft. Then we divided that number by 4 and got 60 sq.ft for the total canopy I needed to cover.

B) Multiply your Canopy Sq. Ft. by 65 (watts)

That’s as simple as it sounds. We took the 60 sq.ft from the canopy and multiplied it by our ideal wattage of 65w/sq.ft

C) Figuring Out Light Distribution For Your Canopy

In our example, we calculated 3,900 watts over a 20ft. x 12ft. canopy inside of your grow room or grow tent. With this particular setup, you’ll need 4x 1,000w HID grow lights kits (4000w total) to cover the entire canopy.

The final step is creating an even distribution of lights to your plants. Don’t make the mistake on stacking them side by side– space the lights evenly across the canopy.

The next step is making sure you ventilate your grow room properly so you don’t overheat your environment.

Now you know how to effectively calculate your plants’ lighting needs. As always, we would like any feedback and recommendation on what you would like us to write about. Leave a comment down below and we’ll reply right away.

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74 thoughts on “How Many Grow Lights Do Your Plants Need?”

    • Hey Vinnie, great question! With T5s, the formula still applies if you’re planning on using fluorescent lights for a full grow cycle. At the minimum, you can do 45-50w per sq/ft. to achieve quality yields (depending on experience). However, you can also train your plants to grow right under the light with less wattage to get a desired yield. Hope this answers your question and let me know if you have anymore! 🙂

  1. In your example above you have the lights running parallel to the short side of the grow canope area. Since the coverage area for a 1000 watt light is a 6’x6′ square, does it make any difference which way the long side of the lights are orentated?

    Also just an fyi, in the example above the calulations and number of lights would be for a 6’x10′ area rather than the 12’x20′ area shown.

  2. I appreciate this information about plants and grow lights. It is good to know that calculating lights depends entirely on the section of the grow room called the canopy. I did not know that this was the place in which plants grew. Something to consider would be to seek professional help when it comes to installing a grow light system to ensure that it function properly.

    • Thank you for taking the time our to read our article on calculating how many grow lights you’ll need to cover your canopy. Whether you’re trying to grow cannabis or cantaloupes, what’s important is making sure you know how many watts of light you need to fill that space, not the entire room.
      You make a great point about seeking professional help when determining how many grow lights you’ll need to fill that room, and how to install them. Granted we offer a complete installation guide on our YouTube channel that takes you step by step through the entire process of installing and hanging your light, we know some people would rather have another grower install them instead. In this case, make sure your trusted grower also knows how far to keep those lights away from each other and, if possible, how to install ducting from one reflector to the next for maximum cooling of your grow light and your grow room

  3. i am a first time grower i plain on buying a 4 foot by 2 foot by 5 foot grow tent would a 2 foot grow light placed in the middle be enough to grow 2 plants

    • That’s certainly a great idea! With that size grow tent your canopy will pretty much be the entire tent, so with those dimensions you’ll want to fill your growing area with about 650w’s of light for both of those plants. You can shoot a little low and get a 600w HPS+MH grow light kit for both of those plants, or you can get closer with a 630w CMH grow light and fill that canopy with a good source of light.
      Make sure you have plenty of ventilation and airflow in there, too, because in a space that size with the plants and lights you’re going to have in there, things are going to get hot and sticky. To avoid problems in your garden, it’s best to make sure you have fresh air flowing throughout your garden.

  4. Growing at my home I prefer to use grow tents. Just buy a complete kit and you don’t have to worry about much.

    • That’s a great idea! If you’re not in a huge collective, or if that collective has special runs of certain strains, then grow tents are going to be the way to go. No need to gut an entire room or risk growing outside in an off-season. Even better, if you want to start off with everything you need an entire grow package is definitely worth it. It keeps the guess work out of piecing it all together, and you get to try different techniques of growing and feeding your plants with gear specifically designed with harvests in mind (nothing like trying to figure out what you need and when you need it all on your own…)

  5. Hi. How did you come up with 60 sq ft? When I mulitiply 12 x 20, I get 240 sq ft. That means you would need 15,600 watts. Am I wrong here? There is a huge difference between 15,900 and 3,900.

    • No problem, Aaron, it happens to all of us. It just looks like you skipped one step: applying the “4 sq.ft. per plant” rule. That means you’ll take 240 and divide it by 4, which the gives you 60sq.ft. Multiply that by 65w’s per sq.ft. and you get a total of 3900w’s needed in that room. From there it’s up to you how you’d like to go about filling the area with space. If you’re looking to go the HPS/MH route you can go with 4x 1000w HID grow light kits, or you can do the same thing with 4x LED grow lights in your grow room. With all that power you’ll need lots of room above your plants, so make sure you have lots of room above your plants to adjust lights as they grow

    • Exactly! The only difference is when you’re using LED grow lights you’ll need to hang them a little higher than you would HID’s. If you need a reference on how high to hang your lights check out our article here and see how far your plants need to be away from your lights source. Nonetheless, you’ll want to make sure your canopy has the correct amount of light covering it at all times, so the same rules apply in terms of wattage requirements

    • Great question, James! Whether you’re using LED grow lights or HID’s (like HPS grow lights or CMH grow lights), the wattage you need over your canopy will need to be measured in the same way. Now, the lumen output of LED’s and HID’s are different, so we can see where you might be a little confused as to measuring lumenosity vs. total wattage. However, to compensate for the intensity all you have to do is raise the light so that it does’t shine so bright on to your plants.

    • Great question! With that grow room setup and 4x 270w HID’s, if you give your canopy 1sq.ft/plant that actually should be just enough watts for your growing area (provided you make your entire growing room floor [4×4 area] the canopy). Now, the amount of plants you can fit in a growing area does depend on the size of the pots you’re trying to use and the overall size of the plant. Start off with 4x plants in there and grow them to harvest- this will give you a gauge of how big your plants will get. From there you’ll be able to see how many more (or less) plants you can fit in that growing area

  6. This is, BY FAR, the best information regarding light needs for a garden! I have seen the usual formulas repeatedly and no one has ever broken it down and explained it this thoroughly. I was using the usual way of calculating and was coming up with close to 5000 watts for a 80 sq foot space (9’x9′) which seemed ridiculous to me. Using your formula that same room needs 1316 watts….this seems much more in line.

    Thanks for sharing gold!
    Morgan

    • We’re glad we could help you out! Calculating how many watts you need in your grow tent can be tricky, especially when you’re growing cannabis. Lots of plants have wiggle room because frankly, quality isn’t a huge factor in production of, say, vegetables (most growers just want to get their produce and keep their grows moving). Cannabis can’t really take much of a hit to get a half-decent yield, so make sure that you calculate the amount of wattage you need for your weed plants to avoid overwhelming them or under-lighting them

  7. So I have a 5x5x6 tent with 2 light of 660W. I plan on growing 12 plants in a 5gal pot using 2 lights. I used your example and it says if I multiple 5×5 which equals 25. Then divide by 4.. giving me 6.25. I multiply by 65watts.. giving me a result of 406W that I should use and will cover the 5x5x6 tent.

  8. Thank you very much for the article. I’m very new at indoor growing. I’m doing right now to start my garden by seed.

    I am learning as I’m going.
    I will come back here for this great advice.

    Thank you very much!

    • Hey Zack!

      We are glad to hear that, we are here Monday-Thursday 8:30am to 5:00p PST and on Fridays until 4pm PST if you need any assistance. Happy Growing!!

  9. So my area is 2×4 (2x4x70 grow tent). That is 8 sq. ft.. Divided by 4, is 2 and multiply by 65 is 130. So I need 130 watts light to cover my area, great going. Look forward to growing with you!

  10. I will be growing my very first plant in the coming weeks and trying to get solid information on what grow lights to use and how much to use. This information is valuable and did what i needed it do do.

  11. I’ve heard that PAR is the only important thing when calculating the type of lights you need, and that watts is simply the power the light draws from the wall.

  12. If i have 4x2x6 grow tent and and i have 300 watts led full spectum.can still put some cfl?and how many watts i need more?

    • Hey There Gerald!

      That is a great question! In regards to the CFL we would recommend keeping the same lighting for your cycle. For example if you were looking for more coverage you can replace the 300w LED for a higher wattage to cover your area. You can also replace the CFL fully to cover the full area as well, we do not suggest having the LED and CFL combined together.

  13. Im wandering about an exhaust fan and carbon filter for odor control. Ok,after everything is hooked up,do you place the filter outside the tent? And if so does the odor stop in the carbon filter? Im new so I want to learn

  14. Great topic, thanks for sharing the infos.
    i was able to calculate the correct amount that i will. i will use two of the S900 LED lights i am stoked!!

  15. I have a 2′ deep x 4′ wide x 6′ tall growing room/tent and i was having a hell of a time figuring out what kind of lights to get. I came across this article and boy did this end my troubles. i figured i need about 2 lights and you helped me out great yall.

  16. How do I find how many plants I can’t fit in a room if I give them 4sq each wouldn’t that be the same math? My room is 10 x 8 so that’s 80/4 = 20. Wouldn’t that mean I can hold 29 pants?

    • Hello Sam,

      Good Going! You will be able to fit around 20 plants. However, it depends on the size of the plants you were looking to grow as well if they were large size plants you can probably fit a little less than 20 but if you were going with smaller sizes then you would be just about good with 20 for that exact size.

  17. Yes I still don’t understand the 4 sq ft rule can you explain that to me better. For example a 4×4 grow area sq ft is 16 ×65 =1040 but using the 4 sq ft method means I would only need 4×65 =260 that doesn’t seem like enough light

    • Hello Andy!

      We would love to explain this a little bit more for you. Once you multiply the 16×65 the number you will get is 1040 and you would take that number and divide it by 2. You will get that to equal to 520 when you are in this range in between you would go with the bigger number. So if you are at 520 which is in between 400w and 600w you would want to go with the bigger wattage of 600w to insure you get the best coverage for you plants. We hope this information was a bit more helpful for you, please let us know.

  18. Hi, I’m new in this industry and it’s even my first grow. Very detailed articles like this have me very interested and makes me want to come back just to see if there are any other tips i can learn. This was a very detailed article and i love how you explained for each light how much we would need for space.

  19. I have a 48×24×60 grow tent how much light & watts should I have in this space first time grower and ineed to be lead in the rite direction?

  20. I’m starting out 4 plants tomorrow and was wondering what type of lights I would need for the vegetative stage. I am planning to put the 4 plants into a tent 4′ by 4′ with a 1000W Led but I assume that initial 1000W LED will be too strong for the seedlings.

    What would you recommend?

    • Hello John!!

      We got you covered!
      You’re right, if you were looking to cover 4 plants in a 4×4 area 1000w would be too much coverage for your plants. In a grow room up to 4×4 we would suggest getting the coverage of 600w HID lighting or either and 600w of LED lighting. The 1000w would be too much heat for your plants and can cause serious damage, swap it to 600w of LED and you will be just fine.

  21. I am interested in using LED’s only in a 4′ x 2′ x 5′ grow tent. What LED wattage would you recommend and do you have ones that can switch from veg growth to flowering?

    • Hello Maffie,

      No need to worry we got you covered although you were fairly close you were off just a bit. Your total wattage would be 140w in this case 140w would not equal to any light in particular exactly so you would need to get the wattage closes to what you will need. In this case you will go for LED with the Advance Spectrum MAX S450 LED Grow Light Panel. If you were looking to go with an HID set up then our 400w Grow Light Kits will also get the job done for you.

  22. Hi, I have a 4’x4′ tent, which I plan on growing four plants in, so my spacing should be correct. My question to you is, in following your formula, I’m getting that I would only need 260 true watts in my tent to achieve optimal coverage. This seems a lot lower than normal. Is my math correct? Is 260 watts optimal? Please let me know if I’ve figured wrong. Just seems really low!

    • Hello Hayden!

      You are correct that is a bit too low for your area and for your plants. What the coverage of the 4×4 tent you would want to make sure your wattage is to around 600w if you were going with the HID lighting. If you were working with LED lighting you would equal that up with 2x S450 Advance Spectrum MAX LED Grow Light Panel. Please do not hesitate to ask us if you need any more information.

  23. Hey there! Im currently using a 8x4x7 tent wth 1000w DH HPS and 1200W Led on 4 plants is it to much light or to less, sorry im not lazy just dont want to mess up the calculations if you would kindly let me know would be greatly appreciated

    • Hey Cody!

      We definitively do understand no worries we would love to help you out. For the size tent you do have the LED wattage that you were using would be too much for your plants. The total wattage that would equal out to about 520w so we would move up to an wattage cover of around 600w. We would recommend using the 600w DE Kit or the S450 Advance Spectrum MAX LED Grow Light Panelthat would equal the same. We hope we were able to help you out a bit today.

  24. I’m getting a 2’x4’x6’ grow tent, if I did the calculations right I should only need a 300 watt led grow light for one plant. I want to do clones later and have 4 plants. If that a good enough light for my grow room

    • Hello Kyle!

      You are correct! If you were looking to get the grow tent of that particular size you would need to have at least a 300w LED grow light. Now, all you would need is one of these s450 Advance Spectrum MAX LED Grow Light Panel. With this grow panel will not only cover the area for you but would also make sure your plants are looking their best.

  25. Great article! This is something that i was struggling with but after reading this i am good to go. I would love to learn more keep them coming. Much appreciated for a fella like me.

    • Hello Chad!

      Yes, to answer your question this would be an overkill to your plants. In a tent size that small you would want to work with about 600w of either HID lighting or LED lighting. We would recommend using this wattage so you do not damaged or kill the plants. We hope this was helpful for you, please do not hesitate to ask us if you need any more further assistance.

  26. Hi,

    Thank you for this great Article.

    Just to be sure, I have a 50*50 grow tent. (length:50 cm and height 50 cm). Whats the best watts led you would recommend for an optimal grow. In other words, how many watts do you think i would need for the best grow considering its only a small grow tent.

    Thank you for you help in advance 🙂

    • Hello Vicky,

      Thank you so much for your question we would love to help you out!
      Although this is a smaller tent you can still get the best coverage for an LED light. You would want the wattage of around 300-600w.
      The S450 Advance Spectrum MAX LED Grow Light Panel would give you all the coverage that you would need with the wattage of 320w that is also equivalent to 600w. We hope this information was helpful for you, happy growing!

  27. Hi there, i have a 4×4 tent. Running a 600w Led Viparspectra. I did the calculations and i ended up with 270watts,The ladies seem to be enjoying it. Especially when the light is a bit further away. Thanks buddy! Great read any advice would be great!

    • Hello Joe,

      What a great question you asked here!
      However, you might want to stay right where you are in regards to the lighting. Due to overkill of the plants if you were to add more lighting in a 4×4 tent it would be to much coverage and heat for the plants. You should be good to go with 960 watt of what you are working with right now.

    • Hello Lee,

      We would love to help you out with that for a 10×20 Grow Room you can fit up to 12 mature plants or up to 30 smaller sized plants. However, in regards to the lighting you would need to go with the 1000w Grow Light Kit to get you started. Now if you were looking to go with LED lighting you would need 2x S810 Advance Spectrum MAX LED Grow Light Panel to equal the lighting that you will need for this size tent of coverage of plants.

  28. What is the proper distance I should have my 600 watt MH optimized vegetative growth light from my plants that are in vegetative? Also can you change a male plant in to a female that just started to sex?

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