Determining what size grow light you need can get a bit complicated.
It depends on so many different factors: the type of lighting you want, the plant(s) you are growing, the size of your grow area, etc.
In this article, I break the issue down and make it as simple as possible.
There are three things you need to know before beginning: the light requirement of your plant(s), the type of light you want to use and the size of your grow area.
1. Light Requirement
For the light requirement, you really only need to know if it has a high light requirement or a low one.
Plants with a high light requirement are generally plants that flower and/or bear fruit, like tomatoes, citrus, orchids, cannabis, etc. Plants with a low light requirement are ones that do not flower, like herbs or leafy vegetables like lettuce.
2. Type of Lighting
If you are unsure which type of lighting you need, go with fluorescent lights if you only have one (or maybe two) plants and LED or CMH lights otherwise. I recommend LED over CMH, because they’re easier to use, they cost less to operate, the generate less heat and they last longer.
3. Size of Growing Area
If you do not know the size of your grow area, measure it. Or measure the plant(s). Or find out how large the plants you plan on growing get.
Once you have those three pieces of information, you are ready to begin.
Scroll down and find the type of type of lighting you plan to use below. Then follow the instructions to figure out the size of light you need and how many lights you need, if more than one.
Contents
How To Determine The Size (And Number) Of Grow Light(s) You Need
I will begin with HPS and MH lighting, then cover CMH, then LED, and finally fluorescent.
What Size HPS And MH Lights Do I Need?
High pressure sodium and metal halide are the easiest type of lighting to figure out (along with CMH below). There are two methods you can use.
By Coverage Area
If you know how large your grow space is (for example, the size of your grow tent), you can easily figure out which light(s) will best cover it. The following are the approximate maximum coverage areas for the most common wattages of MH and HPS bulbs:
- 150 watt: 2 x 2 feet (or 4 square feet)
- 250 watt: 2.5 x 2.5 feet (or 6.25 square feet)
- 400 watt: 3 x 3 feet (or 95 square feet)
- 600 watt: 4 x 4 feet (or 16 square feet)
- 1000 watt: 5 x 5 feet (or 25 square feet)
From that, you should be able to figure out which size light is best for you and how grow lights many you will need.
All else being equal, 600 watt lights are the most efficient (meaning you get more output per watt of electricity used) and 1000 watt bulbs are second best, so you always want to see if those will work for your space first.
For example, say you have a space that is 10 by 20 feet, or 200 square feet. That is perfectly divisible by 4 (the 150 watt bulb) or 25 (the 1000 watt bulb).
That means, you could light the area with 50 bulbs of 150 watts each (200/4=50) or with 8 bulbs of 1000 watts each (200/25=8). The eight 1000 watt bulb option is far preferable.
That said, you could also light the area using 600 watt bulbs.
The area is not perfectly divisible, but 200 divided by 16 gives you 12.5, so you would just use 12 or 13 fixtures and make them fit.
In this case though, I would go with the 1000 watt option, since 8 of them perfectly cover the area.
One thing to note: this is based on the maximum coverage areas for each bulb strength.
That means that the number of bulbs you determine necessary from this method are closer to the minimum light requirement for plants with high light requirements.
The next method makes that clear and gives you better results if you want to give your plants plenty of light to maximize yields.
By Wattage
The other way to figure out your MH and/or HPS lighting needs is to use the wattage requirement. To grow a plant with a high light requirement, like cannabis, you want 30 to 70 watts per square foot of canopy.
I would go for a minimum of 50 watts per square foot. If budget and heat is not an issue, bump that up to 65 watts per square foot.
Let’s look at the example from above again. The one with the 200 square foot area.
If we shoot for 50 watts per square foot, we would need a total of 10,000 watts to cover this area (200 x 50 = 10,000).
That means we would want ten 1000 watt bulbs (10,000 / 1000 = 10) or seventeen 600 watt bulbs (10,000 / 600 = 16.67, which we round up to 17).
You can see that this method says we need more lights than the previous method.
If we work backwards, the previous method would give us a coverage of 40 watts per square foot (8 bulbs x 1000 watts = 80,000 total watts / 200 sq ft area = 40 watts per square foot). That is still plenty of light, but we would get better results with more light.
If you’re not sure which brand is best, read my reviews of the best value HID kits.
What Size CMH Grow Light Do I Need?
Ceramic Metal Halide is even easier to figure out, since there are only two wattages commonly available: 315 watts and 630 watts (which is a fixture with two 315 watt bulbs in it).
The easiest way to think of them is to consider the 315w bulb equivalent to 600w of MH or HPS light and the 630 watt dual bulb fixtures equivalent to 1000 watts.
That means that you can expect a 315 watt fixture to cover up to 4×4 and the 630 watt fixture to cover 5×5. I would go for more intensity and would use a 315w fixture to cover a 3×3 area and a 630w fixture for a 4×4 area.
If you don’t already know which ceramic metal halide light you want, read my reviews of the best value CMH kits.
How Many Watts Per Square Foot For LED Grow Lights?
Watts per square foot is the most common way people determine what size LED light(s) they need, but it is not the only way. It is also not the most accurate way, but it is a good compromise between accuracy and simplicity.
Because the truth is: determining how powerful an LED grow light to get, and how many of them you need, is the most complicated of all types of lighting. This is due to the huge variation in lights and to the fact that most manufacturers flat out lie about the specs of their lights.
That doesn’t mean it’s impossible to figure out.
There are three methods I would consider. I will list them in order from the easiest (and least accurate), to the most difficult to figure out (but also the most accurate).
Using The Coverage Area Provided By The Manufacturer
Most manufacturers provide the coverage area of their lights and you can use that to figure out which light will be able to cover your grow space.
Nice and simple, right?
If only it were that easy. Unfortunately, many manufacturers (most Chinese ones and quite a few American as well) exaggerate the area their light will effectively cover.
Perhaps they give you the coverage area for a plant with a low light requirement (not a high requirement, like marijuana) or they give the coverage for vegging, which is always larger than the coverage for flowering.
This makes it difficult to know if you can trust the manufacturer-provided coverage area. Unfortunately I do not have a good solution for this.
If the light is reviewed on this site, I make sure to point provide the actual area you can expect it to cover. For lights I haven’t reviewed, you’ll have to try to find that info online.
Using The Wattage
To grow a plant with a high light requirement, you want 30 to 40 watts per square foot of canopy area.
Again, there is one huge problem. Most LED brands give both an actual wattage and a theoretical wattage (what the light would consume if the chips were all driven at full power, which they never are).
Important! For this calculation you want to use actual wattage.
This requires you do a bit of research and find out a light’s actual wattage. Most American brand make this information readily available, even if they name their light using the theoretical wattage.
Many Chinese brands don’t even use theoretical wattage. Instead they use some completely made up number. Roleadro’s 2000 watt light, for example, actually draws 230 watts (Roleadro is one of the absolute worst offenders when it comes to blatantly making up numbers).
The good news is that they usually do provide the actual wattage somewhere. They often hide it somewhere far down the page, but it is almost always there.
Once you have the actual wattage, you can use that to figure out if that light will cover your area (or how many of them you need).
Let’s look at an example.
Above, we used a grow space that was 10×20 feet, or 200 square feet. If we want to light that area with LED grow lights, we would need 6000 watts in total if we were shooting for the low end of 30 watts per square foot.
If we were looking to light that area with a BestVA 3000, we would need 10 of them. This light has an actual wattage of 615w, so ten of them would give us 6150 watts in total.
If you have a smaller area and only need a single light, the calculation is even easier.
Say you have a 3×3 grow tent. That makes 9 square feet. At 30 watts per square foot you would thus want 270 watts in total. If you went up to 40 watts per square foot, you would want 360 watts in total.
Thus you would want an LED grow light with an actual wattage of 270 to 360 to light this area. If you’re considering one of these CREE CXB3590 COB lights, the largest one uses 320 actual watts, so it would fit in there perfectly.
Of course, this method does not take into account the efficiency of the light. One light that uses 400 watts may have a much higher output than another light that uses 400 watts.
Wattage is used as a good approximation for the amount of light a fixture will provide, but the actual output will vary greatly from one manufacturer to the next.
For example, the Phlizon COB lights get a large amount of output for the wattage they consume, while competing lights give you far less output for the same wattage. Phlizon lights are simply far more efficient.
And that brings us to the most accurate method: using the actual output.
Using The Output
While this method is the most accurate, it is also the most difficult to use, because many manufacturers do not provide a lot of (if any) good data on the output of their lights. This is especially true with the Chinese brands.
Even if they do provide output info, they often only take a reading dead center below the light, where the output is strongest. They do not give you readings for the rest of the coverage area, usually because the light is very weak around the outside of the area.
All lights will be stronger dead center, but good lights will still get you decent readings around the outside.
I’m sure you’re wondering how output is measured and what amount of output is good.
Light output can be measured in a number of different ways, but the most useful measurement for plants is ppfd, or Photosynthetic Photon Flux Density.
It is measured in umol/m²/s and it tells you how much light is present in the wavelengths that are used by plants for photosynthesis.
This contrasts with another light measurement lumen, which measures the amount for light that is visible to the human eye.
To get optimal results you want plants to get around 300 to 600 umol/m²/s during the vegetative stage and 600 to 1000 umol/m²/s during the bloom stage.
Plants will do fine with less than that, but those are the numbers to shoot for if you want to optimize yields.
At a minimum, you want to get 100 umol/m²/s.
How do you know the PPFD of a light?
Good manufacturers will provide a PPFD footprint (also referred to as a PAR footprint or a PAR chart). These footprints show the PPFD output of a light at various locations in the coverage area.
The best lights will not have sky high readings in the middle (and thus much smaller ones around the outside, even if the manufacturer only provides the figure for the middle), but good readings all around. Even the far corners of the coverage area will ideally be over 100 umol/m²/s, or at least close to 100.
What Size Fluorescent Grow Light Do I Need?
Fluorescent lights only make sense for a plant or two. Any more than that and they become too inefficient.
This article covers how to determine how many fluorescent bulbs you need.
If you don’t feel like reading that, you basically want 60 to 100 actual watts per square foot of growing space.
What Size Grow Light You Need: Final Thoughts
Hopefully this article has helped you figure out exactly what size grow light you need to adequately cover your grow area. There are a lot of factors that go into it, but if you followed this guide, you should now know what you need.
I’m using 8 9.5 watt equivalent 100wcfl daylight and 1 32w led 3color red,blue and white full spectrum so said made by fiat to light up 2ft x3ft and 1 to 3plants is that proper light for weed grow medium yield white wodow?
I doubt that would be enough for 3 plants, but maybe for one. Since it’s mostly CFLs, just give it a try. If you need more lights, it’s easy enough to just add more bulbs.
Stan here thats 1/2 plants I also have in the corners I have 4 cfls the same wattage that would be total 28cfl bulbs @ 9.5 w equivalent to 60w at 800 lumens to light up 2’x3′ I messed up earlier that is all cfls at 9.5w equivalent to 60w 800 lumens x28 bulbs is this proper light
It should be. Just try and add more lights, if needed.
Hi there I’m a beginner / 1st attempt at growing I have purchased the tent and carbon filter fan so far but unsure what lights/how many etc and what else I may need to complete my kit
Tent is 2200x1200x2000
100mm centrifugal fan with built in speed controller
100x500m carbon filter
100x3m ducting
I don’t get what size your tent is. Are those numbers millimeters?
Yes millimeters we use In new Zealand
Please help peps, my centrifugal fan is 150mm I measured wrong
Which is the length, width and height?
2200mm long x 1200mm wide x 2000mm high
I would go with two lights that cover 4 by 4 feet each (1.2 x 1.2 m). The Phlizon 2000 would be a good choice for that: https://growlightinfo.com/phlizon-cob-series-review/
what light do you suggest for 7 x 7 grow room?? I’ll take a few suggestions??thanks
At an absolute minimum, go with 4 of the Phlizon 2000s (link in the comment above), but larger lights would obviously give you better yields. If budget is not an issue, go with 4 SF-4000s.
I’m trying to light up an 8×8 table (various amounts of small plants) so what would I need to buy (LED)?
It really depends on the light requirement of those plants, but basically, you’d want 4 LEDs that each cover a 4×4 area, or perhaps 8 LEDs that cover a 2×4 area. The Mars Hydro TS or SP series lights would be a good option. They have both fixtures that cover 4×4 and ones that cover 2×4, so you have several good options. You can find my review of them here: https://growlightinfo.com/mars-hydro-review/
Hi guys I’m using the optic 6 led in a 3 by 3 tent, just the cobs for veg which equals to around 37 watts per foot the the cobs and led array for flower around 66 watts per foot and am pulling around 17 Oz dried high quality product
Nice results! Thanks for the input!
I have a 4 x 4 area have currently acquired a mars hydro ts 3000 what do you think of my light coverage. Do you think I have too much light for my space to flower at a height of 12 inches.
Will I be in danger of light burn. Have you seen or heard of any results using this light. thanks
You’ll probably need to hang it higher than that, but there’s only one way to know for sure. I’d start higher and gradually move closer until you see burning, then back it off a bit.
Hi steven
im a big fan of your site
Im now using hps lamps 1000 watt 5 x 5 feet
the yields are great but the electricity is expensive and want to change to LED I want a led for growth and flowering but one with the best harvest yielding results
which led lamp can covers 5 x 5 feet with the result about the same as a 1000 watt hps lamp ?
Thanks you
The Spider Farmer SF4000 is the best light on the market (apart from the super expensive US brands) that is similar in spectrum to HPS. We have a review here: https://growlightinfo.com/spider-farmer-sf-led-grow-light-review/
Hi Steven,
I have asked for some info before I wanted to make purchases and I was told that I will not achieve the same harvest as hps 1000 watts with the spider farmer sf400 i have read it is the same like 800 hid now im doubting is the spider Farmer isf4000 good enough to harvest at least 750 grams ??
I doubt it mate, not 750 grams!! Mabie 400/450 grams
Hii Ricardo
thanks ricardo mate
i almost bought it on recommendation of Steven it’s disappointing because I thought he knew a lot about it but i need a led light strong enough That can give me the end result of minimal
700/ 750 gram
Thank you
Well Steve proberly isn’t wrong bro it’s a nice light with nice Samsung led tech for the money, but it’s a little over 400 watts at the wall, and I am yet to see all the 2 grams per watt hype with my own eyes I’ve used the black dogs and the kinds and I haven’t seen it?? Mabie if you are pumping co2 in and everything is so spot on its like a laboratory but most people don’t have that!! Bro if you are looking for that kind of weight I’d go with something that’s close to it at the wall!! Mabie something with grow and bloom options because you won’t need that power for grow so just the grow function for grow then grow and bloom on together for flower using its full output?? Just a idea
Hi Ricardo thank you mate
I almost lost 550 dollars
Because it was recommanded to me
Like it is the same it is a Shame you never can make that comparison
that is exactly what I was looking for a led for growth and flowering that is as strong as a 1000 watt hps and has the same weight outcome I was wondering what kind of led lamp I should buy for a space of 5 x 5 feet at least 4 x 4 feet in growth tent I was told that spider sf4000 is possible but now I am happy that it is not and can be compared to one with a 400 watt hps that is very sad that you can never recommend it to someone who has a 1000 watts hps result
Yes it is sad, the thing is when someone is used to hid lighting and the weight they can achieve switching to led can be misleading, they recommend around 37.5 watts per square foot for led lighting so a 5 x 5 = 25 square foot so round up the watts to 40 per square foot and you will still need 1000 watts power draw!!! I don’t think there is away around the cost of power?? All I can say is led lighting is abit more efficient, abit cooler to run!! That’s it really, mabie try fine tuning your environment or grow style so you are pulling 1 gram per watt before changing to led?? 1 gram x 1000 sounds good to me lol
Ricardo
can you tel me now I know the spider farmer is only cheap in the Bills
Wich led is good to compare with hps led minimum 750 watt
Not in the spectrum only But in the result thats wat I want to see heavy yielding not Some spiderfarmer bulchit thats not even to compare with 400 watt hps 😂😅
Ricardo
can you tel me now I know the spider farmer is only cheap in the Bills
Wich led is good to compare with hps led minimum 750 watt
Not in the spectrum only But in the result
Well it quite a difficult question bro, like I explained I don’t believe in the 2 gram per watt myth unless you have perfection in your grow room and even then I’m unsure if it achievable?? So if you are looking to save on power costs have you thought about a dimmable ballast for your hps?? You could cut the cost by dimming to around 600 watts for veg in a 5 × 5 then upping the power to 750 for flower and try to achieve 1 gram per watt!! It’s quite a saving on power especially if you are running 1000 watts start to finish, if you really want to change to LEDs tho, take a look at the optic range!! Optic 6 optic 8 or optic 8 plus!!! Check the specs especially the power draw I think the optic 6 pulls around 640 watts which is nice!! The problem with led is the more they pull from the wall the more the light costs?? If money is no problem for you just keep looking untill you find one with a power draw close to what you need
Thank you steven
Thank you Ricardo
Thank you Ricardo Im pulling now between 0,8 and 1.,1 gramm per watt using hps realy Great I do basicly not much 😂 Im an old scool Guy And for now I keep it old scool till led lights realy Can compeet with THE hps
Lool good going bro, yesss keep it old skool man, there’s nothing wrong with old skool, my dad used to say, if it’s not broke don’t fix it!!! Loool
Wahahah your dad is right 🙏🏻💪🏼
Hi. I have a 15 x 15 grow room. I have 4 mammoth 10 bar 800w led’s that run 1300 PPFD umol’s per square meter. And 4 DE 945 cmh’s that run 2000 PPFD umol’s per square meter. Both cover a 5×5 canopy space. I’m thinking I only need to run the 4 led’s and 2 of the cmh’s. I’m trying to do no more then 12 plants. I’m trying to maximize the most yield of my space and get the most use out of my lights. How do you recommend I setup my room? How many lights? How many plants? I do run c02 at 1200 ppm’s and have a completely air right room that’s on split ductless controlled environment.
Hi! I just need one for succulents. I have a 2ft by 5 ft a 3ft display that I need proper lighting for. How many watts do you recommend?
I don’t know how much light your plants need, so it’s hard to say. It’s also unclear from your message how large your area is. 2×5?
Well I’m using a rubber maid garbage can lid
Painted at. Installed 8 100 watt equivalent led bulbs 5oook 1500 lumens daylight with 4 20w 5000k 1300lum.comact size cfls plus 4 feit full spectrum standard size bulbs I think it’s like 27ooo lum. Total .not including the feit bulbs at 9w 3300k x4 all placed in that lid with standard bulb splitters finally a 32 and a 30 w full spec. Led panel on the side 1 wht/red the other r/w/b on the walls I have access to 12 more sockets that could be suited with multiple splinters if needed.i have 2 fans, grow erea CLOSET 2×3 8ft.high is this sufficient. Well ….I have 1 gdpurple I just started almost hst. High stress training but Not quit but bent those branches hard slow but sure I got them tied down w/o breaking stock thick &? 7 inches High topped once earlier I’ll top again today thought I’d give her 24hr. Break after that and light defoliation &the transplant was included I took picture I’ll probably give her 2wksor so keeping at 16 on 8off then flush twitch nutrients I’m using earth juice grow,bloom,alaska fish fertilizer, occasionally add some super green nitrogen additives from lucky bamboo call-mag and catalyst food all from earth juice finishing up with beer rabbit sulfur free molasses every other watering with worm casting from meal worm I have for my gecko oh yes and earth juice natural bud & bloom guano (bat shit) would gecko crap work? I dont know hmmm my ph that’s a bitch 6.5/pushing 7.2ish soil well well well I’m trying raised bed organic soil from whitney farms seems to have it all in there I guess but I cut it down with pearlite &vermiculite Pete moss just made airy,fluffy and lite just rite hopefully, I water untill I have oh 2 cups or so run off I’m using 3 gal.plastic planter with extra drainage holes I may cut out the bottom and more potage,that’s it I’ll change bulbs for flower ranging from soft wht.2700k 1500lums. =12000lums led bulbs with 4 2700k 1600lum 23w and the same3300k full spectrum feit led bulbs 4 each and last the panels I’ll switch from blue to wht/red 12/12 so watch a think oh and lay off the nitro change to bloom after flushing with ? 7,8 gal. Of Ro/ph 6.5/7.0 7.2 ish water .so what’s your thought on earth juice products and my self serving mini grow.
Be honest please ps. Temp at 70 ish at nite and as high off 90ish day humidity 50/55ish I’ll bring that down a tad….ish. lol
Thank you stan DeZarn. (Ps) I don’t even smoke well very little,I NEED 2 put down the cancer sticks and ROLL IT UP WITH A BLUNT or a Bong Hit I would be …
I’d like to thank i49. DAM I GOT CARRIED AWAY YES IM LOOKING INTO DIFFERANT LIGHTS THIS WAS SOMETHING TO DO NOW LETS DO IT BETTER -YES- I WOULD LIKE TO SEND A FEW PHOTOS AS SHE PROGRESSES HOW DO I DO THAT…TAKINF ONE NOW 6:00AM TIME TO WAKE HER UP ANYWAY…SHE’S ON TIMERS.
Hi can you help please ive got a 8x4x6 tent running 2 ups 600 whatt bulbs with 8 plants but want to change to led set up what do you suggest not worried about cost of set up thanks
Top choice would be 2 HLG 550 RSPEC: https://growlightinfo.com/hlg-quantum-board-led-grow-light-review/
Second choice: 2 Spider Farmer 4000: https://growlightinfo.com/spider-farmer-sf-led-grow-light-review/
Hi.
I just want to make sure..
Do u use the output watts of an led to calculate the space you can light up, or do you use the usage watts of the led?
Lights do not output watts, so you use the actual wattage they consume.
I’d go for how many watts the light pulls from the wall my friend
Could you recommend me a light that would be fit for a 9 sq feet setup.
Thx
I would go with the Phlizon 2000: https://growlightinfo.com/phlizon-cob-series-review/
Is this good? It is what I have growing my 2 plants, they seem very healthy but are small imo. It’s a small closet grow where I’ve been training
Input Voltage Range: 110V to 130V AC
Frequency Range: 50Hz to 60Hz
Input Power: 30W
Lumen: 2800 lm PF: >0.9
PPFD/1 Feet: 457μmol/m²/s
Beam Angle: 120
Color Temperature: 4000K
If you’re talking about cannabis plants, then they’re going to stay small with that light. If you want them to grow bigger and yield more (or anything), you’ll need more light.
I live in a dark apartment and I want a Bird or Paradise to bloom. I have palm tree that is under 120 watt Full spectrum grow light and over the course of the winter the plant out out three fronds. I was wondering would the same bulb be good for a Bird of Paradise?
https://www.amazon.com/Spectrum-Vegetables-Seedlings-Greenhouse-Hydroponics/dp/B08CW57SP6/ref=sr_1_5?dchild=1&keywords=120+watt+led+grow+light+bulb&qid=1616730772&sr=8-5
Thanks a lot 🙂
hello I have a 4 foot x2 foot x 5 foot grow tent with exhaust fan with philizon 1200 w led do you think this would be to much light for my tent
What’s the actual wattage of the light pal
The 1200W LED grow light actual power is 240- 250 watt . More effective coverage area than any reflector series lights. Perfect fit for a 3x3ft growing area at 24
The 1200W LED grow light actual power is 240- 250 watt .
I have two Bestva 1500 in my 4×4 tent. First grow with LED’s. Will these light’s do the trick! I have five plants in veg,1wk and doing well.
What’s the actual wattage of each light, what power do the pull from the wall
280 watts per unit!
I’m sorry, 220 from the wall!
Ahh that about 48 watts per square, it’s still enough mate just do what I said previously
That’s 62 watts per square foot, that’s plenty of light mate, as they are led you might have to play around with the light to plant distance, as I don’t know much about this brand of lights I’d keep a nice gap in-between untill you get used to them yourself
Thanks Ricardo! Sounds good, I’ll watch the height. Veg at 20″ distance right now,
hi first time grower got 6 50watt led full spectrum bulbs how many plants can i grow with this
5×10 gren house. getting a samsong 250w
I well Ned 2 250w the lite bars tilt in and out .
Dimibel with veg
grow and blume is it good a nuf?
I love this blog! Thank you for all the helpful information. I’m new to gardening and I’m trying to figure out what size grow light I need. Can you help me out?