How many lumens cannabis need can get really confusing.
Kelvin, watts, different light spectrums… there’s so much jargon it can end up getting all jumbled up.
Which one do you need to know to make sure you’re getting your cannabis plants the proper amount of light?
Well, today, I’ll give you a brief lesson on lumens, what they are, how they affect cannabis, and if it’s a metric you even need to be paying attention too!
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What are Lumens?
Simply put, lumens are the measurement of light intensity—how bright light is to the human eye.
The exact definition is something along the lines of a unit of luminous flux.
But as cannabis gardeners, we just need to know that lumens=brightness.
So, is brightness what gets cannabis off? Or is it the wattage of a light bulb?
Well, watts is an increment of energy and doesn’t have anything to do with light output at all.
Wattage is something that growers had used in the past because it made sense when you were growing with high-pressure sodium (HPS) lights.
But now that LED lights have become so prevalent in the growing industry, even lumens are beginning to become an outdated metric—enter PAR.
To read more on LED grow lights, check out the guide here: thcoverdose.com/best-led-grow-lights-for-weed/
Lumens vs. PAR
Photosynthetically active radiation (PAR) is a particular spectral range of solar radiation that ranges from 400 to 700 nanometer.
By GrowLightingExpert (Own work) CC BY-SA 4.0, via Wikimedia Commons
This range often called the peak of the spectrum, is what plants use during photosynthesis.
The reason why PAR is a better metric is that it’s finally something that measures the light wavelengths the plants need to grow!
How cool is that!?
If you’ve ever seen or grown with a LED grow light, then you know that they are not particularly bright.
Instead, they give a range of blue and red lights that are specifically catered to grow plants!
But, the cost of setting up LED lighting in your grow room can be steep—especially if you’re just a small time closet grower.
If that’s the case, lumens will be a good way to know that you're getting enough light.
How Many Lumens for Seedlings
When your plants are in their early stages, they don’t need much light at all.
They are more worried about moisture levels and developing leaves.
It really does require so little you just need to focus on not burning them.
That’s why most growers opt to start their seedlings out under CFLs or small LEDs with full spectrum—a 40W LED light is perfect for seedlings.
Be sure to keep it 18-24” away from the plants and then as soon as they get their first real leaves(1-3 weeks) move them over to your vegetative lights.
The Lumens Cannabis Need During Vegetative & Flowering Stage
When your plants are still growing, even though it takes a lot of energy, they don’t need that much sunlight.
Usually, you should grow at around 50% of the max optimal output needed for your garden (10,000 lumens per sq/ft).
3000-5000 lumens per square foot is ideal.
As soon as they start flowering, they require a large amount of energy to grow their buds, so that’s when we crank it up.
Of course, this all depends on the strain, but it’s a good rule of thumb to jump the lumens per plant flowering up to 7500-10,000.
During these different stages, the plants don’t just require different amounts of lumens but also different light wavelengths.
This is a reason many growers love LED lights.
The good ones let you change the spectrum with the flick of a switch.
Blue/white lights while the plant is a seedling or in the vegetative stage, and red/white lights during flowering.
You can also run both at the same time to give them a continuous flow of the full PAR spectrum.
HPS lights, always produce a higher output of red light making them great for flowering.
Lumens Per Square Foot Grow Room
To put things into perspective, the sun puts out 10,000 lumens per square foot.
When growing cannabis indoors, our goal is to mimic the perfect outside environment.
While this may be the case, you don’t need to hit the 10,000.
Cannabis needs a minimum of 2500 lumens per square foot.
Anything less and it won’t have enough energy to grow.
Remember, more light=more energy.
And what does more energy equal? More bud!
So, if you want to create the optimal environment, you want 10,000.
If you can’t afford enough lighting to get you there, then aim for at least 5,000 lumens per square feet.
Use the calculator below to determine the lumens cannabis need to grow:
If you want to learn how to figure it out on your own, it’s simple.
First, you need to the square footage of your grow area (LxW).
For this example, let's say your grow room is 3’x3’ (3x3=9).
So, to hit 10,000 lumens per square foot, you would need a light that produces 90,000 lumens (90,000/9=10,000).
A typical 600W growing light puts out:
- HID: 85,000 lumens
- HPS: 89,000 lumens
- MH: 55,000 lumens
So, a good light for a 9 square foot grow room is a 600W HPS because you’ll get 9,888 lumens per square foot. You can read more on HPS lights in our buying guide.
Protip: You get the same energy output from a LED grow light that pulls +/-280W with 40% of the heat.
You should now have a better understanding of how many lumens cannabis needs.
- Optimal amount: 7500-10,000 lumens per sq ft
- Good amount: 5000 lumens per sq ft
- Minimum amount: 2500-3000 lumens per sq ft
And you learned about PAR, plus how plants use different wavelengths and lumens at different stages.
But if you have any other questions, please let us know in the comments below!
8 thoughts on “Lumens & Cannabis: How the Two Interact”
OK, so with your math, a 23×9 room will need 2.07 million lumens. If you have a 600W MH light, you’re saying I Need 37 lights in there? That doesn’t seem realistic
Can I ask why you’re going with MH lights? Also, look at a 600w LED light. It’s recommended for a 3×3 grow room. So, with that as the benchmark for 9 sq ft we can figure that: 23×9=207 then 207÷9=23. So using an LED for the manufacturer recommended space will still put you at 23 lights. I suggest you get higher wattage lights for a grow that size.
Yes, I fully agree everyone should follow these recommendations. Definitely need the full 10,000 lumens per square feet for optimal results. And I’d keep those 600’s pretty close too, no more than 12 – 18″ max, or else you’ll suffer lumen fade and your harvest will be horrible. Everyone knows more light, and the closer the light, the better. I actually put my 1000 watt HPS lights 18 – 24″ max from my plants and I get well over 2 lbs per plant everytime of the frostiest and stinkiest bud, no matter the strain for the most part. Great advice Tony!! You really know your stuff.
Thanks! 2lbs per plant is something everyone can be jealous of!
Hey, Awesome Post ,I just want to ask how many LED lights do I need?
My grow room size is 10 ft height and 9 ft Widht!
I am thinking about creating my own light system with leds,Help me 🙂 btw I am growing for the first time and havent smoked Marijuana yet!:( jxt some low grade hash
Thanks! Of course, I’d love to help you out. You’re on the right path going with LED lights. The height of your grow room isn’t tha important because you’ll be constantly adjusting the height of the light to keep them around 18″ from the plants. What is the width of your grow room? The length and width are going to be what determines how much lighting you need. I’ll link to a couple of resources that can help you out. The article on the best LEDs has a lighting calculator that will tell you exactly how much you need.
Hi. Great info on the lighting. Thank you.
I growing outside in 20 square meter tunnel.
How much lumens do I need in led lights to prevent flowering. Sun ok for 10hrs per day. Currently I using fluorescent lights for 8hrs at night but they use too much power.
Ideally, you will have 10,000 just like the sun. I don’t really understand your grow room set up, but I can direct you to our growing calculator that will show you how much you need. You can find it here: https://thcoverdose.com/best-led-grow-lights-for-weed/