What Size LED Grow Light Do I Need?

This is one of the questions that frustrated me most when I started growing with LEDs

There’s so much technical jargon mixed in with marketing falsehoods that choosing the right light is tough.

What Size LED Grow Light Do I Need? For optimum yield, A good rule of thumb is to choose an LED grow light that outputs at least 50w per square foot of grow space. Make sure that the light can draw 50w from the wall (not the combined capacity of the LED’s) Example: a 2ft long x 3ft wide grow space would require a 300w LED grow light

This is a good starting point, but there’s a bit more to it.

Read on to find out exactly how to find the right size of grow light for your setup.

What Size LED Grow Light Do I Need?

The most important bit of equipment in your grow room is your light.

Why? Light is your plant’s food, you need to feed your plants high-quality food for them to produce high-quality yields.

LED lights for weed are now considered the best lighting option by most growers due to their energy efficiency, durability, and full spectrum.

However, with all the options and technical jargon, finding the right light for your project can feel a little overwhelming.

To work out what size of LED grow light you need, you first need to calculate the exact size of your grow space.

The Size Of Your Grow Space

When growing cannabis, a good rule of thumb is to use a minimum of 50 watts of light per square foot of grow space.

We want to flood our plants with light ( food ) so that they have lots of energy to produce big potent buds.

Therefore, we need to calculate the square footage ( length x width ) of our grow space.

We’ll then multiply this number by 50w to give us the rough wattage of light we require.

EXAMPLE: I have a grow space that is 2ft long and 3ft wide. This gives me a square foot grow space of 6 ( 2ft x 3ft ). I would then multiply this number by 50w, which gives me 300w. Therefore, in this example, I would require an LED grow light that was at least 300w.

How Many Plants Can I Grow?

In general, each cannabis plant you grow requires at least 1sq ft of space.

After germination, I like to place one plant in a large 5-gallon fabric container, which has a diameter of roughly 1 sq foot.

My plant will stay in this pot until it flowers.

This allows the plant to grow big and strong without other plants or tent walls obstructing its progress.

Using this as a guideline, I would be able to grow between 1-6 plants in my example above.

Most LED grow lights usually be able to cover about 1- 6 plants.

If you want to grow any more plans than this, you would need to purchase multiple lights to hang side by side ( called daisy-chaining ) to cover the canopy and ensure each plant is getting equal light.

The table below shows how the power of grow light you require increases with the size of your grow space. I’ve also included how many plants each size of grow light will be adequate for.

Note: 1 sq ft grow space = 50 watts LED grow light

Grow Space SizeMinimum Number of Watts RequiredNumber of Plants You Can Grow
2 sq ft (2×1)100w1-2 plants
4 sq ft (2×2)200w1-3 plants
6 sq ft (2×3)300w1-6 plants
9 sq ft (3×3)450w1-9 plants*
12 sq ft (3×4)600w1-12 plants*
16 sq ft (4×4)800w1-16 plants*
20 sq ft (4×5)1000w1-20 plants*

* The size of the grow space in these situations will require you to purchase multiple lights to hang side by side to ensure the entire canopy is receiving light.

Note: The values above are for flowering. During the vegetation stage, you will require around half the power ( 25w ) the best quality LED grow lights come with “Veg and Bloom switch” that optimize the intensity of the light for the vegetation and flowering stages.

Understanding PAR Output

You’ve probably heard about PAR. In simple terms, It’s the part of the light spectrum that your plants use for photosynthesis.

So It’s a pretty big deal.

The higher the PAR output of your light, the more edible light your plants receive.

Take a look at the table below for the optimal ranges of PAR for each stage of your plans life:

StageOptimal PAR Output
Seedling200-400 (μmol/m2/s)
Veg400-600 (μmol/m2/s)
Flowering600-900 (μmol/m2/s)

Ideally, you want to pick a grow light that outputs PAR in the ranges above.

This pretty much rules out any LED grow light below 300w, as they simply don’t have enough power to generate the required levels of PAR.

That said, keep in mind that these are optimal levels of PAR. Using a grow light that has less PAR output than these ranges will not be a disaster, you just won’t max out the potential of your plant.

So, Which LED Grow Light Should You Pick?

If you’re still unsure which LED grow light to go for, then I’ve written a detailed guide to the best LED grow lights that you might find helpful.

Alternatively, the three LED grow lights below are perfect for a small grow set up of between 1 – 4 plants inside a 1 sq ft –  4 sq ft grow space.

free cannabis seeds

Final Thoughts

Hopefully, this article has been able to shed some light ( pun intended ) on how to pick the correct size to grow light for your project.

As long as you know the size of your grow space and how many watts per square meter you need, choosing an LED grow light is pretty simple.

Good luck with your grow!

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James Alexander James Alexander
Hey, I'm James! The founder and head writer here at GreenBudGuru.com. I started this site to share my passion and knowledge of all things cannabis. I used to work as a budtender in central Amsterdam, and have over 7 years of growing experience.

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