What Size Grow Light Do I Need?

As you probably know, the size of the grow lights you have will determine your overall success when growing marijuana. Weed plants need a lot of light to grow successfully; they need it to vegetable, to flower, to grow tall and strong, to form dense buds, and for potency too. Not having enough light can be as disastrous as having too much light.

Determining the size of the grow light is important when it comes to growing weed. Now, the size of the grow light is important, but what is even more important is how strong the light is. How many watts does the light have and what is the lumen output like? It also depends on what other kind of equipment you have. Let’s get to it and answer the question of what size grow light do I need?

What Size Grow Light Do I Need?

Ok, so to be fair, the physical size of the grow light is actually not all that relevant in the long run. The really important part is how strong it is. That being said, the size does still play a role here. However, the importance of the size is related to what kind of bulbs you have. The stronger the lights are, the more wattage they have, and the more lumens they put out, the smaller the size of the light is usable.

For instance, an HID grow light that is 400 watts can put out just as much light (lumens) as an 800-watt fluorescent bulb. Therefore, if you need the same number of lumens, you can choose an HID light that is half the size of a fluorescent light and the results will be the same. It really all depends on the type of bulb you have.

Once again, the physical size of the grow light is not as importance as the strength of it. You could have your whole ceiling covered in fluorescent bulbs and they probably would not put out the same amount of light as half a ceiling covered in HID bulbs.

What stage of growth is the plant in?

One of the more important things to note is that what size and strength of light bulb is required also depends on what stage of growth the plant is in. The younger and smaller the plant, the less light it needs, and vice versa.

Just for the record, plants that are in the second half of their cycle, also known as the flowering cycle when the buds of marijuana begin to develop, require about twice as much strength in light as weed plants in their initial vegetative growth stage.

For example, seedlings, clones, and really young plants in the vegetative state will do fine with 20 or 30 watts per square foot. A weed plant in the vegetative state that is a little larger should get between 30 and 40 watts of light per square foot, whereas a weed plant in the final flowering stage will need up to 60 watts per square foot, or even more.

Generally speaking, 50 watts per square foot of space in your grow-op is the preferred light size/strength needed. So, for example, for a garden that is 2 by 4 feet large, you would need a 400 watt light. However, keep in mind that different types of lights have different strengths. A 400-watt HID light will have the same lumen output as an 800-watt fluorescent bulb.

On the note of lumens, younger plants require at least 2,500 lumens of light per square foot, while flowering plants need as much as 7,500 lumens per square foot, or even more. For this purpose, you need to read the label on the light bulb in question to figure out exactly how many lumens it puts out in a given space.

Other factors to consider

  • If you have lots of reflective materials in your grow room, you won’t need lights that are as big or strong because the reflective surfaces will pick up the slack
  • Generally speaking, the types of lights that get really hot need to be smaller and well vented so they don’t heat up the grow area too much. For this reason, going with a bulb that doesn’t heat up too much is the ideal because you can still go with a bigger one without heating up the area too much


The size and strength of your grow light will determine your overall success and the final yield, so be sure to get the right one.

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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