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.

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 identifying 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 legth 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 an 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.