How to Calculate Grow Light Coverage Area 2021 Update

Light Coverage

Multiple HPS grow light setup in indoor grow room.Knowing how much light your plants need isn’t as simple as choosing any light and then hoping for the best. You’ll have to take into account two important factors:

Plant Coverage — This is pretty self explanatory and means the amount of light each individual plant will receive.
Canopy Coverage — This type of coverage refers to the light that will be spread over the entire group of plants in your grow tent, otherwise known as the canopy.
Whether you’re going with plant coverage or canopy coverage, it’s essential to know how much PAR or lumens you’re going to need for optimal growth. Remember:

Lumens measure the intensity of an HID light, which includes HPS grow lights, MH grow lights, and CMH grow lights.
PAR, or photosynthetically active radiation, is used to measure the intensity of LED grow lights.
You might be asking yourself, “Why do I need to factor lumens/PAR? Can’t I just use a certain number of watts per square foot in my grow room?” While you could just base the amount of necessary light for your plants on the size of your grow room or grow tent, doing so won’t give you enough light intensity for all of your plants.

Plants require a certain amount of lumens or PAR to achieve optimal growth.
As such, it’s best to figure out the exact amount of lumens/PAR the specific plants you’re growing will need.
Notice that we're ditching the whole "x-watts/per sq.ft." method. Now, the coverage area of any given light will have varying PAR/lumens the further out from the center you go. That's why if you have a large growing area and need to use multiple lights, PAR and lumens is going to be especially handy.
This will allow you to figure out how much light they need more accurately than just trying to get a certain amount of wattage per square foot.

Calculate your grow light requirements first

Calculating your grow lighting requirements really is just two steps - determine your plant’s lighting needs (watts/square foot), and your plant canopy dimensions.

A common mistake new growers make is measuring the dimensions of the grow room itself.

What you really should be measuring is the footprint of your plant canopy.

For example, if you are growing in a 5’ x 5’ space but your plant canopy only takes up 4’ x 4’, you don’t need a grow light capable of lighting the 5’ x 5’ space - it would be overkill.

How to calculate grow light wattage requirements

Most flowering plants grow best with 20-30 watts of light per square foot. Don’t make the mistake of thinking more is better. Too much light can lead to bleaching and burning your plants.

Let's say you are growing in a 5’ x 5’ grow space. If your plants need 20-30 watts/square foot, and your canopy takes up 4’ x 4’ of that space, you will need 320-480 watts of grow lighting.

How to calculate LED wattage requirements

If you are planning on using LED grow lights, there are a few things you need to know about calculating your grow light coverage.

Some manufacturers have two different wattage specifications - displayed wattage and actual wattage (power draw).

Because LED grow lights operate so efficiently, manufacturers will display a wattage equivalent to HPS. For example, the Gavita LED 1700e is 646 actual watts, but is equivalent to a 1000 watt HPS grow light.

So, if your 5’ x 5’ growing space requires 320-480 watts of grow lighting, and you choose the LED route, make sure your LED light features an actual wattage of between 320-480.

Calculate HPS or CMH Grow Light Coverage Area:

If you’re planning to use an HPS or CMH grow light, which I only recommend for tight budgets or very large grow spaces, then you’ll have a more difficult time calculating the grow light coverage area. That’s because most HPS grow lights don’t provide coverage charts or coverage area recommendations like LED grow lights do.

With that said, the rule of thumb for HPS grow light coverage area, according to renowned grower Jorge Cervantes, is the following:

How to make your HPS or CMH grow light coverage more efficient:

Use multiple smaller sized grow lights
Use wing style reflectors
When growing in a large area with HPS or CMH grow lights, you can make your grow light coverage more efficient by using multiple smaller lamps compared to one large lamp.

You should also ensure you’re using an open wing style reflector instead of a hood reflector. The wing reflectors spread the light more efficiently than hood reflectors, increasing your HPS or CMH light coverage.

Instead of using one 1000 watt grow light for a 5×5 or 6×6 foot space, try using three 400 watt or 600 watt grow lights in the same space. The benefits are a more uniform distribution of light, and you’re able to place the smaller lights closer to the cannabis plants, leaving them more room to grow.

How do I calculate grow light coverage?

 When Trying to Calculate Grow Light Coverage:

First ask what size grow space you’ll be growing in. I recommend 2×2 feet of space per plant, but there are exceptions to that rule. You can train one single cannabis plant to fill a 3×3 foot grow tent, or you can fit two smaller plants in there. Figure out how you’ll be growing and how much space it’ll require.

Many grow lights are designed to light a specific sized space. For example, ECO Farm grow ligh models that light 2×2 feet, 2×4 feet, 3×3 feet, all the way up to 5×5 feet. It’s not a coincidence that those are the exact sizes grow tents are offered in. Find the model or combination of grow light models that fit your grow space.
 
In order to best calculate grow light coverage, you should find the coverage chart for the grow light you’re considering, and compare it with the size of your grow tent. Take a look at the example below:


This is an example of a 4×4 foot coverage chart provided by a grow light manufacturer. It’s a textbook example of the type of light coverage you’d want in a 4×4 foot grow tent. The light intensity, measured in micromoles, is strong even in the far corners of the measured 4×4 foot area.
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