Calculate Grow Light Coverage – Best Method

Best Methods To Calculate Grow Light Coverage

One of the most common questions I get asked here at 420ExpertGuide is how to calculate grow light coverage area. Determining grow light coverage is one of the best questions to ask when you’re planning to grow cannabis or anything else indoors, because light is directly related to yields, and a weak light will leave you disappointed.

The best method to calculate grow light coverage is to determine what size space you’ll be growing in, and make sure the grow light you’re considering will thoroughly cover that space with sufficient light. Below Ill advise you how to do exactly that.  

Common Grow Light Coverage Questions:

People ask me all sorts of questions when trying to pick a grow light that will cover their grow tent or grow space properly. When trying to determine grow light coverage, you may be wondering questions like:

  • How many watts per square foot of space?
  • How many plants can I grow with a 300 watt grow light?
  • How much area does a 1000 watt grow light cover?
  • What’s the best grow light for two, four, or six plants?
  • How many watts per plant?

Even though these may be among the first questions to enter your mind, I think you need to 

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For example, I’m aware of the 40 watts per plant rule. But watts are no longer an efficient representation of a light’s ability to grow plants. LEDs have different efficiency rates, and different designs (COB, SMD, quantum boards) which all lead to different outcomes.

In addition, watts don’t mean much if the spectrum isn’t right, or if the light isn’t intense enough to flower your plants. You can grow great plants in a 4×4 tent with a 450 watt LED grow light or a fancier 600 watt LED grow light. So which is it?

The Right Questions to Ask 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.

Once you know you’ll be growing in a 2×4 foot or 4×4 foot grow space, the next step is to look at grow lights for those spaces. I wrote articles suggesting my top picks for 2×4 and 4×4 grow tents. So you can skip the work and choose one of those, or continue to scout out a light on your own.

Once you find a grow light, the next question to ask is:

“Does the light cover my entire grow space with light that’s strong enough to grow cannabis?”

There’s a difference between lighting a grow tent, and lighting a grow tent with strong, sufficient light from corner to corner. I wrote all about this in my guide to Understanding Grow Light PAR, PPFD, Wattage, and DLI.

Many grow lights are designed to light a specific sized space. For example, Mars Hydro makes grow light 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:

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4x4 coverage

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.

Let’s take a look at a grow light coverage chart that would be inefficient:

ppfd chartThis grow light manufacturer falsely claims this grow light will cover 4×4 square feet. But as you can see, the red circled numbers are not even enough to get a cannabis plant through the vegetative phase. You might be thinking you can simply lower the grow light and it’ll become stronger. You’re right, but the coverage area would shrink to smaller than 4×4 feet.

What if you already own a grow light and need to calculate its coverage area?

In order to calculate the coverage area of a grow light you already own, you can simply look up the manufacturer provided grow light coverage chart. You can also purchase a PAR meter or LUX meter and measure the light intensity across your grow space. Then compare it with what your plants actually need.

My previously mentioned article about PAR and PPFD describes how many micromoles of light you need, and how you can convert LUX to micromoles if needed.

What if the grow light I’m considering doesn’t provide a coverage chart?

If the grow light you’re considering doesn’t provide a coverage chart, and it’s a low cost light, it’s possible you’re considering an inefficient grow light. Manufacturers of low end grow lights will avoid providing these charts because nobody would buy the grow light if they did.

I recommend sticking to reputable brands like Mars Hydro, Viparspectra, or Spider Farmer. Any brand less expensive than these are less expensive for a reason.

If you’re dead set on buying a more inexpensive grow light model, I encourage you to take a look at the product reviews on Amazon, or on blogs like 420ExpertGuide to determine what amount of space the grow light can be used for.

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:

2×2 feet:3×3 feet:4×4 feet:5×5 feet:6×6 feet:
175 watt 250 watt400 watt1000 watt1000 watt
600 watt

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

  • Use multiple smaller sized grow lights
  • Use wing style reflectors
  • Use an automated Light Mover

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.

You can also consider using a light mover. It’s said using a light mover will increase grow light coverage area up to 25%. It also ensures you’re using your electricity efficiently.


Can you trust grow light coverage recommendations?

Many grow light manufacturers have a unfortunate tendency to exaggerate their grow light coverage claims. I believe it’s because the grow light market is flooded with competition, but also because not everybody is growing cannabis.

Fruit, vegetables, and flowers require less light to grow. Grow light manufacturers are thereby happy to increase the coverage space to meet those plants, but not cannabis. Some high quality large scale manufacturers such as the ones I listed above are more trustworthy in their coverage claims. This is why my grow light coverage recommendations are more conservative than the manufacturer.

The best way to determine whether the stated grow light coverage is accurate is to go ask science what’s right, and not the manufacturer who wants to sell you a grow light.

How do you ask science? You can find the answer by reading my guide to Understanding Grow Light PAR, PPFD, Wattage, and DLI. The science is thoroughly explained here – it’s worth your 10 minutes to read through.

Which grow lights do I recommend for a given coverage area? There are many choices but the models below are reliable and accurate in their coverage area claims. I would take comfort in owning any of these grow lights:

2×2 Feet2×4 Feet3×3 Feet4×4 Feet
Mars Hydro TS-1000Mars Hydro TSL-2000Mars Hydro TSW-2000Mars Hydro TS-3000
Spider Farmer SF-1000Green Sunshine ES300Spider Farmer SF-4000
Viparspectra P2500HLG 550 V2
Optic LED Slim 600H

If you have a space larger than 4×4 feet, you’ll need to combine multiple grow lights to meet your coverage requirements. Most LED grow lights won’t cover larger than 4×4 feet. The grow lights that do are very expensive and not worth the cost compared to combining the more affordable units mentioned above.

In summary, I highly recommend you take an extra 15-20 minutes of your time to learn how to calculate grow light coverage, and ensure the grow light you’re considering offers the proper coverage for your grow space.

How do I calculate grow light coverage?

To calculate grow light coverage, you need to ensure a grow light is strong enough to cover your entire grow space. You can do this by checking the manufacturer provided grow light coverage area chart, or by measuring the light intensity in your grow space and comparing it to what’s needed to grow your plants.


My name is Nick, and I'm a home grower who spends time writing about indoor growing tools and setups. My goal is for 420ExpertGuide to be a one stop shop for indoor growers to find the information they need.