How to calculate grow light coverage | Herbal House NZ (2024)

by matthew monk September 28, 2022

When preparing your grow space, ensuring you have enough lighting to cover the area is crucial. The aim is to give your plants as much light as possible and this can be done in two ways:

- Calculating how many grow lights you’ll need for the size of your plant canopy

- Keeping plants away from sides of the grow space where shadows and low levels of light can occur

Calculating how many grow lights you need is a little bit more complicated than just measuring the size of the room or grow space. Really, you only need to be calculating your plant’s canopy to ensure your lighting covers all areas of the plants – leaves and all!

How to calculate the light coverage before the popularity of LED grow lights

Before the popularity of LED grow lights, calculating how many grow lights you needed was easy: It was simply watts/square foot x your plant’s canopy size.

For example: As a general rule of thumb, plants that have flowers typically need around 20-30 watts of light per square foot. If the size of your plant’s canopy was 4’ x 4’, this would mean you’d need around 320-480 watts.

As you may already know, LED grow lights don’t work in the same way as traditional lighting. Which means basing your calculations solely on wattage may give you a disappointing result.

Does the 40-watt rule fit for LED grow light?

Some people swear by the ‘40-watt rule’ which is 40 watts of light per plant. However, with LEDs, wattage doesn’t correctly express the efficiency of the light and therefore may not optimize your plant’s growing ability. To make it even more complicated, different LED designs (COB, SMD and Quantum Boards) will all have different coverage calculations.

Wattage doesn’t reflect grow light spectrum or intensity

It’s also key to remember that wattage doesn’t reflect the light spectrum or intensity of the grow light, either. You might have found what you think is the right light based on wattage, but find the light spectrum only covers certain phases of the growth cycle.

How to find the LED grow light coverage?

The first thing you need to consider is the grow space, and how many plants you can fit in it comfortably – giving the plants plenty of space around them to grow and expand.

Then, you should be able to find a few different grow light options that match the coverage you need (coverage info can be found in our product descriptions)..

Below, you can find the coverage figures for some of our most popular lights. If you’re looking for something a little different, you might be able to find a grow light coverage calculator online. Alternatively, feel free to contact us:

Calculating 250-watt HPS Grow light coverage

HPS grow light coverage is a little different. Because they’re becoming less and less popular for hobbyist growers. That said, Jorge Cervantes (a renowned grower) has created the following rules for HPS coverage:

• 175w: 2’ x 2’
• 250w: 3’ x 3’
• 400w: 4’ x 4’
• 600w: 4’ x 4’
• 1000w: 5’ x 5’ or 6’ x 6’

These are some of the guidelines for grow light coverage. But, once again, if you’re not completely sure, we’re always on hand to provide advice!