Grow Light Coverage Calculator

Grow lights are expensive gear for gardeners, but no one tells them how much they could yield. Luckily, there’s a scientific way to find out. The Grow light coverage calculator is a handy tool to do it easily.

By matching the grow light’s potential to plant needs, we can compute how many plants we can grow with any grow light.

This calculator does it for you.

Here’s how to use it and how it works.

How to use The grow light Coverage calculator works.

The calculator has information about the popular grow light models in the market and plants usually grown with grow light. It uses this information to calculate how many plants you can grow.

You need to select the grow light model you have (or are planning to buy) and the plant you’re growing. Then, hit compute to calculate precisely how many plants you can grow. The calculator will also tell you why it says this number. If you’re interested, you can do the math to confirm it.

What if I don’t have the grow light model?

We update the grow light model list continuously. But the market is rapidly growing, and your model is probably not in the drop-down list.

Don’t worry. If you know your grow light’s power, efficacy, and coverage, you can manually input them.

Select the “I know the specs” option in the grow light selector drop-down menu. This is the first option on the list. Three more fields will appear. You can manually enter the values for wattage, efficacy, and coverage.

You can get this information from the grow light spec sheet, brochure, or the manufacturer’s website.

Make sure you use the correct units. Efficacy is measured in μmol/j. The manufacturer may also call it PPE (Photosynthetic Photon Efficacy). In the same way, coverage should be entered in square feet. So if the spec says 2’X2′, you should input 4.

Sometimes you might prefer to use regular T5 fluorescent lights to grow seedlings and microgreens. But they come in many sizes. Also, their PPE may vary band to brand.

Related: Why Do You Need Grow Lights [Not Regular Lights]

What if the plant I want to grow isn’t on the list?

You can grow something that is not very popular with grow lights. If you can find the plants’ light needs, you can manually enter them. This is measured in mol/m²/day and known as DLI (Daily light integral)

Finding the DLI would be very tricky. You might have to sift through research papers to find legible values for a plant’s DLI. This is how we’ve determined the values for the different plants. For instance, this research paper, finds out that the optimal DLI for lettuces (iceberg) is 11.5 mol/m²/day.

But you can use these values for a general estimate.

• Leafy greens & herbs: between 12 and 20 mol/m²/day
• Fruiting plants: between 15 and 25 mol/m²/day
• Flowers: between 5 and 12 mol/m²/day

However, these general estimates are not always accurate. For instance, some herbs, like mint and ferns, grow under low-light conditions. In the same way, the light requirements for greens, fruiting plants, and flowers vary from plant to plant.

Related: These Hydroponic-Friendly Plants Need Minimal Light to Thrive.

Example: How many lettuce heads can you grow with Spider Farmer SF1000?

Step I: In the hydroponic way calculator, select Spider Farmer — SF1000 in the grow light drop box.

Step II: Select lettuce in the plant selector.

Stpe III: Hit compute.

You’ll see that you can grow 16 plants. But you can also see a warning that the light it produces is too much. We should dim the light to 40% or reduce the photoperiod to 7 hours.

How does it work?

SF1000 is a popular grow light with an efficacy of 2.5 mol/J.

Every joule of energy you give this grow light becomes 2.5 µmol of photosynthetic active radiation (PAR). Since this grow light has a power of 100W, it can produce 250 µmol of PAR every second. It would have produced (3600*250) 900000 µmol or .9moles of PAR in an hour. If we had kept the lights on for 16 hours, it would have produced (.9*16) 14.4 moles of the same.

The PAR produced has little meaning without considering how much area it covers. Spider Farmer has mentioned that SF1000 has a coverage of 2’X2′ for vegetative growth. Let’s assume we’re following their guidance. Thus, the four sq ft area, converted to m² (.376), is the effective growing area receiving all the 14.4 moles of light. Thus, the growing floor has a photosynthetic photon field density of (14.4/.376) 38.29 moles/m²/day.

Lettuce only needs 11.5 moles/m²/day. Anything beyond 18 moles/m²/day may cause light stress and eventually kill the plant. Thus, the light intensity needs to be reduced. In other words, the grow light needs to be dimmed.

Since we have the option to grow anyway, the only issue now is how many heads of lettuce we can grow. Since a regular lettuce head would take up about one-fourth of a square foot, and the grow light covers four square feet, we can grow 16 heads of lettuce in this space.