The most frequent question I’m asked by indoor growers is what distance above plants to hang a grow light. The short answer is “it depends,” but I lay out the science of it below.

Whether you’re growing cannabis or any other plant that provides fruit or vegetables, the plants require a certain amount of light per day.

The amount of light received is affected by the grow light’s distance above plants.

Science says when you double the distance between a plant and a light source, the light intensity is reduced to 25% the original strength! We can use this to help determine the proper distance to hang grow lights above plants.

Grow Light Distance Chart for HPS / CMH / HID Grow Lights, With Lights at 100% Power:

Above is a grow light distance chart for HPS, MH and CMH grow lights. These are the old style grow lights that use a lot of power and generate more heat than LEDs.

Please note that the distances listed above are assuming the grow lights are running at 100% power. If you dim the light you would be able to hang it closer. How much closer is determined by how much the grow light is dimmed.

Recommended LED Grow Light Distance Above Plants:

Please note that some LEDs like those on the current Mars Hydro FC and Spider Farmer SE grow lights are able to operate at very close distance to plants, such as 10 to 14 inches.

For the safety I did not list any close distances on the chart above because it only applies to a few grow lights. Check your grow light manufacturer website upon purchase to verify recommended distance above plants.

If you’re going to use the recommended grow light hang distances in the chart above, make sure to monitor your plants for signs of heat stress or stretching during the first week or two, and adjust the hanging distance if needed.

Cannabis Seedling Grow Light Distance:

The grow light distance for cannabis seedlings will vary by grow light. The target PPFD (light intensity measured at the leaves) for seedlings is 200 to 250 micromoles.

You can learn how to calculate PPFD by reading my article about it here. Or you can reference the grow light distance chart above for a rough estimate based on the wattage of your grow light.

You can also check the grow light manufacturer website to see if they have a recommended hang height, which will likely be similar to the one listed in the chart above.

Vegetative Growth and Autoflower Grow Light Distance:

The grow light distance for autoflowers and plants in the vegetative growth phase will vary by grow light. The target PPFD (light intensity measured at the leaves) for autoflowers and vegetative growth is 300 to 600 micromoles, assuming an 18 hour light period.

You can learn how to calculate PPFD by reading my article about it here. Or you can reference the grow light distance chart above for a rough estimate based on the wattage of your grow light.

The grow light distance for vegetative growth and autoflowers is the same, because they both grow in 18 hours of light per day.

If you plan to grow on a light schedule different than 18 hours per day, read the article linked above to learn how to calculate the different target PPFD. Longer duration light exposure would require a lower intensity of light, and vice versa.

Flowering Phase Grow Light Distance:

The grow light distance for plants in the flowering growth phase will vary by grow light. It’s fairly safe to assume 18-24 inches distance for grow lights in the 200-400 watt range. Above that power level you’ll be in the 24 to 30 inch range.

Keep in mind a few modern grow lights like the Mars Hydro FC and Spider Farmer SE series are able to be hung at much closer distance due to the design of the LEDs. But this may not be the case for other brands, so I’d recommend using the chart above or checking the manufacturer’s website.

The target PPFD (light intensity measured at the leaves) for autoflowers and vegetative growth is 460 to 920 micromoles, assuming an 12 hour light period. It can be even higher than that, around 1200-1400 micromoles, if you plan on using CO2.

You can learn how to calculate PPFD by reading my article about it here. Or you can reference the grow light distance chart above for a rough estimate based on the wattage of your grow light.

T5 / T8 Grow Light Distance Above Plants:

T5 and T8 bulbs can vary widely in the power output, and therefore the light strength and the hanging distance above the plants can vary. For example, a fixture with two bulbs can output 60 to 100 watts. But a fixture with four or six bulbs can output two to three times at much light.

Low power T5 and T8 grow light setups can be hanged 6 to 10 inches above plants. Higher power T5/T8 grow light setups can be hanged 12 to 14 inches above plants.

How High Should Grow Lights Be Above Cannabis Plants?

Determining the proper distance above plants to hang LED grow lights is a bit more complicated – many have different wattage, and some even have intensifying lenses on top of the LEDs.

For simplicity sake I’ve created the grow light distance chart above by surveying the hang distance required for the most popular LED grow lights of specific wattages, because I know nobody wants to read through this entire article.

The way to properly determine what distance an LED grow light should be from a plant is to determine the light strength at a certain hanging height, and compare that light strength to the light strength required by cannabis plants.

You always want to make sure the plant is receiving the required amount of daily light described in the article linked below.

You can read more about light strength required by cannabis plants in my post here.

When To Adjust Grow Light Distance From Plants?

Increase Light Distance If Plants Are Growing Too Close To The Grow Light:

When your plants mature and grow closer to your grow light, you’ll need to either dim the grow light or adjust the distance from the plants. If you don’t adjust the distance or dim the grow light, you’ll risk burning your plants.

Increase Grow Light Distance If Plants Look Heat Stressed:

You need to adjust your grow light’s height above the plants if the plants are heat stressed. Heat stressed leaves appear to be curling upwards towards the light. Immediately increase the distance between your grow light and your plants if you see this happening.

Decrease Grow Light Distance If Plants Are Stretching:

Plant stems stretch when the distance from the grow light is too far. The plant intentionally stretches its stalk and stems in order to get closer to the light source and receive more intense light.

Genetics can play a role in plant stretching as well, but if you notice this happening, you should hang the grow light closer to the plants. Remember every 12 inches closer to the grow light increases intensity 75%.

While this picture is an extreme example, ideally cannabis plant node spacing should be as tight as possible. If you experience significant stretching, you’ll likely need to top the plant to prevent it from growing too large.

Use The Inverse Square Law To Determine Grow Light Hang Distance Above Your Plants:

The inverse square law is a physics law that can help you to determine how intense your grow light is at a specified distance from your plants, which will help you determine whether you need to hang the grow light closer or further away from your plants.

The inverse square law of light applies to grow lights in that when a plant’s distance from a grow light doubles, the area covered in light increases, but the intensity of the light decreases to 25% intensity of the original distance.

Notice the intensity at 2 meters in the image above is 25% of the intensity at 1 meter. The intensity at 4 meters is 25% of the intensity at 2 meters. This will hold true for your grow light intensity at a distance from 6 to 12 inches, 12 to 24 inches, 24 to 48 inches, etc.

Even though the distances in the image above are meters, the law holds true for any distance. If your starting distance is 1 foot, the intensity will be 25% at 2 feet, and 11% at 3 feet. The intensity will be 25% of the 2 foot reading at 4 feet, and 25% of the 3 foot reading at 6 feet.

Example Of Inverse Square Law To Determine Grow Light Distance From Plants

You only need to know one intensity reading at one distance to determine another intensity reading at another distance.

Say your grow light manufacturer states that your grow light emits 1000 micromoles of light at 18 inches above the plants. You know a more appropriate intensity for your seedlings is 250 micromoles.

You would then double your hang height from 18 to 36 inches, and the light intensity would be decreased to 25% (250 micromoles) of 1000 micromoles, based on the law of inverse squares.

The mathematical formula is Intensity = Light Output/Distance².

Answer These Questions To Determine A Proper Grow Light Hang Height:

If you don’t want to use the inverse squares law or manufacturer recommended height, there are a few main questions to answer in order to figure out the proper grow light height for your grow space. You need to Measure, Convert and Compare.

What amount of light intensity do your plants receive at each height?

For LED Grow Lights:

You need to figure out (roughly) how much light your plants are receiving, and compare that number to what they should be receiving.

Most manufacturers provide light intensity measurements at a few relevant heights above plants. If you can’t find these manufacturer provided light intensity measurements, you can buy a lux or PAR meter and measure it yourself.

Unlike PAR meters Lux meters are very inexpensive. You don’t need a PAR meter because you already know the light is within the PAR spectrum range.

Purchase a Lux meter and measure how much Lux your plants are receiving at the current distance from your grow light.

Then measure the number of Lux and convert it using the Waveform Lighting calculator. Insert the number of Lux, then select Red + Blue + White LED, and run the calculation. You’ll end up with a light intensity number measured as umol/second.

How many micromoles of light do cannabis plants need during each growth phase?”

• Plants in the seedling phase require between 200-300 uMol/second.
• Plants in the vegetative phase receiving 18 hours of light should receive between 306 and 617 uMol/second.
• Plants in the flowering phase receiving 12 hours of light should receive between 460 and 925 uMol/second.

If your plants are currently receiving light intensity within the above range, you do not need to adjust the height of your grow light. Below this range would require moving the grow light closer to the plants. Above this range would require moving the grow light further away from the plants.

Remember, higher light intensity = higher yields. To read more about PPFD and light intensity levels required to grow cannabis, read my full article on the topic here.

For CMH and HPS Grow Lights:

People using CMH or HPS grow lights ask me about Lux and what Lux intensity is good for plants.

Here’s my advice for people using CMH or HPS grow lights: Purchase a Lux meter and measure how much lux the plants are receiving at each height.

Next, you need to convert your Lux reading to micromoles. Luckily, Apogee Instruments, the manufacturer of the light intensity meter created a conversion table for converting Lux to micromoles.

At 50,000 lux, a double ended HPS grow light emits 650 uMols of light per second and about 28 Moles of light during a 12 hour flowering light cycle, or 42 moles of light during an 18 hour vegetative phase light cycle.

That’s more or less on target with what cannabis plants need to grow. So if you’re getting this reading from your HPS light at 30 inch hang height, you’re in a good position. Close to or below 20 moles and you’ll want to lower the lamp until you reach a number up to 40 moles.

Here’s the math: (Your Lux reading * .013 Lux to PPFD conversion rate * 60 seconds in one minute * 60 minutes per hour * hours in your light cycle = Moles of light per day).

Is your CMH or HPS grow light intensity within this range?

• Plants in the seedling phase require between 200-300 uMol/second.
• Plants in the vegetative phase receiving 18 hours of light should receive between 306 and 617 uMol/second.
• Plants in the flowering phase receiving 12 hours of light should receive between 460 and 925 uMol/second.

If your plants are currently receiving light intensity within the above range, you do not need to adjust the height of your grow light. Below this range would require moving the grow light closer to the plants. Above this range would require moving the grow light further away from the plants.

Remember, higher light intensity = higher yields. To read more about PPFD and light intensity levels required to grow cannabis, read my full article on the topic here.

Every grow light’s intensity at a given height is different, that’s why you need to determine these numbers by looking at the manufacturer provided chart or by using a Lux meter. If every grow light was the same, I’d be able to tell everybody to hang their light “xx inches from the plants.”

If your grow light is within the above mentioned range, there’s a good chance it’s hanging at the right level. But there are two more questions to answer to determine what height to hang your grow light.

Is your grow light dimmable?

If your grow light is dimmable, you may be able to dim the light instead of adjusting the light’s distance from your plants. Again, the most critical aspect of this whole process is having a Lux meter handy so you can measure, convert and compare the light intensity levels to the actual light intensity levels your plants need to grow.

If your grow light is dimmable, you can usually adjust the light intensity via a screw or knob on the ballasts.

Another Video About Grow Light Hang Height:

Good Luck! Want to learn more home growing tips? Check out my Cannabis Watering Guide, How to calculate grow light coverage, and How to lower temperature in your grow tent.

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