How Much Light (PPF) Do You Need for Indoor Cannabis?

How much light do you need for cannabis

To set up an efficient home cannabis grow, it is important to determine the optimal size of your grow light. With High Intensity Discharge (HID) lights, such as High-Pressure Sodium (HPS) and Ceramic Metal Halide (CMH) it is possible to use wattage to determine how much light you need for your grow tent. However, LED grow lights can now be significantly more efficient than HID lights, so the wattage rules no longer apply. In this article, we discuss why it is important to match the size of the grow light with the size of the tent, we explore the science of cannabis photosynthesis to determine how much light cannabis plants can use, and we explain how to use Photosynthetic Photon Flux (PPF) to determine the optimal grow light size for your grow tent. We include our Grow Space Calculator to help you to determine the optimal amount of light for your space and estimate the harvest potential.

This article is part 3 of the Coco for Cannabis Grow Set-up Guide.

The Coco for Cannabis Grow Set-up Guide
  1. Grow Tents and Harvest Sizes
  2. How to Evaluate and Compare Grow Lights for Cannabis
  3. How Much Light (PPF) Do You Need for Indoor Cannabis?
  4. Grow Light Calculator (Coming Very Soon)
  5. Airflow, Ventilation and Exhaust Systems for Grow Tents (Coming Soon)
  6. Managing Heat and Humidity in your Grow Tent (Coming Soon)

Matching the Grow Light to the Grow Space

When setting up your indoor cannabis grow, we recommend that you start by thinking about the yield that you would like to be able to harvest each cycle. As we explain in our guide, "Grow Tents and Harvest Sizes", the yield of each grow is limited by the space, so your yield goals should determine the size of your grow tent. The size of your grow tent then determines the amount of light that you need.

You can grow cannabis plants under small lights or large lights. Many growers use less light than they could and still produce decent harvests. However, the efficiency of the grow and the quality of the harvested cannabis is best when the grow lights are matched to the grow space.

Not Enough Light Produces Larf

When the light is insufficient for the space, it can result in lower quality cannabis and more work trimming. Large plants that receive inadequate light will produce a lot of low-quality buds that we call "larf". Many growers mistakenly think that larf is the result of budding sites not receiving light. In reality, larf is the result of a plant that, in total, has more budding sites than energy to develop them. If the plant receives less than optimal light and has a large number of budding sites, it will produce larf.

Too Much Light Is Damaging or Wasteful

It is even more important to avoid giving the plants too much light. As we explain below, there is a limit to the amount of light a plant can use, and excessive light will cause damage. If you have too much light, you could avoid damage by raising or dimming the light. Raising the light wastes energy and reduces efficiency. If you must dim the light, then you are not taking full advantage of your investment. In either case, you would save money and be more efficient if you had lights that were properly matched to the space.

How Much Light Do You Need?

There are various recommendations for how to determine the correct amount of light, however many of them are outdated, not applicable, or based more on marketing than science. We advise you to ignore the manufacturer's claims about coverage area and instead focus on scientific measurements about the amount of light.

Using Wattage to Determine the Amount of Light

With HID lighting (HPS and CMH) it is possible to use wattage to estimate the amount of light. This is because all HID lights are similar in terms of photon efficiency: they convert electricity into usable light at about the same rate. With HID lighting, the rule is about 40 watts per square foot (430 watts per square meter). This means that a 600-watt HID light is adequate for a 4' x 4' tent and a 1000-watt HID light is perfect for a 5' x 5' tent.

LED and HPS Equivalency

High Pressure Sodium (HPS) have traditionally been the preferred HID light for flowering cannabis plants. When LED grow lights came into the market, many manufacturers tried to sell their lights by claiming a "HPS equivalency". Since HPS lights were traditionally measured in terms of watts, LED lights came to be marketed with an "equivalent wattage". However, there has never been a standardized way to make these "equivalencies". Each manufacturer comes up with their own metrics to establish them and most are gross exaggerations. We advise you to completely disregard the manufacturer's claims about "HPS equivalency" or "equivalent wattage" and focus instead on the PPF that each fixture produces.

Using PPF to Determine the Amount of Light

Photosynthetic Photon Flux (PPF) measures the total amount of Photosynthetically Active Radiation (PAR) that a fixture can deliver to the plants. As such, PPF is the proper measurement to determine the total amount of light needed for a grow space. Please see our article, "How to Evaluate and Compare Grow Lights for Cannabis" for a complete discussion of PAR, PPF, and Photosynthetic Photon Flux Density (PPFD).

The PPF rule is 65µmol (PPF) per square foot or 700µmol (PPF) per square meter. To understand why this is the optimal target for the amount of light, it is useful to review cannabis photosynthesis and consider how much light cannabis plants can use.

How Much Light Can Cannabis Plants Use?

It is common to hear that "more light is better" and since many home growers use insufficient lighting for their space, it is often true. However, there is a limit to the intensity of light that cannabis plants can use. If you expose plants to more light intensity than they can use in photosynthesis, it will not increase yield. In fact, too much light can reduce both the yield and the quality of the harvested cannabis.

The rate of photosynthesis and photosynthetic efficiency can be limited by several factors including carbon dioxide, light intensity, temperature, oxygen, water, minerals, age, leaf anatomy and more. In many grow tents, light is the limiting factor. However, as you increase the intensity of the light, other factors like carbon dioxide will become the limiting factor. When photosynthesis is limited by any factor other than light, the leaves reach their light saturation point.

Light intensity that is beyond the saturation point dictated by photosynthesis can damage plant tissue. Therefore, when leaves reach their saturation point, the plant will attempt to protect itself with photoprotection responses. These include things such as chlorophyll or leaf movement, anatomical changes, non-photochemical quenching and thermal dissipation. All these photoprotection efforts by the plant waste energy and can lower yield.

If the plant cannot adequately protect itself from excessive light using photoprotection responses, it will begin photoinhibition. Photoinhibition decreases the rate of photosynthesis and reduces growth and harvest potential. However, symptoms of light stress do not become apparent if the plant is able to cope with the excessive light. Symptoms such as chlorosis occur only when photoinhibition can no longer effectively protect the plant.

Cannabis Photosynthesis: Carbon Dioxide and Light Limits

There are many areas of cannabis science that have not yet had adequate research. Fortunately, photosynthesis is one of the exceptions. In 2008, Chandra et al. published extensive research into cannabis photosynthesis. The data they provide offer the most accurate measurement of how much light cannabis plants can use.

The data from Chandra et al. show that cannabis plants are like many other terrestrial plants. In ambient concentrations of carbon dioxide (CO2), cannabis leaves begin to saturate when the light intensity is 500µmol (PPFD). The limiting factor is CO2. This shows up in the data as the concentration of CO2 within the leaves drops when the intensity of light is above 500µmol. Additional light at this point produces diminishing returns, but it will lead to more photosynthetic activity. However, there is a limit. Cannabis plants begin photoinhibition when the intensity of the light reaches 1000µmol (PPFD). Additional intensity, beyond 1000µmol (PPFD), will lower the rate of photosynthesis and can damage plant tissue.

These limits are largely dictated by the concentration of CO2. Ambient CO2 levels are around 370µmol mol. When CO2 levels are higher, cannabis plants can process more light energy before they become limited. The data from Chandra et al. show that when CO2 concentrations are 750µmol mol, cannabis plants can perform well at a light intensity of 1500µmol (PPFD) without inducing photoinhibition. This allows larger harvests from the same amount of space. However, successfully increasing the concentration of CO2 in the grow space requires sealing the space. The costs of setting up and running a sealed grow space are considerable. Most home growers are better served by using a ventilated grow space and accepting the limits imposed by the ambient levels of CO2. See our guides, "Airflow, Ventilation and Exhaust Systems for Grow Tents" and "Managing Heat and Humidity in your Grow Tent" coming soon.

Optimal Grow Light Size for Cannabis

The data from Chandra et al. confirm that the optimal intensity of light for peak cannabis photosynthesis is between 500 and 700µmol (PPFD). Because the distribution of light is always less than perfect, it makes sense to use the higher figure of 700µmol (PPFD) as the target. This provides the best opportunity to get at least 500µmol (PPFD) to all areas of the canopy while avoiding exposing any area of the canopy to intensities above 1000µmol (PPFD).

700µmol (PPFD) is our target for intensity. To determine the total amount of light that we need in a grow tent we need to convert from PPFD to PPF. PPF is the total amount of light and PPFD is the amount of light that hits one square meter. A grow light that produces a total PPF of 700µmol is, in theory, able to deliver an intensity of 700µmol PPFD to exactly one square meter of canopy space. Lights with less than 700µmol PPF can still deliver an intensity of 700µmol PPFD, but they cover a smaller space. For example, a light with a total of 350µmol PPF could cover one half square meter with 700µmol PPFD intensity light. Similarly, lights that are more than 700µmol PPF can deliver 700µmol PPFD intensity to a larger space.

The Optimal PPF for Cannabis:

Since the optimal intensity is 700µmol PPFD, the optimal amount of light is 700µmol PPF per square meter. This converts to 65µmol (PPF) per Square Foot. To calculate the total amount of light that you need for your grow tent in PPF, simply multiply the square footage by 65 (Sq. ft x 65 = µmol PPF).

Accurately Estimating PPF

There are three ways that PPF is measured or calculated. PPF figures that are reported may be "Calculated Values", or they may be measured through "True Measurement", or "Field Measurement". There are significant differences between them, so to make accurate measurements and comparisons, it is important to understand what type of PPF values you are working with.

True Measurement PPF

The "True Measurement" is the value that is closest to what matters to us as growers. True measurements are done with sophisticated equipment that capture all light and direct it to a sensor for measurement. We use "True Measurements" for HID fixture PPF data. However, true measurements are rare in the LED market because they are very expensive to perform. You are more likely to find "Calculated Values" or "Field Measurement" data about PPF. To make accurate measurements and comparisons, we argue that it is important to adjust these other types of PPF data to an "Estimated True PPF".

Calculated Value PPF

Manufacturers often don't report PPF data at all. When they do report it, it is usually a "Calculated Value" and not an actual measurement. Calculated values are determined based on the diodes in the fixture and assume 100% efficiency. As a result, calculated values are typically significantly higher than a True Measurement of PPF. The degree of discrepancy varies from manufacturer to manufacturer. However, based on known examples of Calculated Values and True Measurement values for specific fixtures, the typical error is about 20%. Our Grow Space Calculator below shows you both the True PPF target and the Calculated PPF estimate which is optimal for different size grow tents or spaces.

Field Measurement PPF

The third type of PPF data that you may come across is "Field Measured" PPF. Field measurements are taken in the field or in a simple testing station with portable sensors. Because of numerous variables that can affect testing in a field setting, these data are usually less reliable than other forms of PPF data. However, with reliable sources, field measured data may be preferable to manufacturer's reported calculated values.

Based on known examples from reliable testing, we can see that field measurement PPF data is typically about 15-20% less than a True Measurement of PPF. Therefore, our Grow Space Calculator and our Grow Lights Calculator estimate that a field measurement will under-count PPF by about 15%. However, Field measured data from one source may be very different than field measured data from a different source, so this can only be a rough estimate.

Grow Space Calculator

Enter the size of your grow tent or space in our Grow Space Calculator below. It will estimate the PPF needs and harvest potential of your grow. To learn more about how we estimate harvest sizes, see our Grow Light Calculator (coming soon).

Grow Space Calculator

Grow Space

Light: Optimal PPF

Calculated: µmol
TRUE: µmol
Field Measured: µmol

Estimated Harvest

Low Benchmark
grams grams
oz oz
- grams
- ounces

Common Grow Tent Sizes
Dimensions Sq.Feet Calculated PPF Optimal True PPF: Low Harvest 0.45g/µmol Benchmark 0.65g/µmol
2' X 4' 8 624µmol 520µmol 234g 338g
3' X 3' 9 702µmol 585µmol 263g 380g
4' X 4' 16 1248µmol 1024µmol 468g 676g
5' X 5' 25 1625µmol 1950µmol 731g 1056g

Common Grow Tent Sizes

Tent Size

2' X 4' 3' X 3' 4' X 4' 5' X 5'

Square Feet

8 9 16 25

Optimal True PPF(µmol)

520 585 1040 1950

Calculated PPF(µmol)

624 702 1248 1625

Low Harvest

234g 263g 468g 731g

Benchmark Harvest

338g 380g 676g 1056g

The Next Steps

Once you determine the amount of light you need, it is time to choose your light fixtures. Be sure to read our guide, "How to Evaluate and Compare Grow Lights for Cannabis".

See the Grow lights that we recommend in our Equipment and Product Guide.

The Coco for Cannabis Grow Set-up Guide
  1. Grow Tents and Harvest Sizes
  2. How to Evaluate and Compare Grow Lights for Cannabis
  3. How Much Light (PPF) Do You Need for Indoor Cannabis?
  4. Grow Light Calculator (Coming Very Soon)
  5. Airflow, Ventilation and Exhaust Systems for Grow Tents (Coming Soon)
  6. Managing Heat and Humidity in your Grow Tent (Coming Soon)

Need Cannabis Grow Equipment?

Check out our Product and Equipment Guide for our Recommended Grow Lights, Grow Set-up Equipment, Germination Supplies and everything else you will need during the grow!

Get Support from our Community!

We have a wonderful community of grower's and you are welcome to join! If you are looking for advice and support throughout your grow, I encourage you to start a Grow Journal in our Grower's Forum. It is a great way to share your experience and ask questions along the way.

We also have a live chat room, which is a great place to ask questions, get quick answers, and chat with fellow growers. We welcome all growers who want to learn, share and grow together!



Author: Dr Photon and Dr Coco

About Our Product Recommendations

At Coco for Cannabis, our mission is to help growers maximize the success of their cannabis crops by providing scientifically accurate information and sharing proven growing practices. The products that we recommend are the actual products that we chose to purchase and use ourselves. We do not accept sponsorship or advertising and will not recommend any product that we would not buy for ourselves. Please see our Product and Equipment Guide.

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The information on should not be considered as financial, legal, or medical advice.
You are responsible for knowing and following the local laws that pertain to cannabis cultivation, possession, and use. Decisions to grow cannabis should be made in consultation with a lawyer or qualified legal advisor. Decisions to use cannabis should be made in consultation with your doctor or medical professional.