How to Determine How Much Supplemental Light to Provide for Indoor Plants

Light is important for plant growth. Without it, there is no photosynthesis, and plants cannot grow and thrive. Light is often the most limiting growth factor when growing houseplants, starting seeds, or nurturing any plant indoors.

Plants under supplemental light — Once you have figured out the light requirement for your plants (DLI) and the type of supplemental lights you wish to use, you can determine the number of light fixtures needed to provide adequate supplemental light. ⁽¹⁾

Frequently the indoor environment does not have enough light for plants to grow and develop well. While indoor plants differ in their lighting requirements, nearly all would benefit from some supplemental lighting during the winter. Many indoor plants benefit from supplemental light year-round.

Once you've determined how much light your plants need (a target DLI) and what type of fixtures you wish to use (LED, fluorescent, etc.), you can build a supplemental light setup perfectly suited to provide the best light for your indoor plants.

The first step to building that light setup is to determine the number of light fixtures needed.

Determining How Many Lights Are Needed

There are a few approaches that home gardeners can take to determine how many lights are needed and how they should be set up including:

Trial and Error
Set Up and Calculate
Calculate and Set Up

Supplemental Light Stand — There are several approaches to determining the number of lights needed, but for most home gardeners it's easiest to set up the lights and observe the plants' growth. Then, make adjustments to the setup based on how the plants are growing. (The "Trial and Error" approach) ⁽²⁾

The best approach to take depends on:

How much information you have
How much time you want to invest
How accurate you wish to be
How many calculations you'd like to do

For most home gardeners the easiest approach is the "Trial and Error" approach.
With the right equipment, the "Set Up and Calculate" approach is a great way to get the best supplemental light conditions with less time spent making adjustments.
When utilizing grow light fixtures that provide important values like PPF, the "Calculate and Set Up" approach allows you to purchase the exact fixture(s) you need.

The "Trial and Error" Approach

To see acceptable growth and development, most indoor plants have a fairly wide range of acceptable light conditions. This means that for many home gardeners, a high level of accuracy or precision is not needed to grow indoor plants under lights successfully. Because of that, trial and error is the most straightforward (and least intimidating) option for home gardeners.

This approach involves setting up a supplemental light system using some general guidelines and then making adjustments based on how the plants respond.

Start by setting up lights utilizing one of the general recommendations below.
Monitor the plant's growth and development over the next several weeks. Adjust according to the recommendations below if acceptable growth is not observed.
Continue monitoring growth and fine-tune the setup until the plants show consistent, healthy growth.

General Recommendations for Light Setups

General Supplemental Light Set-Up Recommendations Using Fixtures Manufactured as Grow Lights
Purchase and install an LED or fluorescent grow light as outlined in the directions in the package and leave lights on for 14 hours a day.

General Supplemental Light Set-Up Recommendations Using Readily Available Fixtures Not Manufactured as Grow Lights
LED: Standard, 4-foot, plug-in shop fixture with full-spectrum (also called "daylight") light, placed 18 inches from the foliage providing at least 3,000 lumens with lights on for 14 hours a day.
Fluorescent: Standard, 4-foot, plug-in, 2-bulb shop fixture with each bulb being 40-watt, T5, high output, and full-spectrum (A.K.A. “daylight”) placed 9 inches from the foliage with lights on for 14 hours a day. (Alternative: Instead of "daylight," use one "cool white" and one "warm white" bulb)

How to Make Adjustments When Growth Under Lights is Not Acceptable

Symptoms Observed	Action Needed	Methods to Make Change
Excessive stretching or elongation Spindly growth Leaf drop Yellowing foliage Absent flowers (if there should be flowers)	Increase Light Intensity	Move the fixture closer to the plant Change the fixture or bulb to one that provides more lumens/higher wattage Install an additional fixture Utilize reflectors or side lights (particularly useful if symptoms are only on parts of the plant furthest from the light source)
Leaf scorch Bleached leaves Leaf drop Excessive reddening of foliage Extremely compact growth	Decrease Light Intensity	Move the fixture further from the plant Change the fixture or bulb to one that provides fewer lumens/lower wattage Remove an extra fixture or bulb
Excessive elongation or stretching Leaf drop Yellowing foliage Absent flowers (if there should be flowers)	Adjust the Wavelength of Light Available	Change the fixture or bulb to one that provides warmer colors (lower Kelvin value) or cooler colors (higher Kelvin value) Provide an additional bulb or fixture in a different color (warmer or cooler) Use a grow light or grow bulb instead of a traditional fixture Wavelength rarely needs to be changed when using grow lights
Absent flowers or fruiting (if there should be flowers/fruit)	Adjust the Day Length	Adjust the timer to turn lights on for more or less time without going less than 8 hours or more than 16 hours Ensure other lights in the area (room lights, lamps, street lights, etc.) are not "polluting" the dark period, preventing flowering

The "Set Up and Calculate" Approach

This approach can be straightforward for home gardeners who want more accuracy and do not want to take the time to see how the plants react over the course of several weeks or months. It is also helpful if you already have the light fixtures, have limitations on how the supplemental lights can be set up, or already have your lights in place.

Homemade setup with plastic shelves By MeganBetteridge AdobeStock_418224794 — The "Set Up and Calculate" approach is useful when you already have supplemental lights in place, and you want to make adjustments without waiting weeks or months to see how the plants will respond. ⁽³⁾

This approach involves setting up the lights, using a light meter to measure light intensity, using that measurement to calculate the DLI, and then making adjustments to get the light levels in the desired range.

Set up lights as desired or by utilizing the general guidelines above.
Measure the lux or footcandles produced utilizing a light meter placed at foliage level.
- Measure with a light meter or smartphone app
Calculate the PPFD by multiplying the lux (or footcandles) value by the conversion factor provided by the manufacturer or found in online tables or calculators.
- PPFD = lux × conversion factor
- Convert footcandles to lux by multiplying the footcandles by 10.764. (1 footcandle = 10.764 lux)
- Alternatively use a light meter that measures in PPFD. Be sure it's calibrated for your specific type of light (LED, fluorescent, etc.)
Calculate the DLI by multiplying the PPFD by the number of hours the lights are on and 0.0036.
- DLI = PPFD × # hrs × 0.0036

If the calculated DLI is too low or high, make adjustments and use your light meter to recalculate the DLI. Once you've got your DLI in an acceptable range, you are good to start growing.

Do a lot of the terms and concepts above look unfamiliar? Learn more in this article: Important Considerations for Providing Supplemental Light to Indoor Plant.

The "Calculate and Set Up" Approach

This approach is particularly helpful if you have not yet purchased your equipment, know the PPF value of the fixtures, and have a precise target DLI. Use these values to determine the supplemental light needs and then purchase and build your supplemental light setup.

Use your target DLI and the length of time your lights are on to determine the PPFD needed.
- PPFD = DLI / (# hrs × 0.0036)
Utilize your PPFD, the PPF value from the manufacturer, and the size of the growing area to determine the number of lamps needed.
- # of fixtures = (PPFD x growing area) / (PPF per fixture x CU)
  - CU is the typical degradation or light lost from fixtures to plants. It's usually around 70-80% (use 0.80 for this value if it's estimated)
  - Be sure the growing area is calculated in square meters (not square feet!). (1 m²= 10.7639 ft²)

This calculation may not be perfect because there are a few factors that can be hard to determine before the lights are set up such as the CU value. Additionally, the values will change based on the height and angle of the fixtures. This makes the size of the growing area difficult to accurately calculate. This is especially true if the lights illuminate an area larger than the square footage of your growing area.

What to Do When You are Missing Necessary Numbers?
The biggest limitation to this approach is that frequently all the necessary values needed to make the calculations are not available.

Don’t confuse "PPF Efficacy" with "PPF." Sometimes manufacturers provide the PPF efficacy for the fixture (measured in µmol/watt or µmol/joule). This value shows the efficiency of the fixture, not the amount of light provided by the fixture that is useful to the plant (aka PPF). PPF and PPF Efficacy are not interchangeable.

While lumens are frequently provided by the manufacturer, often the PPF value for the light fixture is not. Lumens and PPF are not equivalent - one measures the amount of light perceived by the human eye and the other measures the amount of light emitted that is usable by plants. There is no universal conversion factor to convert lumens to PPF because every fixture emits a different array of wavelengths.

You can approximate the PPF using the lumens, but it will only result in a rough estimation. However, for many home gardeners, this level of accuracy is acceptable. To estimate PPF, use an online calculator or divide the lumens by a conversion rate of 60. (Note: the conversion rate for different light sources can vary widely from as low as 40 to as high as 75 or 80. This makes using 60 an estimation, but it can give you a place to get started.)

Once you are finished with the calculations and have the setup built, it is good to double-check your work using the "Set Up and Calculate" approach and make some adjustments from there (if needed).

Do a lot of the terms and concepts above look unfamiliar? Learn more in this article: Important Considerations for Providing Supplemental Light to Indoor Plant.

Seedlings Under Lights — By determining the number of lights needed, you can ensure your seedlings grow to their full potential. ⁽⁴⁾

Key Terms & Ideas

Daily Light Integral (DLI) - the amount of PAR delivered over an area in a 24 hour period of time (mol/m²/day). DLI basically tells you how much photosynthesis can occur.
Footcandle - A unit of measurement that indicates light intensity defined as the number of lumens over one square foot. (1 fc = 10.76 lux)
Intensity - Refers to the amount of light produced or emitted. Intensity is measured in several different ways but the most useful for growing plants is PPF and PPFD.
Kelvin - A unit of measurement that indicates the approximate color of the light source. Lower numbers are redder in color and higher numbers are bluer.
Lumens - a measure of the amount of light given off as perceived by the human eye.
Lux - A unit of measurement that indicates light intensity defined as the number of lumens over one square meter. (lux = lumens/square meter)
PAR (Photosynthetically Active Radiation) - The specific range of light wavelengths plants use to photosynthesize.
Photoperiod - The physiological response of a plant (such as flowering) to the length of the night or a dark period.
Photosynthetic Photon Flux (PPF) - The amount of PAR being provided by the light source. (Essentially, the amount of light being provided that is useful to the plant.) Measured in micromoles per second (µmol/s).
Photosynthetic Photon Flux Density (PPFD) The amount of PAR being provided over a surface area. (Essentially, the amount of light useful to plants that actually reaches the leaves.) PPFD is measured in micromoles per square meter per second (µmol/m²/s).
Watts - The amount of energy used by the fixture.
Wavelengths Used By Plants - Plants use a specific range of wavelengths in the light spectrum to photosynthesize (referred to as PAR: Photosynthetically Active Radiation). Plants primarily use red and blue wavelengths in the visible light spectrum.

More Information

Photo credits: 1: Aaron Steil; 2: Aaron Steil; 3: Megan Betteridge/AdobeStock; 4: Aaron Steil

Authors:

Aaron Steil

Last reviewed:

December 2023