PAR Lighting Calculator

Feb 14, 2016 @ 2:05am

Plants require light to function, but they are sensitive to light at different wavelengths than humans, and that makes calculating the lighting required for indoor plant growth difficult, especially when all the light bulbs out there are rated in lumens, a measure of how bright they appear to humans. So I made a calculator to determine the actual PAR rating of a light setup.

So why do this? Lights are routinely measured in lumens, lumens is a unit that describes the brightness of a light as we would see it, so a bright UV light is zero lumens, a dim green light can be many lumens, while a light that puts out less total light, but is red, may actually be less lumens. This is because the human eye is more sensitive to green than red.

Similarly, plants use light via chlorophyll to grow. chlorophyll is sensitive mostly red and green light. Studies have been done to determine how well plants grow under various wavelength of light. These studies can be used to develop a photosynthetically active radiation spectrum that describes how well a plant grows under different wavelengths of light. This spectrum is known as the PAR action spectrum and is used to determine the intensity of a light as a plant "sees" it. This is usually expressed as the photosynthetic photon flux in µmol/s, or how many photons a plant can use from the light (chlorophyll cares about the number of photons received, not watts). The PPF of a light is similar to it's lumens, it's the total light given off considering how usable it is for a plant or human respectively.

The intensity that a plant receives is dependent on the fixture and distance from the light. Normally we see this expressed in lux or foot-candles. It is a measure of how much light hits an area. For plants we use PPFD, photosynthetic photon flux density. This is usually expressed in µmol/m^2*s, various plants require different levels. When someone says their PAR light level is 200, they mean the PPFD is 200µmol/m^2*s.

When growing plants indoors PPFD is not all that matters, it's also important to determine the daily light integral (DLI), this is a measure of the total PPFD received over the course of the day, when outside this accounts for things such as clouds, shade at various times of the day, and the fact that the sun appears dimmer in the morning and evening and brighter at noon. The DLI is one of the best measurements for determining if a plant is getting the proper amount of light over the course of a day for proper growth. For artificial lighting, the DLI can be increased by increasing the photoperiod, or how long the lights are on per day. Many plants use the photoperiod of the light to detect the season, and they will flower in response to the photoperiod. Many temperate species require that the photoperiod is varied to simulate the seasons. Ideal values of DLI vary by plant, Measuring Daily Light Integral in a Greenhouse, but for a low light plant 5mol/day may be sufficient while a full sun plant may require 20+mol/day.

]My calculator calculates all of the above numbers, it can even convert between lumen and PPF and it can convert between lux and PPFD. I created it because with all the LED lights coming out the industry has come out with LED grow lights, these use high efficiency single wavelength LEDs picked to coincide with the peaks on the PAR spectrum. Unfortunately their advertising states what LEDs they have and talks all about their spectrum, but they skip any talk of actual brightness, probably because of how hard it is to explain it. My calculator can calculate the PAR ratings for these lights, as an example this is the light I got, they list that is has 12 LEDs, and is 12W and has three each of 660nm, 630nm, and 460nm LEDs. That defines the spectrum for that specific light. On my calculator you can enter a custom spectrum of "660:3, 630:3, 460:3" to specify a light with three equal brightness LEDs of each of those wavelengths. Then you can input the radiant flux (typically 20-30% of the power consumed for an LED) and get the PPFD for your setup, or you can measure the light in Lux with an accurate light meter and it will give you the PPFD (in practice this probably won't work, most light meters are NOT accurate when measuring colored light, if your light meter is accurate for all colors this will work). Hopefully this helps people trying to figure out the lighting levels required for their brand new LED grows lights (or whatever light they happen to be using).


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Apr 6, 2017 @ 8:59pm


Aug 6, 2019 @ 8:40am
Thank you

Squicky bill

Jun 8, 2020 @ 11:17am
Thanks lad, i've been struggling since an hour to find a good definition, this is good, thanks men


Aug 5, 2020 @ 5:32pm
The Par Dli Cal link is down, can you please fix it


Aug 5, 2020 @ 6:37pm
My power is out thanks to the Hurricane Isaias, so hopefully 6-24 hours I get powtand it comes back... I'll probably move the link soon so this doesn't happen again.


Feb 28, 2021 @ 12:09am
I put together some css for your calculator to prettify, hope you don't mind, and concatenated the files into a single HTML file, I posted on github and made sure the licenses are included: and

Just because I found your work super helpful and interesting, wanted to see it was preserved and used. Cheers!


Sep 23, 2022 @ 7:10pm
Hi, for the input I put in ppf 50, beam width 60 degrees, height 31.5485 inches. This outputs PPFD 92.5, but shouldn't the output PPFD be 76.1 ppfd?

Because at that height, the diameter illuminated should be 36 in, resulting in an illuminated area of 0.66 sq m, and 50 ppf/0.66 sq m = 76.1 ppfd.

Is my math wrong, or are you calculating the max ppfd in the center of the beam instead of the average across the entire beam angle? Thanks!


Sep 4, 2023 @ 7:43am
Do you have source for the wavelength to PPFD conversion?


Sep 4, 2023 @ 9:05am
Pavi, one of the links went to the PDF, but I guess they took the PDF down. You can find it by punching the link into the Internet archive, I'll update the link

This is the source though


Sep 5, 2023 @ 1:27am
Thank you so much!


Apr 3 @ 11:26pm
According to the linked paper by McCree, quantum yield peaks at 620nm. So why does 660nm light give a ppfd of more than twice that of 620nm light? Test It yourself, the numbers don't seem to match the action spectrum.


Apr 4 @ 1:55am
that's because there are lumens involved in calculation instead of watts. Lumen is a photometric unit, flux of the source in visible light to human eye. Watt is radiometric unit for radiance, considering all wavelengths. 660nm is not as well visible to human eye than 620nm.
Flux at 660nm would usually be reported as watts or milliwatts, because output as lumens would be too small.


Apr 4 @ 3:06am
Hey Paivi, thank you for the quick response. I thought that was already taken into account by the calculator, oops. So for wavelengths other than around 550nm (the green line on the graph), one should use wattage instead? But then the question as to the lights (in my case LEDs) photosynthetic photon efficacy (PPE) arises. But who cares its just an estimate right? Thank you again!


Apr 4 @ 3:24am
for LEDs, PPE is usually already reported in datasheet, as well as dominant wavelength. Then you may choose these parameters to use for calculator. You will receive higher PAR for 620nm. Lumens and lux have larger difference because of conversion according to wavelength explained earlier.


Apr 4 @ 8:09am
Hey Paivi,
Datasheets have luminous efficacy (lm/w), but unfortunately I dont have access. So yeah, with this you can find the scaled luminous flux of each diode using the link you sent. The link didnt work before, but now it did and is extremely helpful and should be pasted on the calculator homepage imo.


Apr 4 @ 8:16am
Hey Jacob,
LED datasheets are public and can be found on manufacturers' websites.
LEDs for visible light report output as lumens but wavelengths let's say <400nm and >600nm are reported as milliwatts, and LEDs optimized for horticultural lighting report PPF.


Apr 4 @ 8:43am
Sorry, I mean that I need to request for the specific LED part number because there are different versions of the red 5050 SMD LED.