I will show you the way to calculate your Lumens per square inch (LSI).  This method does NOT account for reflection, refraction, absorption, type of light source, reflectors of fixture, age of lights (intensity loss or spectrum shift), spectrum quality, depth of tank, nor any other factor that will reduce or increase lighting in your tank.  This How To is ONLY to give you an idea of how much light you possibly have. The best way to know how much light you have is not by calculating your light but measuring your light with a PAR meter.  If you dont need this kind of accuracy, then this How To can be helpful.  AND NO there is NO true factor (constant factor) to get from LSI or LUX  to PAR.  And no this does not guarantee your plants to thrive.


If you wish to purchase a PAR meter that was made for less than 100 dollars you go to to MisterGreen on  his DIY par meter progress and link to contact him is found at the bottom of this post.

After understanding how to manually calculate your LSI, you can download my edited version of Brokefoot’s excel calculator to automatically calculate your LSI, and even LUX at the substrate or any height; AQUARIUM LIGHTING CALCULATOR 1.1 (LED fftv edit).  If you wish, skip all the discussion and go straight to the automatic calculator at the end of this post.

For a more indepth article to read about lighting for both FW and SW tanks click here


GLOSSARY (partial lay person version)


  • Lumens -  measurement of visible light (Lm)

  • LUX - measurement of lumens per square meter (Lm/m^2).  This measurement is usually weighted to the sensitivity of the human eye.  This is why PAR is favored.

  • PAR - Photosynthetically active radiation (µmol photons/m^2/sec  or  µmol m^–2 s^–1 )

  • PUR - Photosynthetically usable radiation; this is nearly the same as PAR but when measuring PUR one is cutting out the green and yellow spectrum almost entirely leaving the two spectrums that are roughly in 400nm - 525nm and 625nm - 700nm ranges.  This is still measured in (µmol photons/m^2/sec  or  µmol m^–2 s^–1 )

  • WPG - measurement of power of the light source per gallon (watts/gallon or wpg).  This is a very flawed measurement due to the nature of the units in this ratio.  Watts refers to power put into the light NOT the intensity of the light itself, since all the power that goes into the light source is NOT entirely converted into light.  Notice the heat that comes from a light?  Thats wasted power (watts) not converted to light which leads us to how much light actually comes from the power put into the light; Luminous efficacy

  • Luminous efficacy - how well a light source gives light relative the power put into it measured in lumens per watt (Lm/watt or Lm/w)

  • nm - read nano meter is a measurement of radiation wavelength (eg Light)

  • LSI - measurement of visible light spread over the area in question (Lumens per square inch  or Lm/in^2)

  • in^2 - this is read as inches squared

  • refraction - light that goes through a transparent or semi transparent object (eg glass tank walls or lids)

  • DW - Driftwood

Now lets do some calculations

My tank is 36”L x 15”W x 20”H.  Because the back of my tank has a built in sump, and taking useful internal measurements, that leaves me with 35.5”L x 10”W x 19.5”H to illuminate.

The area to illuminate is the LENGTH X WIDTH

35.5in x 10in = 355in^2  (read as 355 inches squared; which is in units of AREA)

Now lets look at the light source

My lights are four 36” 39 watt T5HO tubes.  These types of lights give 88Lumens/watt (it may be less or more, this depends on the manufacturer).  This is generic ratio that some agree on. To get total lumens we multiply

4 x 39watts x 88Lumens/watt = 13,728 Lumens


LSI is in the units Lumens per square inches, therefore we divide our total lumens by the area we calculated before

13,728 Lumens    =   38.7   L    = 38.7 LSI  (the excel doc has my tank as an example)

                               355 in^2             in^2

To understand what range my light falls under I will be using REXGUIDE website (found by clicking here) which has the high range for lighting which has far higher ranges than what Tom Barr and other old timers for planted tanks agree on.  The newer generation agrees to the higher range.  I personally use the higher range because when using the older timers range I noticed significantly less growth and less color (other than green) of the plants.  Here is a version of the newer generations range.

Lighting                              LSI                     Plants you can grow

Very low light                     7 - 11                 you can grow some plants

Low light                           12 - 17                you can grow a fair amount of plants

Medium light                      20 - 25               you can grow most plants

High light                           28 - 32               you can grow all aquarium plants

Very high light                   33 +                   you can grow all aquatic plants with no doubt

Compare your LSI to these ranges or the range on the excel calculator (which is another range that some agree on)

Due to keeping this simple I will NOT talk about the factors that can increase or decrease your LSI nor how it affects LUX.  There is more math involved which scares many.  If you wish to discuss these you may but you can find many arguments against LSI and LUX as a way to determine your lighting.  To reiterate this is not the best way to know your lighting, but it is an attempt to give our community a way to calculate our lighting (not measure like PAR).




Download the attached excel document at the end of this post.  In order to view this document directly from your computer you must have a program to open the document type.  Another way to view the document is by using Google Drive (previously known as Google Docs). You can upload the attachment to your Google Drive and play with it there if you do not have Microsoft Excel, OpenOffice, or some excel viewing AND editing program (you must be able to edit the excel to use the calculator)


  • When typing in your dimensions for the tank, use inches

  • When you want to know the LUX at a certain depth (eg middle height) then change the depth number

  • When you want to know the LSI, type in the INTERNAL measurements of the BASE of your tank.  For example: my tank is 36”L x 15”W x 20”H    my tanks internal measurements are  nearly 35.5”L x 10”W x 20”H

    • I measure the internal dimensions of L and W only since thats where the plants are going to be, because the plants are not IN the glass/acrylic, so we subtract the thickness of the glass/acrylic

  • LED range is due to some of the lowest and highest Lumen/watt ratios that I’ve found currently on the market with both manufactured fixtures, diy fixtures, and also using LEDs bulbs from HomeDepot (which give some of the lowest Lumen/watt ratios).  Highest is Cree XM-L U2 (10 watt) leds ran at 2.9 Volts @ 0.7Amps (2.03 watts) which give 147.8 Lm/watt

  • T8 tubes with magnetic ballasts give a lower Lm/watt which you see in the table.  T8 tubes with electronic ballasts give the higher Lm/watt

  • T5 NO (normal output) give the higher Lm/watt  while the T5 HO give the lower Lm/watt

  • Spiral CFL at 6500K color temperature usually give more Lm/watt, but remember alot of the spectral range is in green which is bright to the human eye but not used as well by aquatic plants.

The original calculator was provided by Brokefoot on   The link to his page with the original post and original excel calculator can be found by clicking here

If anyone wishes to make their own PAR meter you can diy it by following Hoppys post here


And if anyone wishes to purchase a DIY made one ask this MisterGreen here

you can also see his posts and progress on making the DIY par meter here

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Replies to This Discussion

@ Justins questions 

"What is the best light for plants?
What do i need to look for when getting a light? 
par or wpg?"

  • The common agreement for lighting is to have a lighting that has a spectrum with a good amount of blue and red (with a bit of green and yellow for human eyes to like the light; this is aka CRI - color reproduction index)
  • Lighting with a spectrum that is geared toward plant growth we can actually not need as much lighting, which when we can start talking about PUR or PAR.

If you can find lights that give PAR ratings then thats awesome.

The old timer range for PAR is this

  • Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
  • Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance   algae problems
  • High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems

this range is obtained by reading here

the young generation uses a range like this

  • 15-80 PAR for low light plants
  • 50-150 PAR for moderate light plants
  • 100-200+ PAR for high light plants

this range is obtained here

Plants can adapt to many light ranges, which is why I believe we have such a huge argument about agreeable ranges between old and new tankers.

  • BTW while we can talk about WPG, but I think we shouldn't  Because we can have different intensity of light as we go deeper in to the tank, how does that then make sense when putting plants higher in the tank (eg on DW high in the tank) as opposed to the very bottom (eg carpeting plants which get the lowest lighting)  This is why using PAR is so great.  With a PAR meter we can measure the lighting on the DW high in the tank, or the carpet at the base of the tank.  

ware did you take this from

I didn't take it from anywhere. I used the knowledge that's out there as well as knowing math and dimensional analysis (essentially making sure the units are correct) to come up with what I have.



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