HOW TO Calculate your tank lighting (in LSI) - The Planted Tank Forum
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post #1 of 4 (permalink) Old 04-08-2013, 10:13 AM Thread Starter
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HOW TO Calculate your tank lighting (in LSI)


(LSI, LUX, & WPG included in MS Excel Calculator)

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 (fudge factor is manufacturer, reflector quality, etc dependent). And no this does not guarantee your plants to thrive.

**EDIT ** Remember that the reflectors on your fixture GREATLY affect your calculations. There are some grossly generalized factors that I have posted in the manual calculation below.

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). Or if you wish, skip all the discussion and go straight to the automatic calculator at the end of this post in the attached file.

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 (Ámol photons/m^2/sec or Ámol 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 (Ámol photons/m^2/sec or Ámol 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

**EDIT** These total lumens assumes you have the absolute PERFECT reflectors. If you have crappy reflectors you may have a reduction in lumens by 1/4 to 1/6 (assuming low reflectivity properties to better reflective properties in that order). If you have very good reflectors you may have a drop in lumens by a 400 with a variance of +/- 200 Lumens depending on your reflectors and position of tubes in the reflector.


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 /355 in^2 = 38.7 LSI /in^2 (the excel doc has my tank as an example)

**EDIT** - PLEASE understand that this calculation gives you total lumens over the area in question, provided that ALL THE LIGHT provided by the fixture is TOTALLY contained within the tank. We all know that this is NEVER true, so you're realistic LSI will be LESS than calculated. Once all the affecting factors (reflection from reflectors, reflection off surfaces, refraction through and out of the tank, etc) are at least recognized, then we can try to take them into account and arrive to an LSI that is less riddled with error. But to make this slightly more approachable I am not including these factors currently (but maybe in the future with more collaboration).

To understand what range my light falls under I will be using something similar to REXGUIDE website (found by clicking here) which has the high range for lighting which has far higher ranges than what Tom Barr and some 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) )
  1. Very low light 7 - 11 you can grow some plants
  1. Low light 12 - 17 you can grow a fair amount of plants
  1. Medium light 20 - 25 you can grow most plants
  1. High light 28 - 32 you can grow all aquarium plants
  1. 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 topics, go ahead, 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 some brands put phosphors that output alot of the spectral range is in green and yellow, 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

Q & A
Question by Justin on FFTV
"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.


Some of the information in the links are old. If you can find new information that people can agree on that modifies this information or links, please let me know. I will include the changes as an edit and mention your name near the edit. I wish for this thread to be a collaboration for those of us who do NOT have access to a par meter. If you do have a par meter but wish to participate in helping this becoming a valued tool, then by all means I welcome your input.
Attached Files
File Type: zip aquariumlightcalculator1.1(UCRDave-LED-fftv-edit).zip (6.2 KB, 160 views)


Last edited by ucrdave; 04-09-2013 at 08:23 PM. Reason: Some generalized factors put in manual calculation.
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post #2 of 4 (permalink) Old 04-09-2013, 06:03 AM
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Isn't it self evident that if you put 108 watts of T5HO bulbs over a 55 gallon tank the amount of light you get depends on how good the reflectors are. All of the lumens produced by the bulbs do not reach the tank unless you have good reflectors, so more than half of the lumens are wasted if those are just bare bulbs. Any calculator of this type just has to include a reflector factor to be of any use at all.

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post #3 of 4 (permalink) Old 04-09-2013, 06:08 AM
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Great article. I need to read this several times to master it.

Patience once drove a man insane

Thanks for your time
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post #4 of 4 (permalink) Old 04-09-2013, 12:32 PM Thread Starter
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Originally Posted by Hoppy View Post
Isn't it self evident that if you put 108 watts of T5HO bulbs over a 55 gallon tank the amount of light you get depends on how good the reflectors are. All of the lumens produced by the bulbs do not reach the tank unless you have good reflectors, so more than half of the lumens are wasted if those are just bare bulbs. Any calculator of this type just has to include a reflector factor to be of any use at all.
Yes, having a reflector factor is necessary, however I cant calculate my way toward a factor and have confidence in the result. If I were to have measurements, then I could calculate a factor for specific types of reflectors. This kind of information would then allow calculators such as these to be much more useful.

**EDIT** General fudge factors have been put in place for the manual calculation, however they are not included in the auto calculator untill confirmation of my factors seem reasonable. These changes should make your example go boom into the wind (╯░□░)╯︵ ┻━┻

If one understands that with different reflectors one can increase or decrease the intensity of light over a surface, then one could subtract or add more lumens from ones' total to achieve a calculation with more confidence. Unfortunately, many people do not understand which factors affect the final outcome. PAR meters take the guess work and laborious calculation off the table. With PAR meters we can see great work in compilation of measurements such as yours and others on this site.

Hoppy (or anyone), can you direct me to a link that gives some basic factors for the most common reflectors for t5HO fixtures that are trusted by many? I would like to include it in the calculator and main body of this thread.

I hope that the manual calculation in this post helps to make this calculation approachable...


Last edited by ucrdave; 04-09-2013 at 08:36 PM. Reason: inclusion of fudge factors
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calculator, lighting, lsi, lux, par

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