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Home / Technical Articles / An example of calculating the number of indoor lighting fixtures
An example of calculating the number of indoor lighting fixtures
An example of calculating the number of indoor lighting fixtures (on photo: Columbus, OH Ohio History Center reading room; by army.arch via Flickr)

Input information

These are the input data for the following calculation:

  1. An office area has length: 20 meter; width: 10 meter; height: 3 meter.
  2. The ceiling to desk height is 2 meters.
  3. The area is to be illuminated to a general level of 250 lux using twin lamp 32 watt CFL luminaires with a SHR of 1.25.
  4. Each lamp has an initial output (Efficiency) of 85 lumen per watt.
  5. The lamps Maintenance factor (MF) is 0.63 ,Utilization Factor is 0.69 and space height ratio (SHR) is 1.25.

Calculation in 8 steps

1. Total wattage of fixtures:

Total wattage of fixtures = Number of lamps x each lamp’s watt.
Total wattage of fixtures = 2 × 32 = 64 Watt.

2. Lumen per fixtures

Lumen per fixtures = Lumen efficiency (Lumen per Watt) x each fixture’s watt
Lumen per fixtures = 85 x 64 = 5440 Lumen

3. Number of fixtures

Required number of fixtures = Required Lux x Room area / MF x UF x Lumen per fixture
Required number of fixtures = (250 x 20 x 10) / (0.63 × 0.69 × 5440)
We will need 21 fixtures

4. Minimum spacing between each fixture

The ceiling to desk height is 2 meters and space height ratio is 1.25, so:
Maximum spacing between fixtures = 2 × 1.25 = 2.25 meter.

5. Number of required rows of fixtures along with width of room

Number of rows required = Width of room / Max. spacing = 10 / 2.25
Number of rows required is therefore = 4.

6. Number of fixtures required in each row

Number of fixtures required in each row = Total Fixtures / Number of rows = 21 / 4
Therefore, we have 5 fixtures in each row.

7. Axial spacing between each fixture:

Axial spacing between fixtures = Length of room / Number of fixtures in each row
… and that would be: 20 / 5 = 4 Meter

8. Transverse spacing between each fixture:

Transverse spacing between fixtures = Width of room / Number of fixtures in row
… and that would be: 10 / 4 = 2.5 Meter.

5 things to conclude

Calculated room for the number of lighting fixtures
Calculated room for the number of lighting fixtures
So far, we have been calculated the following:

  • Number of rows with lighting fixtures = 4
  • Number of lighting fixtures in each row = 5
  • Axial spacing between fixtures = 4.0 meter
  • Transverse spacing between fixtures = 2.5 meter
  • Required total number of fixtures = 21

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Raimond
    Sep 21, 2023

    Good day.
    Very good material and well presented.
    I am working on the design of mini (500-600w) solar power plants.
    This material helped a lot.
    Thank You.

  2. Paul frank
    Jul 26, 2023

    It is very interesting and brief except the mistakes as satendra had highlited but everything is fine. I like it

  3. Satendra Maurya
    Apr 11, 2023

    It’s good, but under calculation many mistakes are there like step 4 – 2*1.25 = 2.5 but there is 2.25 & step 8 – instead of 10/5 there is 10/4. Correct it. Except calculation mistake everything is really too good. Thank you so much for explanations.

    Mar 28, 2023

    It is very good.

  5. Ajay
    Feb 23, 2023

    The 8th step answer was wrong I guess. Because “width of room/no.of fixtures in row is the formula right? Then 10/5 is the numbers right?

  6. Akindele R
    Jan 27, 2023

    I appreciate this page because it gives me the full package of what I’m looking for. Thank your very much.

  7. Kiran Y
    Nov 28, 2022

    Dear sir can you explain industrial lighting same as step by step procedure ….

  8. Mohammed
    Nov 16, 2022

    Really, it’s so nice

  9. Elizabeth
    Sep 17, 2022

    I need a villa with full lighting calculation for it’s room and elactrical installations on AUTO CAD too, can you send me on my email now, please

  10. Kehinde John Daramola
    Sep 14, 2022

    Thanks for this calculation. This is really helpful for me.

  11. Jason KARAKE
    Jul 07, 2022

    the 21st has no place we equip with none odd fixtures and It’s good.

  12. Jason KARAKE
    Jul 07, 2022

    It’s good

  13. Kaneki Kun
    Feb 19, 2022

    In this example, we calculated the number of lighting fixtures as 21 but, why we only placed 20 in the figure (as 5×4)? Can anyone please explain?

  14. Ali
    Jan 11, 2022

    We have weeding hall
    Give me please your recommendations
    We well make led lamp spots.
    Ceil and sides white colour

  15. Jeremiah kyalo
    Sep 28, 2021

    A very interesting notes

  16. Mothilal
    Aug 18, 2021

    How does the each lamp has an initial output (Efficiency) of 85 lumen per watt

    Jun 07, 2021

    fitting fixture 17 pcs if height 3 meters how much fixture will be required.
    pls, reply.

  18. syed abdul rahman shah
    Oct 11, 2020

    how can i get utilization factor, initial lamp lumen and SHR please a little beat explanation

  19. Murugesan
    Jun 29, 2020

    The ceiling to desk height is 2m and space height ratio is 1.25 ? Can you explain the 1.25

  20. MST. J . KHATUN
    Apr 12, 2020

    A 2000M Long 16M Bridge is to illuminated including 260M approach road on both sides of the bridge by 120W Led lumanaries in the octagonal pole height 12M.FIND THE no of led fittings and pole required?

  21. Bas
    Mar 09, 2020

    Heey Guys,

    I have challenge you guys could might help me with. Right now i am working with LEDs with a dominant wavelength of 650nm for one of my projects. The problem is, is that i need a irradiance strength of 30 mW/cm2 ( only from the 650 nm +/- 10nm bandwith ) with a distance from my LED to the table of 10 cm. I can make Lux calculations, but Lux is the energy of the LED from all bandwiths and not only the 650 nm +/- 10nm range. Do you guys now the formula’s for this calculation or the software were i can calculate this with?

    Gr. Bas

  22. Sunil John
    May 22, 2019

    The room height is not considered and This Calculation result is different when calculated in Lighting Software like Dialux and Relux

  23. Aravinthan M
    May 22, 2019

    how can i get utilization factor and SHR please a little beat explanation

  24. Sachin
    Mar 18, 2019

    Distance of fixtures along width is 2.5 m as per the calculation. But the diatance of fixtures along length is 4m based on the number required is 21.But if we do so distance between fixtures along the length is 4 m which is above 2.5m. Won’t this effect illumination lenght wise?? Because as per SHR maximum allowed distanve between light fixtures is only 2.5m..??

    Oct 30, 2018

    how can i get utilization factor,initial lumen in a lamp and SHR please a little explanation

  26. Ravi GM
    Jun 24, 2018

    Dear Jignesh Parmar,
    i am Ravi. i need one information. the above lighting calculation how did you get the Each lamp has an initial output (Efficiency) of 85 lumen per watt. please exaplain me. i am not getting this value.
    ravi gm

      Mar 13, 2020

      I believe you can do area of the room X with Lux of the room to get Lumen.

      do, every lighting fittings comes with Lumen per wattage of the light fittings

    Apr 29, 2018

    How to calculate the no of fixtures required shed length 59 meters x width 28 meters x height 9 meters with hi bay fitting

    • Bhavin Patel
      Jun 13, 2018

      1 lx = 1 lm/m2
      Area = 59 (Length) *28 (Width) = 1652 Sq. Meters
      Average Lumen Required for the Application: i.e: 150
      Total Lumen = 150 Lumen * 1652 Sq. Meters = 2,47,800 Lumens
      Now Find Fitting Lumen = Say 100W, (150Lumen/Watt= 15000 Lumen Fitting we would like to use
      No. of Fittings = Total Lumens/ Lemen per Fitting = 247800/15000= Approx. 17 Nos. of Fitting
      We also have to consider Height of Installation with Beam Angle of Light which can Increase or Decrease no. of Fittings
      Please do not hesitate to contact us for further information if required to [email protected]
      Thanks in advance,
      Bhavin Patel

      • Vijayakumar.J
        Dec 19, 2018

        Room Height not considered in calculation, How to considered Room height in illumination manual cal.

  28. Majd
    Mar 16, 2017

    Hey, i was wondering how to find the Utilization Factor for a light?

  29. rajeev
    Jan 19, 2017

    dear sir

    i need to know the no of lights ( high bay required) for a shed having lenght,breadth and height of 73 mts 33 mtrs and 9 mtrs respectively. the highbay have 130 lumens per watt at source 120 beam angle and are of 100 watts . the lux level is of 250 . can you please let me know the no of fixtures requird


    • Poojashree
      Jan 30, 2020

      Shall we give distance irrespective of because diffusers are designed if I alligned to diffuser line the distance is x_4x -x is there if I’ll not arrenge like that light fixture looking like odd in the ceiling how can I do now

  30. divya
    Sep 06, 2016

    can u help how to calculate illumination factor for 4 tube lights of 40 watts inside and outside led bulbs are putting on a hub how to calculate the spacing

  31. jamshid
    Jun 15, 2016

    How i can calculate SHR in lighting calculation

    May 06, 2016

    i need to know what is the relation between No. of Light and lumens while increasing mounting height of the light.

    also what is the recommendation for industries. how can i able to find Spacing Height Ratio? Where to i got this?

  33. moubani banerjee
    Oct 14, 2015

    20x 400w son floodlights are mounted on a 12pole at the middle of each of the side of a square area of 40m side. calculate utilization factor ,the average illumianace on the area, the illuminance at any corner point. GIVEN : the total initial lamp lumen is 50 klm, D.F=0.8, MF=0.75, loss factor=0.9

  34. koua
    Jul 21, 2015

    It’s verry interesting!

  35. Emmanuel
    Jul 15, 2015

    Your work is very good . I appreciate it so much . My question is , if u are really keen to share your knowledge in the field of electrical Engineering , how would one communicate to you ?

  36. Said
    Apr 27, 2015

    Thank you very much.

    • Mostafa Elsayeed
      Mar 21, 2018

      Your clarification and description is very interesting and simple, thanks too much, in this regard I want to ask some questions:

      1. should i take into consideration the clean factor (1.27-1.33), ie the values of numbers will be multiplied with this factor and therefore no of luminaries will increase.

      2. for a certain space, it should be identified the color of ceiling, wall& floor and material type to get the reflecting factors and g with room index through the data table of the chosen lamp to get UF and this data no longer be available for the new models of lamps! how can we deal with this issue? appreciate your reply.

  37. dawsam abdallah
    Sep 29, 2014

    how can i download this in pdf form

  38. awny
    Sep 23, 2014

    How can I download this article format PDF
    Thank you very much

  39. mohammed nizamuddin
    Sep 01, 2014

    Number of rows with lighting fixtures = 4
    Number of lighting fixtures in each row = 5
    Required total number of fixtures = 21


  40. mohammed nizamuddin
    Aug 31, 2014

    how to download this one?

  41. kavi
    Aug 29, 2014

    tiny mistake in figures..

    step 4: spacing should be 2.5 instead of 2.25…1.25×2=2.5..
    step 5: the calculation sticks to 10/2.5=4

  42. Alexander Nowak
    Aug 19, 2014

    Nice demonstation of a classical calculation without the help of a fat light planning software.
    But let me please make some comments: A lighting level of 250 Lux for an office area is not compliant to standards. For offices you should plan with 500 Lux instead.
    Also I would consider to use not CFL but linear fluorescent lamps (T5), which have much higher efficiency of approx. 100 Lumen per Watt. Than you get also a better MF of 0,8 and this together reduces the number of fixtures.

      Oct 30, 2018

      how can i get utilization factor, initial lamp lumen and SHR please a little beat explanation

  43. ugbana joseph
    Aug 18, 2014

    Thanks a lot its useful mostly to me because am still a fresher in the field of engineering.

    • ronak gala
      Oct 10, 2019

      Sir if I post a picture could you tell me how to solve this sum

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