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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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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.

36 Comments


  1. Jeremiah kyalo
    Sep 28, 2021

    A very interesting notes


  2. Mothilal
    Aug 18, 2021

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


  3. NAZMUL HASAN
    Jun 07, 2021

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


  4. syed abdul rahman shah
    Oct 11, 2020

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


  5. Murugesan
    Jun 29, 2020

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


  6. 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?


  7. 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


  8. 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


  9. Aravinthan M
    May 22, 2019

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


  10. 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..??


  11. TADELE MELESE
    Oct 30, 2018

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


  12. 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.
    thanks
    ravi gm


    • OBANKIE
      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


  13. RAJAN VARMA
    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.


  14. Majd
    Mar 16, 2017

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

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