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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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About Author


Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.


  1. Majd
    Mar 16, 2017

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

  2. 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


  3. 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

  4. 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?

  6. 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

  7. koua
    Jul 21, 2015

    It’s verry interesting!

  8. 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 ?

  9. Said
    Apr 27, 2015

    Thank you very much.

  10. dawsam abdallah
    Sep 29, 2014

    how can i download this in pdf form

  11. awny
    Sep 23, 2014

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

  12. 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


  13. mohammed nizamuddin
    Aug 31, 2014

    how to download this one?

  14. 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

  15. 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.

  16. ugbana joseph
    Aug 18, 2014

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

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