Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

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

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
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?

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

three  +  4  =  

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge