Get the essence of electrical engineering in the form of specialized HV/MV/LV technical articles, electrical guides, and papers.

# An example of calculating the number of indoor lighting fixtures

Home / Technical Articles / 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

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

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. ### 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 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

1. Murugesan
Jun 29, 2020

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

2. 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?

3. 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

4. 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

5. Aravinthan M
May 22, 2019

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

6. 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..??

7. Oct 30, 2018

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

8. 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

9. 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
Bhavin Patel

• Vijayakumar.J
Dec 19, 2018

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

10. Majd
Mar 16, 2017

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

11. 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

thanks

• 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

12. 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

13. jamshid
Jun 15, 2016

How i can calculate SHR in lighting calculation

14. SUBRAMANI
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?

15. 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

16. koua
Jul 21, 2015

It’s verry interesting!

17. 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 ?

18. 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.

19. dawsam abdallah
Sep 29, 2014

20. awny
Sep 23, 2014

Thank you very much

21. 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
TOTAL NUMBER OF FIXTURES SHOULD BE 20 OR 21?? IF 21 PLEASE EXPLAIN ? ACCORDING TO DATA AT THE BOTTOM SIDE (ALSO MENTIONED IN 1ST 2 LINES OF COMMENT) I THINK SO TOTAL NUMBER OF FIXTURES SHOULD BE 20.

REGARDS

22. mohammed nizamuddin
Aug 31, 2014

23. 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

24. 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

25. 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

### Subscribe to Weekly Digest

Subscribe to free HV/MV/LV technical articles, electrical guides and papers.