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

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.

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. 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
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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