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How Many Light Fixtures Do I Need? – Ultimate Guide

lux level palces


How many would I need for an indoor sports court 26’x35’ with 19’ ceilings?

Have a 40’x 48’ shop and light would be mounted at 16’ many lights?

I have 30 x34 garage with 10-foot high ceiling. I currently use 4 x 8-foot dual tube fluorescent fixtures. How many 100 watt fixtures will I need?

Have you encountered these problems when purchasing lamps? This article won’t let you become a lighting expert but will give you guidance for your next purchase.

 #1 What is Lumen, Lux and Footcandle?

It happens far too often. You walk into a store, pick up a light bulb or a handheld spotlight and look at the specifications- only to find that one says lumens, another says lux,the last one says Footcandle. The short answer is that they’re all separate standards for measuring different kinds of lighting effects.  Let’s break it down into the long answer to dispel a few more shadows.


First, let us give up the misunderstanding that the wattage is equal to the brightness. Although there is a direct correlation between the wattage and brightness of incandescent bulbs, this is not the case for LED products. Since LEDs consume much less power (watts), it is not possible to compare the brightness of wattage-based incandescent lamps and LED bulbs.

Therefore, a significant factor to pay attention to when looking for the brightness of LED lights is lumens. Lumens is the measure of brightness and tell us how much light a particular lighting product emits.

lumen per watt

For reference, a typical incandescent 50 W bulb emits approximately 700 lumens.

50W LED bulb emits approximately 5000 lumens

LED has a better lumen price!


lux and candle

Lux (lx) measures illuminance, which is the amount of light on a surface per unit area.  A single lux is equal to one lumen per square meter.  If the lamp displays its brightness as a measurement of lux, it usually lists a distance from the bulb since any change in distance or bulb type changes the lux level.  As an example, if you place a 100 lumen bulb in a floodlight that shines on only one square meter of surface, that surface will be lit at 100 lx.  However, if you back the floodlight away to shine on four square meters, the surface is now lit with 25 lx.


footcandle to lux

In America, we still use the figure of merit of footcandle, which is a value similar to lux but is lumens per square foot not per square meter. There is a simple relationship to convert from a square foot in inch terms to a square meter in inch terms, gives us a factor of 10.764. We use this factor to convert from FC to lux; 1 FC = 10.764 lux. I think that understanding this relationship goes a long way in getting comfortable with these lighting terms.

A good way to remember the differences between lumen vs lux (FC) is:

  • Lumens are how much light is given off
  • Lux and Footcandle are how bright your surface will be
  • 1 FC( footcandle) = 10.764 lux



#2 Factors affecting Lux (Footcandle)

Besides the lumens of the lamps themselves, what other aspects will affect lux? Beam angle, mounting height and LLF.

Beam beam

The beam angle is the angle of radiation for a light source. For example, a beam angle on 120 degrees means the light has a shape of a cone and its borders form a 60-degree angle with the center axis. A change in the Beam angle affects the luminous intensity (lux) of a light source but not the luminous flux (Lumens).

Installation height


The lamps with the same lumen and light angles produce different lux at different heights, and the effective illumination area is also different. In the figure, the illuminator has a light diameter of 1.2m at a height of 1m and a light density of 737 lux. At a height of 3m, the illumination diameter is 3.6 m, but the light density is reduced to 82 lux.


Light Loss Factor is the product of several factors, each depreciating light output at a certain point in time in the future compared to initial light output. When a light fixture is activated, it produces light which must leave the lamp, then the fixture then reaches the workplane where it is needed. Along the way, a number of operating and environmental conditions interfere with the transmission of light, resulting in wasted layers. The LLF of LED is usually between 0.8-0.95.


#3 Common and Recommended Light Levels Indoors

lux level

The outdoor light level is approximately 10,000 lux on a clear day. In the area closest to windows of the building, the light level may be reduced to approximately 1,000 lux. In the middle area, it may be as low as 25 – 50 lux. Additional lighting equipment is often necessary to compensate for the low levels.

Earlier it was common with light levels in the range 100 – 300 lux for normal activities. Today the light level is more common in the range 500 – 1000 lux – depending on activity. For precision and detailed works, the light level may even approach 1500 – 2000 lux.

The table below is a guide for recommended light level in different workspaces:

recomended lux level


#4 Take out your tape measure


  1. Measure and calculator to calculate the area of ​​the area using a tape. Measure the length and width of each rectangular portion of the area. You multiply the length and width of each part to get the area of ​​each part. Add all the areas together to find the total area.
  2. Measure the height of the fixture installation. Because the larger the distance between the light source and the illuminated area the lower the illuminance will be. We only need this step when to accurately simulate the scene using lighting design software. We do not use this parameter in a rough calculation method.
  3. Where you want to use these fixtures. Look up the above table to determine the lux level. 1 footcandle = 10.764 lux
  4. Collecting luminaire parameters, such as watt, lumen, beam angle. You can find it in the product description. If you can not find LLF, 0.8-0.95 will be your reference.


#5 Calculate lumen and lighting fixtures

Method 1: On your manuscript paper – Easy but Rough

In this example, we will use a 12*14 (40*60 ft) meter and a 4.2 meter high Classes.  Classes’ general lighting industry standard is 250 lux per square meter. UFO lamps used: 150watt; beam angle: 120; lumen: 17000; LLF (Light Loss Factor) = 0.8

Classes total area = 12 * 14 = 168 square meters

The actual total required lumen=168*250/LLF=46666 lumen

Number of lamps required = 46666/17603 = 2.65

Then we need 3 or 4 units 17000-lumen lamps

Open the Omni calculator and enter the above numbers into the corresponding place for automatic calculation or you can calculate it manually. (Omni calculator does not include the LLF in the calculation).

Method 2: Lighting design software – Complicated but Accurate

In the above example, we did not take into account the light angle and mounting height. Because what we are talking about here is a rough calculation method. But when we use lighting simulation software, the two parameters are important. The picture below shows the actual lighting effects of the above example I simulated with DIALlux. UFO lamps used in UFO lamps used: 4*150watt; beam angle: 120; mounting height: 4.0m; lumen: 17603/unit; LLF (Light Loss Factor) = 0.8

lighting design-2

lighting design

The simulated average illuminance is 345lx > 250lx. This is completely in line with our needs. So the rough calculation method above is sometimes worthy of your reference. But when you need accurate calculations, professional lighting design software will be your best partner.




  1. Lumens to watts calculator 
  2. Lumen to lux calculator

Understand three concepts

  • Lumen
  • Lux(lx)
  • Footcandle(FC)

A good way to remember the differences between terms is:

  • Lumens are how much light is given off
  • Lux and Footcandle are how bright your surface will be.

4 Factors affecting lux

  • The lumen of the lamp
  • Beam angle
  • Installation height
  • LLF (Light Loss Factor)

How to calculate how many lumen and lighting fixtures?

Before we do this, we need to read Common and Recommended Light Levels Indoors to determine the level of illumination in the workspace.

  1. On your manuscript paper – Easy but Rough
  2.  Lighting design software – Complicated but Accurate

Keep in mind, however, if you have specifically dark colored walls and furniture or if you’re using fixtures with shades, you’ll need approximately additional lumens per square foot. Finally, preference will play the largest part in your decision. If you like the room to be especially bright, you may want to add an additional 10 to 20% to our numbers. In fact, the best approach For most spaces is to aim high and install dimmers to bring the light level down to desired levels.

Although I am not an electrician, I am still happy to solve the lighting problem for you. Leave your problems in the comments.

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