Lighting Calculation
Figure out how much light you need in each room and how many fixtures you'll need.
By Meg Escott
If the idea of doing a lighting calculation is a bit offputting, trust me. It isn't difficult. I promise. On this page you can find out what lighting levels you'll need in different rooms in your home and for different activities. Then we'll go through an example and you'll see that it's not so hard.
Lighting units
We can't get much further talking about lighting before we take a look at how we measure light. Here's a few words you'll need to know to make the rest of this page make sense.
Lumen
A lumen (symbol lm) is a measure of the total amount of light visible light emitted by a source in any particular direction. That's keeping it simple. For a more precise definition Wikipedia can tell you more about what a lumen is.
Lux  the metric system
Lux is a measure of illuminance which basically means it's a measure of how much light there is over a given surface area. One Lux (lx) is equal to one lumen per square meter.
1 lx = 1 lm/m^{2}
Footcandle  the imperial system
Footcandle is also a measure of illuminance for those of you who prefer to work in feet. One Footcandle (fc) is equal to one lumen per square foot.
1 fc = 1 lm/ft^{2}
Converting between Lux and Footcandles
If you need to convert between Lux and footcandles (values given to precision of 2 decimal places):
 1 Lux (lx) = x Footcandles (fc)
 1 Footcandle (fc) = 10.76 Lux (lx)
Here's a handy illumination converter.
Lighting Calculation  Step 1
Find out how many lux or footcandles you need
The lighting table below tells you how many lux or footcandles you need in each room or for various tasks. I've also thrown in some examples of moonlight and daylight for interest.
The values given in the table aim to assure safety, comfort and charm. There are times when more light may be available such as when there is bright sunlight spilling through a window or when more light is desirable such as for a small task.
Activity  Lux  Footcandles 

Direct Sunlight  32000–100000  2300  9300 (approx) 
Daylight (not sun)  10000–25000  930  2300 (approx) 
Full moon (clear)  1  0.1 
Kitchen ambient  108  10 
Kitchen task  538  50 
Dining  54  5 
Living Space  54  5 
Living Space (task)  323  30 
Desk lighting  431  40 
Bedroom ambient  54  5 
Bedroom reading  431  40 
Bedroom dressing table  431  40 
Bathroom ambient  54  5 
Bathroom task  323  30 
Laundry  323  30 
Circulation  54  5 
Sewing  538  50 
Garage/Workshop  108  10 
Small detailed task  1076  100 
We're going to work out the amount of light required for a kitchen.
Using the table above we can see that we need
 108 lux (or 10 footcandles) for ambient lighting
 538 lux (or 50 footcandles) for task lighting for food preparation
Lighting Calculation  Step 2
Work out how many lumens you need
Ambient lighting in the kitchen
In this step we need to work out the area of the kitchen.
Let's say we have a 4m x 3m kitchen we multiply these two numbers together to get an area of 12 square meters.
To get the number of lumens we multiply the lux requirement from step 1 by the area.
This gives us 12 x 108 = 1296 lumens.
If you're working in feet, calculate the area in square feet and use the footcandle value from the light table.
Task lighting in the kitchen
Here we're going to work out the surface are that needs to be lit for food preparation.
Let's say we want to light the countertop which measures 4m x 0.6m. That makes the area 2.4 square meters.
To get the number of lumens the calculation is 2.4 x 538 = 1291 lumens
If you're working in feet, calculate the area in square feet and use the footcandle value from the light table.
Lighting Calculation  Step 3
Work out how many bulbs you need
The number of lumens that a light bulb emits is usually labelled on the packaging. Remember that the lighting fixture you have in mind will determine what type of bulbs you are going to use.
Number of bulbs for ambient lighting
Let's say we were going to use a fluorescent light source for the ambient lighting. Fluorescent tubes that emit 800 lumens each are available. One tube wouldn't be sufficient, but two tubes would give 1600 lumens which would be plenty for our 1296 lumen requirement.
Number of bulbs for task lighting
Let's say we want to use LEDs for the under the cabinet lighting and we've found a bulb we light which gives out 350 lumens.
So we need 1291/350 = 3.6 which means we need 4 of the bulbs to light the countertop.
That's it  we're done!
Lighting Calculation  Summary
Here's a summary and a calculator to help you.
 Use the light table to find out how many lux or footcandles you need.
 Calculate the amount of lumens you need. Multiply the area in square meters by the lux, or the area in square feet by the footcandles.
 work out how many bulbs you need. Divide the number of lumens by the number of lumens delivered by each bulb.
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