Lighting Calculation

Figure out how much light you need in each room and how many fixtures you'll need.

If the idea of doing a lighting calculation is a bit off-putting, 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/m²

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/ft²

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.

Note for mobile users - to view the table you'll have to switch to full website mode.

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.

Lumens labelled on light bulb packaging

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.