Lighting Calculation

Lighting Calculation Terms

The lighting tables you'll be referencing will tell you how much light you need for each room or task in either footcandles (imperial measurements) or lux (metric).

Let's first define a few lighting terms.

Candela: One candela is equivalent to the illumination from one standard candle. (There is a far more technical definition if you're interested at Wikipedia's candela page.)

For those working in the imperial system:
One footcandle is the amount of illumination on a surface created by a light source of one candela that is a foot away from the surface.

In the metric system:
One lux is the amount of illumination on a surface created by a light source of one candela that is a metre away from the surface.

When you purchase light bulbs there will generally be two numbers of interest on the packaging. One is Watts which measures the power draw of the bulb. The other is lumens.

For those using feet, one footcandle is equal to 1 lumen/square foot.

For those using metres, one lux is equal to 1 lumen/square metre.

So in order to calculate your lighting needs for a given room, you check an illumination chart for the optimal number of footcandles or lux for a given task and then multiply by the square footage (or metres) of the room to obtain the number of required lumens.

Below is a chart for basic tasks and room functions. Below the table you'll find an example home lighting calculation for a kitchen.

If you are using a very small screen or smart phone, rotate your device to landscape view to read the table below.

How to Calculate Lighting for a Kitchen

Note: The lighting calculation example below is calculated using the imperial system (feet). If you are working in metres, simply exchange the footcandle numbers for the appropriate lux numbers from the table and calculate your room area in square metres.

Let's do a lighting calculation for a 10 by 12 foot kitchen as an example. For our basic general kitchen lighting, we know from the table above that we'll need 20-50 footcandles. For food preparation, we'll want more like 50 to 100 footcandles.

Let's start by calculating the area of the kitchen. By multiplying the length and width of our kitchen together we get 10 feet X 12 feet = 120 square feet.

Now to calculate the required lumens for the kitchen we multiply the number of footcandles (let's take the dimmest general lighting level of 20 footcandles first) by the square footage. For this we'll need 20 footcandles X 120 square feet = 2400 lumens.

For the maximum foot prep level of 100 footcandles, the calculation would be 100 footcandles X 120 square feet = 12,000 lumens.

For compact fluorescent lights (CFL) the illuminance tends to be about 40 to 70 lumens per Watt of power draw (incandescent lights are more like 10-17 lumens/Watt). For our example let's use 20 Watt CFLs rated at 1200 lumens.

So for our lowest light requirement of 2400 lumens, the calculation would be:

2400 lumens / 1200 lumens per bulb = 2 bulbs

For our brightest light requirement of 12,000 lumens, the calculation would be:

12,000 lumens / 1200 lumens per bulb = 10 bulbs

That seems like a lot of lights but if you consider all the light possibilities for a kitchen: dimmable recessed lights, some under cabinet lights, the light on the stove top vent hood and a few track or hanging lights right above an island or prep counter, you could reach that ten bulb level.

For some however, this level of 12,000 lumens may be simply too bright. For a more personalized home lighting design, do a few quick calculations in your current home to determine the light level in a given room. Compare the light level of that room to the tasks shown in the table above. If you feel the light in that room is inadequate, bring in a few extra lamps from other rooms until the light seems right. Add up the number of lumens from all the bulbs in the room and then calculate the number of footcandles you now have in that room. Compare this number to the chart above to get a feel where in each range you prefer your lighting.

Keep in mind that any kind of shade over the light fixture, whether it is a lamp shade or a colored glass pendant over the bulb will lower the number of lumens output for that bulb.

To achieve the variation in light level required between the general kitchen lighting level and the food prep lighting level, you can group your lights on a few different switches. Under-cabinet lights are often on a separate switch as are the lights in the stove vent hood. You can also have any fixtures directly over an island or peninsula style counter on their own switch.

Some or all lights can also be put on dimmer switches.

If you are doing your home lighting design and the above calculations seem too frustrating, check out LightCalc Lighting Software which will do all the lighting calculations for you.