How much light does it take to illuminate a room? Most designers and architects use two fundamental principles of lighting design when planning and specifying the lighting of any space:
- the qualitative (or aesthetic) aspect of light
- the quantitative (or engineering) aspect of light
Calculating for total lighting required is considered quantitative.
The lumen method is the method most commonly used to calculate the total light output required for your space. Using this formula, you first determine the intended use of the space and then measure its area.
This guide will help you understand what the Lumen Law is and how to calculate it. You can also use the lumen calculator below to determine how many lumens you need based on room size and other key variables.
Useful Lighting Terms
Lumen
Lumen output is a measure of the total amount of visible light emitted by a light source. It is also commonly referred to as brightness or light output.
Point of reference: A standard 100-watt incandescent light bulb produces approximately 1,500-1,700 lumens. Technically, a 600 lumen LED provides the same amount of light as a 600 lumen incandescent.
LED lights offer a higher color rendering index (CRI), so while they can more accurately show the color of the object being illuminated, they don’t provide as much light.
Watts
Wattage is a measure of how much electricity (or energy) a bulb consumes to achieve its lumen output.
Each type of light source—LED, fluorescent, halogen, or incandescent bulb—has a different lumen-to-watt ratio. If a 100-watt incandescent lamp produces 1,500 lumens and a 10-watt LED lamp produces the same brightness, then the 10-watt LED bulb may claim to have 100-watt equivalent power and energy efficiency.
Here is the lumen watts chart. Note that these ratios may vary slightly, even between different LED products.
Foot Candles
Foot candles are a primitive measurement system for measuring the intensity of light on a one square foot surface from a uniform light source. In other words, a foot-candle is the light measured one foot from the candle.
Considering the human-centered principle of lighting design, IES (the largest association of professional lighting designers) provides candlepower charts to illustrate how many candlepower humans need to comfortably perform tasks in different spaces.
For example, for dishwashing, they recommend that your lighting provide 20 foot-candles of light at 2 feet 6 inches from the floor. This is also known as a horizontal goal.
How to Calculate the Total Lumens Needed
- Determine room size in square feet. Multiply the length by the width of the room to get the area of the room. For example, if the room is 10 feet wide and 10 feet long, the room will be 100 square feet.
- Establish candlepower requirements for your application. Lighting requirements vary by the type of room (also known as the application) being illuminated. For example, a bathroom or kitchen needs more foot-candles than a living room or bedroom. Once the intended use of the space has been determined, browse this candle chart to see the candle requirements recommended by IES for your application.
- Multiply the square footage of the room by the candlepower requirement. For example, a 100 square foot living room requiring 20-foot candles will require 2,000 lumens. A 100 square foot restaurant that requires 40-foot candles will require 4,000 lumens.
We’ve created a helpful guide to help you understand this calculation in more detail.
Footcandle Requirements for Various Applications
Commercial Lighting Footcandle Requirements
Residential Lighting Footcandle Requirements
Calculate the total lumens needed for the room
Let’s review how to gauge how much light a space needs. Multiply your room square footage by the candlepower requirements. For example, a 20-foot candle in a 100 square foot living room would require 2,000 lumens. A 100 square foot restaurant that requires 40-foot candles will require 4,000 lumens.
Download a Free Footcandle Chart »
Consider Lumen Lighting Variables
variable ceiling height
NOTE: Only read this section if your ceiling height is higher than 10 feet. If your ceiling height is less than 10 feet, the lumen method above and the lumen calculator below are sufficient to determine the light output required for your application.
If the ceilings in the space are high, this must be taken into account in the lumen calculations. You can do this by using the footcandle multiplier.
To calculate the recommended candle multiplier, use the following formula:
footcandles (fc) = cd ÷ h
cd = candlepower
h = distance between the lamp and the horizontal target
Once you determine your desired footcandle level, you multiply that by your room’s square footage.
variable wall color
If you have particularly dark walls and furniture, or if you use shaded fixtures, you’ll need about an extra 10 lumens per square foot.
lumens to candela conversion
A candela is a unit of measurement for luminous intensity. It expresses the level of light intensity relative to the light emitted by a candle of a particular size and composition.
A historical candle equals 1.02 candela. In modern usage, candela is sometimes used as a synonym for candela.
Lumen output = C/0.07958
Consider Personal Taste.
Personal preference will play a big role in determining how much light you need in your space. If you want the room to be extra bright, you may want to add an extra 10-20% to our numbers and install a dimmer to adjust the light to the desired level.
When the lighting in a space is well designed, you notice the room and the objects in it. In other words, you notice what the light illuminates, not the lighting product itself. Poor or insufficient lighting design manifests itself as hot spots, dark spots, and unexpected shadows.
Good or efficient lighting design accounts for the total general and task lighting needed for a space. Dynamic or superior lighting design can affect the qualitative human experience. It illuminates vertical (not just horizontal) visual impressions such as walls – and ceilings with upward lighting to minimize shadows and dark spots for smooth, streamlined and evenly distributed light.
