How much light is enough? Most designers and architects use two foundational principles of lighting design when planning and specifying lighting for any space —the qualitative (or aesthetic) aspect and the quantitative (or engineering) aspect of light. Calculating for total lighting required is considered quantitive.
Use the lumen method, the most commonly used light output formula, to calculate the total light output needs for your space. First, establish the intended use of the space, then, measure its square footage.
Here are a few key terms.
Lumen output, also known as brightness or light output, is a measure of the total quantity of visible light emitted by a light source. The reference point: a standard 100-watt incandescent light bulb produces about 1,500 to 1,700 lumens. Strictly speaking, 600 LED lumens provides the same amount of light as 600 incandescent lumens. LED lights provide higher Color Rendering Index (CRI), so, while they more accurately reveal the colors of the subject being lighted, they don’t provide more light.
Wattage is a measure of how much electricity (or energy) a light bulb consumes to achieve its lumen output. Each type of light source—LED, fluorescent, halogen or incandescent—has a different lumen-per-watt ratio. If a 100-watt incandescent light produces 1,500 lumens, and a 10-watt LED light does the same, the 10-watt LED light may claim 100-watt equivalency and energy efficiency.
Here’s a lumen-to-wattage ratio chart. Please note that lumen-per-watt ratios may slightly vary, even from LED to LED products.
Footcandle is the original measurement system for light intensity on a one square foot surface from a uniform source of light. In other words, a footcandle is the light measured one foot away from a candle. Considering the human-centric principles of lighting design, the IES, the largest society of professional lighting designers, provides recommendations of how many footcandles of light humans need to perform varying tasks and comfortably occupy various spaces.
For example, for washing dishes, they recommend that your lighting provides 20 footcandles of light at two feet, six inches off the floor. This is also referred to as the horizontal target.
The Lumen Method
Calculating total lumen output required for your application
- Determine room square footage. Multiply the length times the width of the room to get the room square footage. For example, if the room is 10 feet wide and 10 feet long, the room square footage will be 100 square feet.
- Establish the footcandle requirements for your application. Lighting requirements vary depending on the type of room being lit. For example, a bathroom or kitchen will require more footcandles than a living room or bedroom. Once you establish the intended use of your space, browse this light level chart for the IES recommended footcandle requirement for your application.
- Multiply room square footage by the footcandle requirement. For example, a 100 square-foot living room, which needs 20 footcandles, will need 2,000 lumens. A 100 square-foot dining room, which needs 40 footcandles, will require 4,000 lumens.11 See PDF and complete foot candle index.
Here are some suggested footcandle recommendations
By use, space and application
|Offices: Average Reading and Writing||50-75|
|Offices: Rooms with Computers||20-50|
|Auditorium / Assembly||15-30|
|Hospitals: General Areas||10-15|
|Labs / Treatments Rooms||75-100|
Determining how much light you need for any room is simpler than you probably imagined. Multiply your room square footage by the footcandle requirement. For example, a 100 square foot living room, which needs 10-20 foot-candles, will need 1,000-2,000 lumens. A 100 square foot dining room, which needs 30-40 foot-candles, will need 3,000-4,000 lumens.21 See PDF and complete foot candle index.
Note: Only read this section if your ceiling height is taller than 10ft. If your ceiling height is below 10ft, the Lumen Method above and the calculator below are sufficient tools for determining the required light output for your application.
To calculate for your recommended footcandle multiplier, use the following formulas:
recommended 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 square footage.
Our calculator assumes a standard 8-foot ceiling. It then uses averages and recommended light levels to calculate for different ceiling heights. Please note, these are estimations. For exact analysis, please consult a lighting design professional.
If you have especially dark colored walls and furniture or if you’re using fixtures with shades, you’ll need roughly an additional 10 lumens per square foot.
Candlepower is a unit of measurement for luminous intensity. It expresses levels of light intensity relative to the light emitted by a candle of specific size and constituents. The historical candlepower is equal to 1.02 candelas. In modern usage, candlepower is sometimes used as a synonym for candela.
Lumen output = C/0.07958
For example, 200 candlepower would equal: L = 200 candlepower / 0.07958. Do the division using a calculator: 200 candlepower / 0.07958 = 2 513 lumens.
Alcon Lighting creative director and co-founder David Hakimi works to achieve efficiency in lighting, affording architects and lighting designers and engineers the ability to maximize ideal lighting. David takes pride in the company’s design, energy and building knowledge, tracing the commitment to add value to lessons learned from his Southern California lighting trade and craftsman father. David aims to assure that each customer complies with codes and achieves project goals with artistry, economy and excellence.