Lumens Calculator: How to Determine Total Required Lumens for Your Space

How much light is enough to light a room? Most designers and architects use two foundational principles of lighting design when planning and specifying lighting for 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 most commonly used for calculating the total light output needed for your space. With this formula, you first establish the intended use of the space, then you measure its square footage.

This guide will help you understand what the lumen method is and how to calculate it. You can also use the lumens calculator below to determine how many lumens you need based on room size and other key variables.

Table of contents:

1. Helpful Lighting Terms
2. The Lumen Method – How to Calculate Lumens
3. Footcandle Requirements for Various Applications
4. Summary: Calculating Total Lumens Needed for a Room
5. Factoring in Lumen Lighting Variables
6. Lumens Calculator
7. Accounting for Personal Taste

Helpful Lighting Terms

Lumen

Lumen output is a measurement of the total quantity of visible light emitted by a light source. It’s also commonly known as brightness or light output.

The reference point: A standard 100-watt incandescent light bulb produces about 1,500-1,700 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

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 bulbs — has a different lumen-to-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 bulb may claim 100-watt equivalency and energy efficiency. 

Here’s a lumen-to-watt chart. Please note that these ratios may vary slightly, even between different  LED products.

Foot-candle

Foot-candle is the original measurement system for light intensity on a one square-foot surface from a uniform source of light. In other words, a foot-candle 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 a footcandle chart on how many foot-candles of light humans need to perform tasks comfortably in different spaces.

For example, for washing dishes, they recommend that your lighting provides 20 foot-candles of light at two feet, six inches off the floor. This is also referred to as the horizontal target.

The Lumen Method – How to Calculate Total Lumens Needed

1. Determine room size by 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.
2. Establish the footcandle requirement for your application. Lighting requirements vary depending on the type of room being lit, also known as the application. For example, a bathroom or kitchen will require more foot candles than a living room or bedroom. Once you establish the intended use of your space, browse this footcandle chart for the IES-recommended footcandle requirement for your application.
3. Multiply the room square footage by the footcandle requirement. For example, a 100 square-foot living room, which needs 20 foot-candles, will need 2,000 lumens. A 100 square-foot dining room, which needs 40 foot-candles, will require 4,000 lumens.

We’ve created a helpful guide to How Many Lumens You Need that walks you through this calculation in more detail.

Footcandle Requirements for Various Applications

Commercial Lighting Footcandle Requirements

Residential Lighting Footcandle Requirements

Summary: Calculating Total Lumens Needed for a Room

Let’s recap how to gauge how much light you need for a space. Multiply your room square footage by the footcandle requirement. For example, a 100-square foot living room, which needs 20 foot candles, will need 2,000 lumens. A 100-square foot dining room, which needs 40 foot-candles, will need 4,000 lumens.

Factoring in Lumen Lighting Variables

Ceiling Height Variable

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 lumens calculator below will be sufficient for determining the required light output for your application.

If you have high ceilings in a space, then you must account for that in your lumens calculation. You do that with a footcandle multiplier.

To calculate for your recommended footcandle multiplier, use the following formulas:

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.

See Also

What is Architectural Lighting?

Wall Color Variable

If you have especially dark colored walls and furniture or if you’re using light fixtures with shades, you’ll need roughly an additional 10 lumens per square-foot.

Lumens to Candlepower Conversion

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

Lumens Calculator

This free calculator can help you calculate  the lumens needed to light a room. You can then shop for lighting that adds up to the total lumens you need.

Please note that the calculator assumes a standard 8-foot ceiling. It then uses averages and recommended light levels to calculate for different ceiling heights. Also note that these are estimations.  For exact analysis, please consult a lighting design professional.

Accounting for Personal Taste

Personal preference will play a large role  in determining how much light you need in a space. If you like the room to be especially bright, you may want to add an additional 10-20% to our numbers and then install dimmers to adjust the light to desired levels.

When lighting is properly designed in a space, you notice the room and the objects in it. In other words, you notice what the lighting illuminates, not the lighting products themselves. Bad or deficient lighting design shows up as hot spots, dark spots and unintentional shadows.

Good or efficient lighting design accounts for total general and task lighting required for a space. Dynamic or superior lighting design factors the qualitative, human experience. It lights for vertical (not merely horizontal) visual impressions, such as walls — as well as ceilings with uplighting to minimize shadows and dark spots for smooth, streamlined and evenly distributed light.

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