How to calculate the intensity of light needed for your rooms
How do we know when we have enough light? That’s a tough enough question, but when you have to calculate how much LED lighting you need to create a well-lit space, it can be a bit more complicated.
Whether it’s LED lighting in architecture used in commercial or residential settings, here we’ll show you how to determine how many LED lumens you’ll need to adequately illuminate your environments.
A measure of the light emitted by a source, whether LED, fluorescent, halogen or incandescent. This is also known as “brightness” or “light output.” Your point of reference: a standard 100-watt incandescent bulb produces about 1,500 to 1,700 lumens (lm).
Not a measure of brightness; but a measure of how much electricity (or energy) a bulb consumes to achieve its brightness. Each type of light source, LED, fluorescent, halogen or incandescent has a different lumen per watt ratio. Below we are going to use lumens as a measure to make sure we have enough light for a space.
Since we have combined watts and lumens, it is easier to talk about bulbs in terms of watts. So, if a 100-watt incandescent bulb produces 1,500 lumens and a 10-watt LED does the same, the 10-watt LED can say “100-watt equivalent” on its label.
Here is a wattage equivalency chart, but keep in mind that lumen per watt ratios may have slight variation, even from LED to LED products, covering architectural lighting applications such as linear pendant lighting, non-trim LED recessed lighting, or linear recessed lighting.
The number of lumens a bulb generates for each watt it consumes. The higher the number, the more efficient the bulb. For example, lighting products that have earned the ENERGY STAR label are highly efficient, meaning they offer the same features while using less energy.
But: how much light is enough?
Determine the area of the room
Multiply the length by the width of the room to get the area. For example, if the room is 10 feet wide and 10 feet long (approximately 10 feet wide and 10 feet long), the square footage would be 100 square feet (9m²).
Determine foot candles by type or use of the room
The illuminance unit of measure “foot-candle” refers to how bright a light is one foot (approx. 30cm) from its source. Lighting requirements/needs vary depending on the type of room being lit. For example, a bathroom or kitchen will require more foot-candles than a living room or bedroom.
Determining the lumens needed
A lumen is a unit of light measurement. To determine the lumens needed, the area of the room should be multiplied by the area of the room according to the required need for its use. For example, a living room of 100 square feet (approx. 9m²), which needs 10-20 footcandles, will need 1,000-2,000 lumens. A dining room of 100 square feet (approx. 9m²), which needs 30-40 footcandles, will need 3,000-4,000 lumens.
A quick summary
For an average space of 250 square feet (approx. 23m²), you’ll need about 5,000 lumens as your main light source (20 lumens x 250 square feet). In the dining room, you’ll want about 30 lumens per square foot at the table (you’ll want to see the food, but not examine it), so if your table is 6 x 3 feet (2m x 1m), that’s 540 lumens.
Keep in mind, however, that these numbers are for typical conditions. If you have very dark colored walls and furniture or if you use fixtures with shades, you will need approximately 10 additional lumens per square foot. We based our calculations on 8-foot ceilings. Finally, personal preference will play the biggest part in the decision. If you like the room to be especially bright, you can add an additional 10 to 20% to our numbers. In fact, the best approach for most spaces is to oversize and install dimmers to reduce the light level.