CALCULATING LIGHTS FOR INTERIORS

Key Terms

Lumens

A measurement of light emitted by a source, whether it’s LED, Fluorescent, Halogen, or Incandescent. This is also known as “brightness” or “light output.” Your reference point: A standard 100-watt incandescent light bulb produces about 1,500 – 1,700 lumens.

Watts


Not a measure of brightness; instead, it’s a measure of how much electricity (or energy) a bulb consumes to reach its claimed brightness. Each type of light source, LED, Fluorescent, Halogen, or Incandescent has a different lumen-per-watt ratio. Below we’re going to use lumens as a measurement to make sure we have enough light for a space.


To calculate the lighting wattage needed in any room:

Multiply width of room x length of room x 1.5 = amount of total minimum wattage needed (assuming an eight foot ceiling). 

Divide by 60 to get the number of 60-watt equivalent bulbs required.

Areas where high-detail, task-specific work occurs or rooms in which elderly people reside, for example, require higher levels of light. Multiply the width by the length of the room and multiply the result by 2.5 to estimate higher light-level requirements. A room 10 feet wide by 12 feet long would need 300 watts under this formula.

We can disperse lighting wattage around the room to achieve both task lighting and ambient lighting as appropriate. A 10-by-12-foot room, for example, lit for average lighting -- using the 1.5 multiplier -- may divide the required 180 watts among a reading lamp of 60 watts and three track lights of 40 watts for ambient lighting. Adding a ceiling fan with another 60 watts light ensures that you have more intense light should you need it, though you would be slightly shy of the 300 watt high-light figure.