Calculation Methods for the Number of Lighting Fixtures Needed

Technology and Calculations

• From single rooms to multi-level commercial spaces, calculating the number of light fixtures necessary to illuminate the entire area doesn't have to be difficult. Although there are many different ways to calculate for lighting needs, there are simple online and software based tools available to assist with this process. If you don't feel like putting pencil to paper, try an online calculator tool such as Philips' CalcZone, DMS Lighting's Lighting and Fixture Calculator or A-Wall's Light Fixture Calculator. While these online tools are best for smaller or home-based projects, computer software such as Elite's Zonal Cavity Lighting Calcs is an option for large or commercial needs.

Zonal Cavity Method

• The zonal cavity method is a well-used calculation tool for deciding on the amount of light, or fixtures, necessary for a specific space. Calculations done in this method take into account the overall room cavity, or area, for both regular and irregularly shaped spaces. The results of a zonal cavity calculation tell you how many lights or how much illumination you will need for your room's specific width, length and depth. For a regularly shaped room, the zonal cavity formula typically equals five times the room's depth times the sum of the room's width and length divided by the product of the width and length.

Footcandles

• One of the key points to determining the number of lighting fixtures that you need is assessing how much light falls on the room's spaces. Understanding what a footcandle, or the measurement of light on a surface, is a vital part of calculating how many fixtures any room can accommodate. As this measurement is equivalent to one lumen for every square foot of space, you can use the lighting fixtures lumen rating to calculate footcandles. Dividing total lumens by the room's square footage will give you the number of footcandles that the lamp produces. Although there is no steadfast number of footcandles allowable for any specific room space, most residential rooms need approximately 15 footcandles per square foot of space.

Considerations

• Any lighting calculation method is dependent on several factors. These considerations include your overall square footage, shape of the room, natural or ambient light filtering in from windows or outside spaces, watts per light bulb in use and number of bulbs per fixture. For example, you try an online estimator that calculates your fixtures needs at three per your entire room space. Don't run out just yet and buy any three fixtures. Instead, look at the wattage of the bulbs that you are placing into the fixtures as well as the number of bulbs. There is a vast difference between three fixtures each with one 40-watt bulb and three fixtures each with three 60-watt bulbs.