A luminaire is a complete light fixture including a lamp or lamps, ballasts (if needed), housing and accessories. Properly spacing luminaries maximizes their ability to light a given area. Each model of light fixture produces a different effective area of lighting, meaning that the number of light fixtures needed to light a given space varies depending on the model. Before calculating the spacing between luminaries, you need to have already figured the number of luminaries that are necessary to light the area.

Measure the length and width of the area to be lit, then multiply the two numbers to calculate the square area.
Example: A room is 18 feet long and 15 feet wide. Multiply 18 feet by 15 feet to get 270 square feet.

Divide the space's square area by the number of luminaries to be installed.
Example: You plan to install four light fixtures in a room that is 270 square feet. Divide 270 square feet by four to get the answer: 67.5 square feet.

Calculate the square root of the number you found in Step 2, and round the answer to the nearest tenth to convert that number from square feet to feet. The resulting number is the distance you should space your luminaire.
Example: The square root of 67.5 is 8.2158. The rounded answer is 8.2 feet, or 8 feet 2.4 inches.

Check your selected light fixtures' manufacturer data to find the maximum allowed spacing for that model. This distance is the farthest apart you can space the selected luminaries while maintaining a usable level of light. For used light fixtures or ones that are missing this data, contact the manufacture directly.

Compare the distance you calculated in Step 3 with the manufacturer's maximum spacing requirements. If your distance is less then the manufacturer's maximum spacing, your lights will provide adequate illumination. If your distance is farther than the manufacturer's recommendation, add one or more light fixtures and recalculate the spacing.
Tips & Warnings
 Doublecheck your mathematical answers for each step before proceeding to help eliminate mistakes.
 Spacing light fixtures in a commercial project or any publicly accessible area should be handled by a professional to ensure safety codes are met.
Related Searches
References
 Lightsearch: Light Guide: Useful Formulas
 "Electrical and Lighting"; J. Trost; 1999
 Photo Credit light image by zina from Fotolia.com