Room Lighting Calculator
A room lighting calculator determines how many luminaires are needed to achieve a target illuminance level in a room using the IES zonal cavity (lumen) method. Proper lighting design ensures adequate visibility for tasks, reduces eye strain, and meets energy code requirements. The calculation requires the target illuminance (foot-candles or lux), room dimensions, the lumens per fixture, and efficiency factors (coefficient of utilization and light loss factor). This tool calculates the total lumens required, divides by the output per fixture adjusted for efficiency, and rounds up to the nearest whole fixture count.
Zonal cavity (lumen) method
Total lumens = Target FC x Room Area / (CU x LLF)
Number of fixtures = ceiling(Total lumens / Lumens per fixture)
CU (coefficient of utilization) accounts for the fraction of fixture lumens reaching the workplane. LLF accounts for lamp aging, dirt, and maintenance. Use photometric data from the luminaire manufacturer for precise CU values.
Frequently asked questions
How many lumens do I need per square foot?
The required lumens per square foot (foot-candles) depend on the task. General ambient office lighting: 30-50 foot-candles (fc). Detailed work or drafting: 50-100 fc. Hallways: 10-20 fc. Retail display: 50-150 fc. Kitchen countertops: 50-70 fc. The IES Lighting Handbook provides recommended illuminance levels for all applications.
What is the zonal cavity method for lighting design?
The zonal cavity (lumen) method calculates the average illuminance level on the workplane using: E = (N x F x Cu x LLF) / A, where E is illuminance (foot-candles), N is number of luminaires, F is luminous flux per luminaire (lumens), Cu is coefficient of utilization, LLF is light loss factor, and A is room area.
What is the coefficient of utilization (CU)?
The coefficient of utilization (CU) is the fraction of luminaire lumens that reach the workplane. CU depends on the luminaire type, room cavity ratio (RCR), and ceiling/wall reflectances. CU typically ranges from 0.4 to 0.8. The luminaire manufacturer's photometric data provides CU values for different room conditions.
What is a light loss factor (LLF)?
The light loss factor (LLF) accounts for the reduction in light output over time due to lamp lumen depreciation, luminaire dirt, and other factors. A typical combined LLF for well-maintained LED luminaires is 0.80 to 0.90. Older fluorescent systems may use LLF values of 0.65 to 0.75.
How do I convert foot-candles to lux?
1 foot-candle = 10.764 lux. To convert: Lux = Foot-candles x 10.764. US lighting codes and standards typically use foot-candles; international standards (IEC, ISO) use lux. The IES Lighting Handbook provides recommended values in both units.
Official sources
- Illuminating Engineering Society: IES Lighting Handbook - Illuminance Recommendations and Zonal Cavity Method.
- U.S. Department of Energy: DOE Energy Saver - Lighting Technology and Design.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.