Coefficient of Utilization Calculator


About Coefficient of Utilization Calculator (Formula)

The Coefficient of Utilization (CU) is a measurement of lighting system efficiency. It tells you how much light produced by each fixture actually illuminates an area. CU values can be expressed in either decimals or percentage, with higher numbers signifying greater effectiveness.

Calculating the coefficient of utilization requires several factors, including luminous flux from a lamp, its distribution in terms of luminance, surface reflectance in the room and its dimensions.

The formula to calculate this CU is:



  • CU is the coefficient of utilization
  • LRP is the luminous flux reaching the desired plane (lumens)
  • LU is the total luminous flux emitted by the fixture (lumens).

The total light flux received by a working plane is calculated by multiplying the lamp’s luminous flux by its distribution in lumens, then multiplying that figure by the surface reflectance of the room. Thus, for any given lamp, its total luminous flux must equal that produced.

This formula can be simplified as follows:

CU = (Coefficient of Utilization of Luminaire) x (Room Surface Reflectance).

The Coefficient of Utilization for Luminaire is determined by the lamp, fixture and room dimensions. You can obtain this coefficient from manufacturer’s tables or software calculators.

Overall, the coefficient of utilization calculator is an invaluable tool in calculating the efficiency of a lighting system and can assist architects, lighting designers, and engineers optimize their designs for maximum illumination performance and energy efficiency.

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