Lighting Design Calculator

Calculate luminaire quantities and layout grids using the Zonal Cavity Method. Select space categories, luminaire types, and room dimensions to get average lux levels, uniformity ratios, and lighting power density with compliance checking.

IESNA · EN 12464-1 · AS/NZS 1680

Design Parameters

Room Dimensions

Surface Reflectances & Maintenance

Results

Luminaires Required
25
Layout Grid
5 x 5
Average Lux
546 lx
Total Wattage
875 W
Min Lux
327 lx
Max Lux
764 lx
Uniformity Ratio
0.60
LPD
10.94 W/m²
Room Cavity Ratio
2.81
CU
0.546
Target Lux: PASS (546 >= 500 lx)
Uniformity: PASS (0.60 >= 0.60)

Luminaire Layout (5 x 5)

10.0 m8.0 m

About Lighting Design Calculator

The lighting design calculator determines how many luminaires a room needs and lays them out on a grid using the Zonal Cavity Method (the lumen method). It is used by lighting designers and electrical engineers to hit target illuminance per IESNA, EN 12464-1, and AS/NZS 1680 while checking uniformity and lighting power density.

Enter the room dimensions, mounting and workplane heights, surface reflectances, the luminaire type, and a target lux level. The tool computes the room cavity ratio, the coefficient of utilization, the number of luminaires, the average maintained illuminance, the uniformity ratio, and the lighting power density in real time.

How It Works

  1. Compute the room cavity ratio RCR = 5 * hrc * (L + W) / (L * W) from the cavity height and room dimensions.
  2. Interpolate the base coefficient of utilization from the RCR and adjust it for the ceiling and wall reflectances.
  3. Solve the lumen method for the number of luminaires N = (E * A) / (F * CU * MF) and round up.
  4. Arrange the luminaires on a grid, recompute the average maintained illuminance, and check uniformity and lighting power density against the standard.

Worked Example

A 10 m by 8 m office (area 80 m2) with mounting height 3.3 m and workplane 0.8 m gives a cavity height of 2.5 m, so RCR = 5 * 2.5 * (10 + 8) / 80 = 2.8125. With ceiling reflectance 0.8 and wall reflectance 0.5 the coefficient of utilization is 0.5458. For a 4000 lm luminaire, maintenance factor 0.8, and a 500 lx target, N = (500 * 80) / (4000 * 0.5458 * 0.8) = 22.9, rounded up to 23 and laid out as a 5 by 5 grid.

Formulas

Room cavity ratio
RCR = 5 * hrc * (L + W) / (L * W)
Adjusted coefficient of utilization
CU = baseCU * (0.8 + 0.2*rho_c) * (0.7 + 0.3*rho_w)
Lumen method (number of luminaires)
N = (E * A) / (F * CU * MF)
Lighting power density
LPD = (N * W) / A

Standards & References

  • IESNA Lighting Handbook
  • EN 12464-1
  • AS/NZS 1680

Frequently Asked Questions

What is the lumen method?

The lumen method (Zonal Cavity Method) finds the number of luminaires N = (E * A) / (F * CU * MF) needed for a target average illuminance, where F is the lumen output, CU the coefficient of utilization, and MF the maintenance factor.

How is the coefficient of utilization found?

The base CU is interpolated from the room cavity ratio, then adjusted for ceiling and wall reflectances. A higher RCR (a tall, narrow room) lowers the CU because more light is lost before reaching the workplane.

What uniformity ratio is acceptable?

The tool checks the minimum-to-average illuminance ratio against the standard: EN 12464-1 expects at least 0.7, while IESNA and AS/NZS 1680 use 0.6. A higher ratio means more even lighting across the workplane.

What is lighting power density?

Lighting power density (LPD) is the installed lighting power per unit floor area, LPD = (N * W) / A in W/m2. Energy codes cap LPD by space type, so a lower value indicates a more efficient design.