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How Many Lights for an Office at 500 Lux

Offices are commonly designed around 500 lx on the working plane for desk tasks, which is the figure given for writing, typing and reading in the European lighting standard EN 12464-1. The lumen method turns that target into a number of luminaires once you know the room size, the luminaire output and two factors that account for room geometry and ageing.

The calculator below is preset for a 12 × 8 m open office with the luminaires 2.05 m above the desk height (a 2.8 m ceiling and 0.75 m desks), 3600 lm LED panels of 30 W, a utilisation factor of 0.6 and a maintenance factor of 0.8. Replace the utilisation factor with the value from the luminaire maker's table for your room index and reflectances.

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The lumen method spreads N luminaires evenly over the room so that the working plane receives the target illuminance E. Keep the spacing S within the luminaire's spacing-to-height ratio for even light.

For estimation and education. Not a substitute for a licensed engineer, the applicable codes (IEC 60364, NEC/NFPA 70, BS 7671, local regulations) or manufacturer data. Read the disclaimer.

Inputs

Room

m
m
m

Luminaire height minus working-plane height, e.g. 2.8 − 0.8 m

lx

For example, offices are commonly designed around 500 lx at the task (EN 12464-1 / ISO 8995-1). Take the value for your task from the standard that applies.

Luminaire

lm

For LED luminaires, enter the luminaire flux and 1 lamp

From the maker's UF table for the room index and reflectances

0.8 is an example for LED luminaires in a clean room

W

Circuit watts from the datasheet; the default is an example

Result

Lighting (lumen method) — Engineerly. Estimate for education only; verify against applicable codes and manufacturer data.

Luminaires needed
28
Maintained illuminance achieved
504lx

28 luminaires give 504 lx, meeting the 500 lx target.

Room area
96m²
Room index K
2.34
Exact number before rounding
27.778
Suggested layout (rows × columns)
4 × 7 = 28
Spacing along the length × across the width
1.71 m × 2 m
Installed lighting power (layout)
840W
Lighting power density
8.75W/m²

Working, with your numbers

  1. 1. Room area
    A = L × W
    A = 12 m × 8 m
    = 96 m²
  2. 2. Room index
    K = L × W / (Hm × (L + W))
    K = 96 / (2.05 × (12 + 8))
    = 2.341
  3. 3. Number of luminaires (exact)
    N = E × A / (Φ × n × UF × MF)
    N = 500 lx × 96 m² / (3,600 lm × 1 × 0.6 × 0.8)
    = 27.778
  4. 4. Number of luminaires (rounded up)
    N = ⌈N_exact⌉
    N = ⌈27.7778⌉
    = 28
  5. 5. Maintained illuminance with the rounded number
    E = N × Φ × n × UF × MF / A
    E = 28 × 3,600 × 1 × 0.6 × 0.8 / 96
    = 504 lx
  6. 6. Suggested layout
    rows × columns ≥ N, columns along the length
    4 × 7
    = 28
  7. 7. Installed lighting power
    P = N_layout × P_luminaire
    P = 28 × 30 W
    = 840 W
  8. 8. Lighting power density
    LPD = P / A
    LPD = 840 W / 96 m²
    = 8.75 W/m²

Assumptions

  • Lumen method: average maintained illuminance on the working plane over the whole room area.
  • Utilisation factor 0.6 and maintenance factor 0.8 as entered.
  • Mounting height is measured from the working plane to the luminaires, not from the floor.
  • Columns run along the room length and rows across the width; the first and last luminaires sit half a spacing from the walls.
Room: 12 m × 8 m, Hm 2.05 m
Target illuminance: 500 lx
Luminaire: 1 × 3600 lm, 30 W
UF / MF: 0.6 / 0.8

Check before you rely on this

  • The utilisation factor depends on the luminaire's light distribution, the room index and the ceiling, wall and floor reflectances. Take it from the maker's UF table for your luminaire; 0.6 is only an example.
  • The lumen method gives the average illuminance on the working plane. Uniformity, glare (UGR), cylindrical illuminance and emergency lighting need a point-by-point calculation, for example in DIALux or Relux.
  • Keep the luminaire spacing within the maximum spacing-to-height ratio given by the maker, or the light will be uneven.
  • Estimation and education only. Not a substitute for a licensed engineer, the applicable code (IEC 60364, NEC/NFPA 70, BS 7671 or local regulations) or manufacturer data.

Notes for this case

  • The lumen method gives the average illuminance; uniformity and glare need a point-by-point calculation in lighting design software.
  • Arrange luminaires so the spacing does not exceed the maker's maximum spacing-to-height ratio.
  • Daylight and controls reduce energy use but do not change the installed number needed for night-time.

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How these formulas were checked: methodology and testing.