Factory Lighting Calculation: Number of Luminaires and Power with a Worked Example
Short answer
Factory lighting is calculated with the lumen method: number of luminaires = (target lux × area) ÷ (luminaire lumens × utilisation factor × maintenance factor). For example, in a 40 × 25 m hall with an 8 m ceiling, about 27 luminaires of 20,000 lumens (UF 0.7, MF 0.8) are needed for 300 lux; for uniformity the layout is rounded to a grid of 30 luminaires and the result is verified with lighting software.
In factory lighting, the answer to "how many luminaires do we need?" is found not by guesswork but in a few simple steps. In this article we carry out the calculation for a production hall from start to finish. The values are examples; in a real project the luminaire's photometric file and the dimensions of the space are used.
Example: a 40 × 25 m production hall
| Data | Value |
|---|---|
| Area | 40 m × 25 m = 1,000 m² |
| Ceiling (mounting) height | 8 m |
| Working plane | 0.85 m |
| Task | Rough assembly → target 300 lx |
| Environment | Moderately dusty → maintenance factor 0.8 (with a cleaning schedule) |
| Luminaire | Industrial LED, 20,000 lm, 140 W (example) |
Step 1: Room index
The mounting height is measured above the working plane: Hm = 8 − 0.85 = 7.15 m.
k = (40 × 25) ÷ (7.15 × (40 + 25)) = 1,000 ÷ 464.75 ≈ 2.15
Step 2: Utilisation factor
The utilisation factor (UF) is the proportion of the light leaving the luminaire that reaches the working plane. It is read from the photometric table according to the room index, the ceiling, wall and floor reflectances and the luminaire's light distribution. In this example we assume UF = 0.7 for an industrial luminaire that concentrates light downwards from a high ceiling.
Step 3: Number of luminaires
N = (300 × 1,000) ÷ (20,000 × 0.7 × 0.8) = 300,000 ÷ 11,200 ≈ 26.8 → 27
For an even layout, the 27 luminaires are rounded to a grid that fits the proportions of the space: 6 rows × 5 luminaires = 30 luminaires. The spacings are 40 m ÷ 6 ≈ 6.7 m and 25 m ÷ 5 = 5 m; whether these spacings suit the mounting height (the maximum spacing the luminaire allows) is checked against the photometric data.
Step 4: Installed power and comparison
| LED (example) | Metal halide 400 W | |
|---|---|---|
| Luminaire power (incl. ballast) | 140 W | ≈ 430 W |
| Number of luminaires | 30 | 30 (for similar light) |
| Installed power | 4.2 kW | ≈ 12.9 kW |
| Power density | 4.2 W/m² | ≈ 12.9 W/m² |
To see the annual energy and cost difference with your own operating hours, you can use our LED savings calculator.
Step 5: Verification with software
The lumen method gives the average illuminance. Shadows from racks and machines, the drop in the corners and uniformity (Uo) can only be seen in lighting software using the luminaire's photometric file. The calculation report should include the average illuminance, the minimum value and the uniformity for each area.
Typical target values
| Area / task | Illuminance (lx) | UGR | Ra |
|---|---|---|---|
| Rough assembly | 300 | ≤ 25 | 80 |
| Medium precision | 500 | ≤ 22 | 80 |
| Fine assembly | 750 | ≤ 19 | 80 |
| Warehouse, storage | 100 | ≤ 25 | 60 |
The values in the table are typical targets from EN 12464-1; in a project, the table in the current edition of the standard and the specific conditions of the space should be used.
Conclusion
A factory lighting calculation has five steps: target, room index, utilisation and maintenance factors, number of luminaires and verification with software. If you would like a calculation and layout plan for your facility, take a look at our lighting design service.
Frequently Asked Questions
How is the number of luminaires calculated for factory lighting?
With the lumen method: number of luminaires = (target lux × area) ÷ (luminaire lumens × utilisation factor × maintenance factor). The result is rounded to a regular grid and verified with software.
What is the room index?
It expresses the geometry of the space in one number: k = (length × width) ÷ (mounting height × (length + width)). The utilisation factor is read from the luminaire's photometric table according to this value and the surface reflectances.
What maintenance factor should be used in a dusty factory?
About 0.8 is usually used in clean environments; in dusty and dirty environments lower values are used, depending on how often the luminaires are cleaned.
How much power is saved by converting metal halide luminaires to LED?
For the same illuminance the installed power usually falls clearly; in the hall of the worked example, installed power drops to about one third compared with the 400 W metal halide solution.