CALCULATORS // LIGHTING
Estimate the average maintained illuminance (lux) for a single room using the lumen method. This gives a rough design-stage figure — not a substitute for a full photometric calculation using manufacturer data.
ESTIMATED AVERAGE ILLUMINANCE
— lx
Enter room dimensions to see a result.
What each figure means
Two units keep coming up: lumens measure how much light a luminaire produces (its rated output); lux measures how much of that light actually lands on a surface once the room is accounted for. This calculator uses the standard lumen method formula:
E = (N × F × UF × MF) ÷ A- E Average illuminance (lux)
- N Number of luminaires
- F Luminous flux per luminaire (lumens)
- UF Utilisation factor
- MF Maintenance factor
- A Room area (m²)
- Room length & width
- The room’s floor dimensions in metres. Together they set the room area (length × width), which is the divisor in the formula. Example: the defaults below use a 6m × 4m room.
- Luminaire height above floor
- How high above finished floor level the luminaire itself is mounted. Combined with the working plane height, this sets how far the light has to travel to reach where it’s needed — which affects how efficiently it does so.
- Working plane height
- The height of the surface you actually need lit — not always the floor. Typically 0.8m for a desk, or 0m for floor-level areas like corridors or warehouses. The gap between this and the luminaire height affects how efficiently light reaches it.
- Light output per luminaire
- This is F in the formula above. The total light output of one fitting, in lumens, taken from its datasheet — not its wattage, which is energy consumed rather than light produced.
- Number of luminaires
- This is N in the formula above. How many fittings are installed — doubling the fitting count, with everything else unchanged, doubles the estimated lux.
- Luminaire type
- Sets UF in the formula above — the utilisation factor, the fraction of a fitting’s rated lumens assumed to reach the working plane rather than being absorbed by walls, ceiling, or the fitting itself. A higher UF (e.g. a high bay at 0.75) gives a proportionally higher lux result for the same lumen output. The gap between the luminaire height and working plane height, relative to the room’s floor area, also nudges this figure slightly — a proportionally taller gap makes light less efficient at reaching the working plane.
- Room surface reflectance
- A modifier on the utilisation factor based on how much light nearby surfaces bounce back into the room. Light selects ×1.10 (a 10% increase to UF); Medium leaves UF unchanged; Dark selects ×0.85 (a 15% reduction) — pale surfaces reflect more of the light that would otherwise be absorbed.
- Maintenance factor
- This is MF in the formula above. An estimate of how much a luminaire’s light output will have dropped by the end of its life, from lamp ageing and dirt build-up. Example: if you need 300 lux maintained throughout the light’s life and the maintenance factor is 0.8 (80%), it will only be putting out 80% of its original brightness by the end — so the calculation already builds that 20% future loss in. To guarantee a target lux level over the full life of the installation, you’d add more fittings or choose higher-output luminaires, rather than sizing only for how bright they are on day one.
- Room area
- This is A in the formula above. Length × width — e.g. 24 m² for a 6m × 4m room. The combined output of all fittings is spread across this area, so a larger room gives a lower average lux for the same lighting, and a smaller room gives a higher one.
- Estimated average illuminance (lux)
- This is E in the formula above — the answer. It’s the average amount of light landing on the working plane across the whole room, in lux. The higher the number, the brighter the room.