What are the CIBSE recommended lux levels?

CIBSE (Chartered Institution of Building Services Engineers) publishes recommended maintained illuminance levels for different room types: General offices = 300 lux, Drawing/CAD offices = 500 lux, Classrooms = 300 lux, Retail = 300-500 lux, Warehouses = 150 lux, Corridors = 100 lux, Workshops = 300-500 lux, Operating theatres = 1,000 lux, Domestic living rooms = 150 lux, Domestic kitchens = 300 lux. These are measured on the working plane (typically 0.7m above floor).

What is the lumen method formula?

The lumen method calculates the number of fittings: N = (E × A) / (F × UF × MF). Where: E = required lux level, A = room area (m²), F = lumens per fitting, UF = utilisation factor (typically 0.4-0.7, depends on room index and surface reflectances), MF = maintenance factor (typically 0.8 for clean LED, 0.7 for industrial). The utilisation factor accounts for light absorbed by walls, ceiling, and floor — lighter surfaces give higher UF.

What is the room index and how does it affect lighting?

Room index (K or RI) describes the proportions of the room relative to its height: K = (L × W) / (Hm × (L + W)), where Hm is the mounting height above the working plane. A large, low room has a high room index (more light reaches the working plane). A tall, narrow room has a low room index (more light is absorbed by walls). Room index directly affects the utilisation factor used in the lumen method — higher RI = higher UF.

What maintenance factor should I use for LED lighting?

Maintenance factor (MF) accounts for lumen depreciation over time and dirt accumulation on fittings. For LED fittings: Clean environment (office) = 0.8, Normal environment = 0.75, Industrial/dirty = 0.65-0.7. LED luminaires maintain about 90% of initial lumens at 50,000 hours (L90B10). The MF is the product of lamp lumen maintenance, luminaire maintenance, and room surface maintenance. Using 0.8 for general commercial LED is standard practice.

How do I arrange fittings in a regular grid?

Once you know the number of fittings, arrange them in a regular grid for even illumination. For a rectangular room: aim for a spacing-to-height ratio (SHR) of 1.0-1.5 × mounting height between fittings. Edge fittings should be half this distance from walls. For example, in a 6m × 4m room with 12 fittings: 4 rows of 3 fittings, spaced 2m × 1.33m apart, with 1m × 0.67m from walls. Uniformity should be checked — minimum lux should be at least 80% of average.

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Lux Level Calculator — CIBSE Lumen Method

Calculate the number of light fittings needed to achieve CIBSE recommended lux levels using the lumen method. Covers offices, warehouses, retail, and domestic rooms.

Room Type

CIBSE recommended lux level

Fitting Type

Luminaire lumen output

Room Length

Internal room length

Room Width

Internal room width

Ceiling Height

Floor to ceiling

How We Calculate This

This calculator uses the CIBSE lumen method to determine the number of light fittings required to achieve the recommended illuminance level.

Lumen Method Formula

N = (E × A) ÷ (F × UF × MF)

Where E = lux, A = area (m²), F = lumens per fitting, UF = utilisation factor, MF = maintenance factor

Room Index

K = (L × W) ÷ (Hm × (L + W))

K < 1.0: Small or tall room — low UF
K = 1.0-2.0: Typical office/classroom
K > 3.0: Large open-plan — high UF

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.