Lux Level Calculator — CIBSE Lumen Method

Workplace illuminance in the UK is governed by BS EN 12464-1:2021 (interpreted by the CIBSE/SLL Code for Lighting 2022). Typical maintained levels: General office work — writing, typing, reading, data processing (DSE) = 500 lux; office filing, copying and circulation = 300 lux; CAD office = 500 lux; technical drawing by hand = 750 lux; general classrooms = 500 lux (BS EN 12464-1:2021 raised this from the former 300 lux; 300 lux is now only a permitted dimmed level for young children); general retail = 300 lux, supermarkets = 500 lux; warehouses = 150 lux; corridors = 100 lux; general workshops = 300 lux, fine/electronics work = 500 lux; commercial kitchens = 500 lux; general hospital ward = 100 lux (examination ~1,000 lux per CIBSE LG02 2019); operating theatre general = 1,000 lux. Domestic rooms are not covered by BS EN 12464-1 — values such as living room 150 lux, bedroom 100 lux and domestic kitchen 300 lux are typical UK guidance only. All values are maintained illuminance on the working plane (typically 0.7 m above floor).

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.

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.

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.

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. Check uniformity: BS EN 12464-1:2021 requires task-area uniformity Uo (minimum ÷ average) of at least 0.6, with at least 0.4 in the immediate surrounding area and 0.1 on background walls and ceilings. Use our Lighting Layout Calculator to plan the exact fitting positions for your room.