Illuminance and number-of-luminaires calculation (lumen method, EN 12464-1)

Preliminary lighting design in seconds

How many luminaires are needed for a 40 m² office? For a 60 m² classroom? For a 200 m² precision workshop? The lumen method, codified in the standard EN 12464-1 (indoor workplaces), gives the answer in a single formula: N = Em × A / (Φ × UF × MF). This tool applies the method in real time, lets you choose from 27 room types standardized, offers 4 architectural layouts typical values and draws a to-scale SVG layout to visualize the result immediately. Ideal for a quote, a preliminary design, a quick sketch check or as support for a client discussion.

The calculator does not replace a photometric simulation DIALux evo or ReLux for project validation (uniformity Uo, glare UGR, point calculations, inclined surfaces). It provides the estimated average illuminance Em and the power density in W/m² to compare with SIA 380/4, and a layout preview exportable as an image. To automate this work in AutoCAD with generation of the dimensioned layout plan and the luminaire schedule, see the command CALEP of the module ElectroCAD Tools.

How the lumen method works

La méthode des lumens est la procédure historique de pré-dimensionnement de l'éclairage intérieur, normalisée depuis les années 1960 et reprise en l'état dans EN 12464-1 pour le pré-projet. Elle repose sur un bilan énergétique simple : le flux lumineux total utile sur le plan utile (Em × A en lumens) doit égaler le flux émis par les luminaires (N × Φ) corrigé par deux facteurs cumulés, le facteur d'utilisation UF (qui modélise les pertes optiques entre le luminaire et le plan utile) et le facteur de maintenance MF (qui modélise la dégradation dans le temps).

• N : number of luminaires (to round up to the next integer and adapt to the layout)
• Em : target average illuminance on the working plane, in lux (EN 12464-1 table)
• A : room area, in m² (A = L × W)
• Φ : nominal luminous flux of a luminaire, in lumens (manufacturer datasheet)
• UF : utilization factor (unitless, 0.28 to 0.78)
• MF : maintenance factor (unitless, 0.60 to 0.90)

The calculator also displays theroom index k = (L × W) / (Hu × (L + W)), where Hu = H − h is the useful mounting height. This index is the input to the manufacturers' UF tables: the larger k is, the better the room exploits the emitted flux (UF increases). A k of 1 characterizes a typical office room; below 0.6, the room is too tall or too narrow and the UF drops sharply, requiring more luminaires to compensate.

Em values by room type (EN 12464-1)

EN 12464-1 sets a minimum maintained illuminance on the working plane according to the visual task. Here are the main values built into the calculator dropdown, together with the UGR limit (discomfort glare) and the minimum colour rendering index Ra:

For the exhaustive list (more than 250 entries), refer to the full text ofEN 12464-1 (2021 edition) or to the Em data sheets by usage category. The values above are task-area illuminances; the surrounding-area and background illuminances follow ratios of 0.5 to 0.75 depending on the context.

Choosing UF (utilisation factor) correctly

UF measures the share of the flux emitted by the luminaires that actually reaches the working plane, after reflections off the walls, ceiling and floor and after absorption by the room. It depends on four parameters: the photometric distribution of the luminaire (direct, indirect, direct/indirect, asymmetric), the reflectances ρp/ρm/ρs (ceiling/walls/floor, typically 0,7/0,5/0,2 for a light office, 0,5/0,3/0,1 for a dark workshop), the useful mounting height Hu = H − h and theroom index k. The manufacturer provides a UF table (k, ρp, ρm, ρs) for each luminaire.

Typical UF values for preliminary design, for a modern direct LED panel (LOR ≈ 1) with light reflectances 0.7/0.5/0.2: ≈0.49 in a narrow/tall room (k≈0.6); ≈0,62 in an ordinary office (k≈1) ; ≈0.78 in a wide, low room (k≈2); up to ≈0.92 for a large hall (k≈5). An indirect luminaire or one with a very closed diffuser goes considerably lower. The mode Auto (k) of the calculator looks up an 11-step table (k 0,6 → 5,0) aligned with manufacturer charts and the utilance values calculated by Relux/DIALux. The mode Manual covers the 0,40-0,95 range for atypical cases. In the absence of a manufacturer datasheet, use 0,70 as the default for a normal commercial room with direct LED lighting.

Choosing MF (maintenance factor) correctly

MF incorporates four cumulative degradations between commissioning and the next maintenance intervention: LLMF (Lamp Lumen Maintenance Factor, luminous flux depreciation of the sources, typically 0,90 at 25 000 h for L80 LED), LSF (Lamp Survival Factor, failures before maintenance, negligible for modern LED), LMF (Luminaire Maintenance Factor, optical soiling of the luminaire, highly environment-dependent) and RSMF (Room Surfaces Maintenance Factor, degradation of the room surfaces). The calculator offers 3 discrete levels: Clean 0.80 (office, school, healthcare, annual cleaning); Normal 0.70 (clean workshop, light industry, retail, housing); Dirty 0.65 (dusty industry, foundry, agriculture, intensive kitchen).

The 4 available layouts

The calculator offers four layout patterns that cover 80 % of practical cases in residential and commercial buildings (the CALEP module of ElectroCAD Tools offers 30 for the final design, see our layout guide).

Regular grid (rows × cols)

Default pattern. Luminaires are placed on a regular grid with spacing dx in X and dy in Y. The wall distance is typically dx/2 and dy/2. Note: open-plan office, classroom, meeting room, laboratory. The calculator optimizes rows × cols to respect the room's L/W ratio, which makes dx ≈ dy and guarantees maximum uniformity with the simple lumen method. Empirical validity rule: spacing S ≤ 1.5 × Hu (otherwise the uniformity Uo drops below 0.6).

Staggered (odd rows offset)

Odd rows are offset by dx/2 relative to even ones. Advantage: better perceived visual uniformity (the eye no longer sees the grid), recommended in retail space, entrance hall, restaurant, reception area. With a staggered grid, dx can typically be increased by 10-15 % at equivalent uniformity Uo, i.e. saving 10 % of luminaires in large rooms.

Parallel lines

Continuous rows aligned with the long axis of the room. Indication: corridors, circulation halls, linear shops, workshops with aligned machines, garages, car parks, changing rooms. Advantage: consistency with the circulation axis and the orientation of workstations (the operator's shadow is cast behind them, never in front). For workshops, rows parallel to the main machine axis are the arrangement required by good industrial lighting practice.

Inner perimeter

Luminaires placed only along the four walls, at a constant distance from the edges. Indication: indirect lighting coves, wall-washing in retail and hospitality, rooms with a central technical ceiling (sprinkler, duct), exhibition halls. Estimated average Em lower at the centre of the room, to be supplemented by localized direct lighting or a ceiling downlight. An ambience and accent pattern, typically to be combined with a reduced downlight grid (« double layer » mode in CALEP).

Limitations of the lumen method

The lumen method gives theaverage illuminance Em on the working plane, c'est-à-dire la valeur intégrée sur l'ensemble de la surface du local. Elle ne fournit pas : l'uniformité Uo (rapport Em min / Em moyen, exigée ≥ 0,6 par EN 12464-1 sur la zone de tâche), l'éblouissement d'inconfort UGR (calculé par lancer de rayons depuis la position de l'observateur), l'éclairement vertical Em,v sur les façades de meubles ou les visages, les éclairements ponctuels (point par point), les calculs sur surfaces inclinées (plans de travail inclinés type CAO/dessin, gradins, escaliers), et la modélisation du mobilier qui obstrue le flux.

Rule of good practice: lumen method for pre-design, quoting, preliminary design and sketch verification ; photometric simulation (DIALux evo, ReLux, AGI32) for final design validation, the dossier OIBT, the SIA 380/4 energy dossier and any binding tender.

Power density and SIA 380/4

SIA 380/4 (Swiss standard “Electrical energy in buildings”, 2017 revision) sets target and limit values for lighting power density in W/m², distinctes de l'éclairement Em (qui est une exigence d'EN 12464-1, normative pour le confort). Valeurs limites cibles SIA 380/4 typiques : 6 W/m² bureau standard, 8 W/m² école, 10 W/m² commerce, 12 W/m² industrie légère. Le calculateur estime la puissance installée à partir d'une efficacité LED de référence de 110 lm/W (LED tertiaire récente). Pour un bureau 500 lx en 8×5 m, on aboutit typiquement à 5-6 W/m², soit pile dans la cible SIA 380/4. Au-delà de 10 W/m² pour un bureau, le projet est non-compliant in energy terms and requires either more efficient luminaires (130-150 lm/W), a reduced Em, or a sensor-based flux dimming strategy (occupancy + daylight harvesting).

Automate luminaire layout in AutoCAD

This web tool covers preliminary design and costing. For the final design with automatic generation of the dimensioned layout plan, luminaire schedules and tender documents, the command CALEP d'ElectroCAD Tools applies the same EN 12464-1 methodology directly in AutoCAD: selection of room polygons (or import from the architect's plan), automatic application of Em per room type from the CRB CAN 2026 database, choice among 30 architectural layouts (see hexagonal and parallel-line algorithms), placement of luminaire DWG blocks with title block, material take-off for quotations and Excel/CSV export to the CAN service catalogue for the CRBX SIA 451 tender.

Discover ElectroCAD Tools CALEP module

Frequently asked questions