Lighting Lux Level Calculator — Get the Right Light Level for Every Room

Lux (lx) is the SI unit of illuminance. It measures the amount of luminous flux (light) falling on a surface per unit area. One lux equals one lumen per square metre. It is the primary measure used by lighting designers, architects, and electricians to specify how bright a space needs to be for its intended use.

Getting lux levels right is critical for compliance, occupant comfort, and energy efficiency. Too little light causes eye strain, reduces productivity, and creates safety hazards — particularly in workplaces where detailed tasks are performed. Too much light wastes energy, increases running costs, and can cause glare that is just as problematic as insufficient light.

In the UK, lighting design is governed by CIBSE (the Chartered Institution of Building Services Engineers) recommendations and the European standard BS EN 12464-1 for indoor workplaces. These standards specify the maintained illuminance — the minimum average lux level on the working plane — for every type of room and task. The Elec-Mate calculator suite includes all these values built in, so you do not need to look them up manually.

CIBSE publishes recommended maintained illuminance values for a wide range of room types and tasks. These values represent the minimum average lux level that should be maintained on the working plane throughout the life of the installation, accounting for lamp depreciation and dirt accumulation on luminaires and room surfaces.

The table below shows the most commonly referenced values. These are used across commercial, industrial, educational, healthcare, and domestic lighting designs:

These values should be treated as minimums. Many designers specify higher levels for premium fit-outs or where the client has specific requirements. For cable sizing on lighting circuits, the total wattage derived from the lux calculation feeds directly into the circuit design.

The lumen method is the standard approach for calculating the number of luminaires needed to achieve a target lux level. The formula is:

Each variable in this formula has a specific meaning and source. The required lux (E) comes from CIBSE or the project specification. The area (A) is the floor area of the room in square metres. The lumens per luminaire (F) come from the manufacturer's photometric data for the chosen fitting. The utilisation factor (UF) accounts for how efficiently light reaches the working plane, considering room shape, surface reflectances, and luminaire light distribution. The maintenance factor (MF) accounts for light loss over time due to lamp depreciation, luminaire dirt, and room surface deterioration.

Elec-Mate's calculator handles the entire formula. You enter the room dimensions, select the room type, input the luminaire lumen output, and the calculator returns the number of fittings needed. It also calculates the achieved lux level if you want to check a fixed number of luminaires against the target. This pairs well with the maximum demand calculator when sizing the supply for a lighting-heavy installation.

The room index (RI or K) is a dimensionless number that describes the proportions of the room relative to the mounting height of the luminaires. It directly affects how efficiently the light from the luminaires reaches the working plane:

A high room index (typically above 3) means the room is large relative to the mounting height — light reaches the working plane efficiently, and the utilisation factor is high. A low room index (below 1) means the room is small or the ceiling is very high — more light is lost to walls, and the utilisation factor drops.

The utilisation factor (UF) is read from the luminaire manufacturer's UF table using the room index and the reflectance values of the ceiling, walls, and floor. Typical reflectance values are 70% for a white ceiling, 50% for light-coloured walls, and 20% for the floor. Elec-Mate includes standard UF lookup values, so you select the room surface colours rather than looking up tables manually.

Understanding room index is also important when working with voltage drop calculations for long lighting circuit runs in large commercial spaces.

The maintenance factor (MF) accounts for the gradual decline in light output over the life of the installation. Three factors contribute to this decline:

• Lamp lumen depreciation (LLD) — all light sources lose output over time. LED lamps typically retain 80% of their initial output at their rated life (L80), giving an LLD of 0.8. Fluorescent tubes may have an LLD of 0.85 at mid-life.
• Luminaire maintenance factor (LMF) — dirt accumulation on the luminaire optic, diffuser, or reflector reduces light output. A clean environment (office) might give an LMF of 0.9, while a dirty environment (factory, kitchen) might give 0.7.
• Room surface maintenance factor (RSMF) — walls and ceilings darken with age, reducing the amount of reflected light. Typically 0.9 for clean environments and 0.8 for industrial spaces.

The overall maintenance factor is the product of these three: MF = LLD x LMF x RSMF. For a typical office with LED luminaires, MF = 0.8 x 0.9 x 0.9 = 0.65. For a clean room with frequent maintenance, it could be 0.8 or higher. The conduit fill calculator and lighting calculator work together when planning wiring routes for large lighting installations.

This requirement applies to every AC final circuit feeding light fittings in a dwelling — whether new-build, rewire, or an addition to an existing installation. The regulation uses the word 'shall', making it a mandatory obligation, not a recommendation.

Beyond the RCD requirement, Reg 314.1 requires every installation to be divided into circuits to prevent a single circuit failure causing a hazardous situation. Lighting is explicitly identified as the primary example: failure of a single lighting circuit could cause loss of escape route illumination. In commercial and industrial premises, this means a single lighting circuit serving an entire floor is likely non-compliant — multiple circuits (or sub-circuits via a lighting distribution board) are required so that failure of one circuit does not plunge the whole floor into darkness.

When specifying recessed LED downlighters, check that the luminaire carries the correct marking for installation on or near combustible surfaces (refer to Table 55.3 of BS 7671 and Reg 559.3). Insufficient ventilation around tightly fitted downlighters is one of the most common installation defects found on EICR inspections.