Question 1

How do I calculate the number of lights needed for a room?

Accepted Answer

Use the lumen method: multiply the required lux level by the room area, then divide by the product of the lumens per fitting, the utilisation factor (UF), and the maintenance factor (MF). The formula is N = (Lux x Area) / (Lumens x UF x MF). The utilisation factor depends on the room index (a measure of room proportions) and the reflectance of the ceiling, walls, and floor surfaces.

Question 2

What is the room index and how do I calculate it?

Accepted Answer

The room index (RI) describes the proportions of a room relative to its height. It is calculated as RI = (L x W) / (Hm x (L + W)), where L is room length, W is room width, and Hm is the mounting height (ceiling height minus work plane height). A low room index (below 1.0) means the room is tall relative to its floor area, making light distribution less efficient. A high room index (above 2.5) indicates an efficient, well-proportioned space.

Question 3

What is the utilisation factor in lighting design?

Accepted Answer

The utilisation factor (UF) is the proportion of the total light output from the fittings that actually reaches the working plane. It accounts for light absorbed by room surfaces and the distribution pattern of the luminaire. UF depends on the room index, surface reflectances, and the luminaire type. Typical values range from 0.3 (small dark room) to 0.7 (large light-coloured room with efficient luminaires).

Question 4

What maintenance factor should I use for LED lighting?

Accepted Answer

For LED lighting in a clean indoor environment, a maintenance factor of 0.8 is standard. This accounts for gradual lumen depreciation over the LED lifetime (typically 80% of initial output at L80 rated life) and light loss from dust accumulation on the fitting. For dirty or industrial environments, use 0.7. For very clean environments with sealed IP65 fittings, 0.9 may be appropriate.

Question 5

What are the recommended lux levels for different rooms?

Accepted Answer

BS EN 12464-1 and CIBSE guidelines recommend: offices and classrooms 500 lux, kitchens and retail 300 lux, living rooms, bathrooms, and warehouses 150 lux, bedrooms, corridors, and stairways 100 lux, workshops 500 lux. These are minimum maintained illuminance levels on the task area. Higher levels may be needed for detailed work or for occupants with reduced visual acuity.

Question 6

How does ceiling height affect the number of fittings needed?

Accepted Answer

Higher ceilings reduce the room index, which in turn reduces the utilisation factor, meaning more fittings are needed to achieve the same lux level. As light travels further from the fitting to the work plane, it also spreads over a wider area, reducing intensity. For ceilings above 4 metres, high-bay fittings with narrow beam angles are typically needed. For standard 2.4-2.7m domestic ceilings, LED panels or downlights work well.

Question 7

What spacing should I use between light fittings?

Accepted Answer

The maximum spacing to height ratio (SHR) depends on the luminaire distribution. For general-purpose LED panels, the maximum spacing should not exceed 1.5 times the mounting height to avoid dark patches between fittings. The calculator determines an optimal grid layout that distributes fittings evenly across the room area. First fittings should be placed at approximately half the spacing distance from the walls.

Question 8

How do surface reflectances affect lighting levels?

Accepted Answer

Light-coloured surfaces (high reflectance) bounce more light onto the work plane, increasing the utilisation factor and reducing the number of fittings needed. A white ceiling (reflectance 0.7) and light walls (0.5) can increase UF by 30-50% compared to a dark ceiling (0.3) and dark walls (0.1). This is why painting a room lighter colours can noticeably improve lighting levels without changing the fittings.

Room Type	Lux Level	Typical Fitting	Notes
Office / classroom	500	LED panel 600x600	Task area; 300 lux for immediate surround
Kitchen	300	LED panel or downlight	Counter areas may need supplementary task lighting
Living room	150	LED downlight	Ambient; supplement with table/floor lamps
Bedroom	100	LED downlight	General; bedside task lighting 300 lux
Bathroom	150	LED downlight (IP44+)	Mirror area 300 lux recommended
Workshop	500	LED batten	Bench areas may need 750 lux
Warehouse	150	High bay LED	Minimum on floor; racking areas may need more
Retail	300	LED panel or track	Display areas up to 500 lux
Corridor / stairway	100	LED bulkhead or downlight	Emergency lighting also required

Lighting Design Calculator

How This Calculator Works

Quick Reference -- Illuminance Requirements

Practical Notes

Frequently Asked Questions