Calculate how many light fittings you need per CIBSE standards
Good lighting is one of the most important aspects of any room, affecting everything from comfort and mood to productivity and safety. In the UK, the Chartered Institution of Building Services Engineers (CIBSE) publishes recommended lux levels for different room types and activities. A lux is the unit of illuminance -- it measures the amount of light falling on a surface. Getting the right number of light fittings ensures your room is properly illuminated without being uncomfortably bright or wastefully over-lit. With the widespread adoption of LED lighting, homeowners and designers now have access to highly efficient fittings that produce excellent light quality at a fraction of the energy cost of older technologies. LED downlights, panels, and pendants come in a wide range of lumen outputs and colour temperatures, making it possible to tailor the lighting to each room's specific needs. However, the sheer variety of options can make it difficult to know how many fittings you actually need. This calculator uses the lumen method, which is the standard approach for general lighting design in domestic and commercial settings. It takes into account the room size, the target lux level recommended by CIBSE for your room type, the lumen output of your chosen fitting, and correction factors for utilisation and maintenance. The result tells you how many fittings you need and estimates the running cost based on current UK electricity prices.
To calculate the number of light fittings needed: 1. Enter the room length and width in metres. For L-shaped or irregular rooms, calculate each rectangular section separately and add the results together. 2. Enter the ceiling height. The standard UK ceiling height is 2.4 metres for modern builds, but older properties may have higher ceilings of 2.7 metres or more. Higher ceilings require slightly more light output to achieve the same lux level at the working plane (typically 0.75 metres above floor level). 3. Select the room type. Each room type has a CIBSE recommended lux level: kitchens need 300 lux for safe food preparation, living rooms need 150 lux for comfortable general use, bedrooms need 100 lux, bathrooms 150 lux, home offices 400 lux for screen and paperwork, garages and hallways 100 lux. 4. Select the fitting type and enter the lumens per fitting. A typical LED downlight produces around 800 lumens (equivalent to a 60W incandescent bulb). LED panels used in kitchens and offices typically produce 3000-4000 lumens. Pendant lights vary enormously depending on the lamp used. Check the packaging or product datasheet for the specific lumen output. 5. Review the results. The calculator shows the target lux level, room area, total lumens required (accounting for utilisation and maintenance factors), the number of fittings needed (rounded up to a whole number), total wattage, and an estimated annual running cost based on 4 hours of daily use at the current average UK electricity rate of approximately 34 pence per kilowatt hour. 6. When positioning fittings in the room, space them evenly in a grid pattern for the most uniform illumination. A common layout for downlights is to divide the room into equal zones, with one fitting centred in each zone.
The calculator uses the standard lumen method for lighting design: Required lumens = Target lux multiplied by room area, divided by the utilisation factor multiplied by the maintenance factor. The target lux level comes from CIBSE recommendations: living rooms 150 lux, bedrooms 100 lux, kitchens 300 lux, bathrooms 150 lux, offices 400 lux, garages 100 lux, and hallways 100 lux. The utilisation factor (UF) represents the proportion of light that reaches the working plane. In domestic rooms with average reflectances (light-coloured walls and ceiling), a UF of 0.5 is typical. This means approximately half the light output from the fittings is absorbed by surfaces or directed away from the working plane. Professional lighting design would calculate the UF more precisely using the room index and reflectance tables. The maintenance factor (MF) of 0.8 accounts for the gradual reduction in light output over time due to lamp depreciation, dirt accumulation on fittings, and reduced surface reflectances as rooms age. This means the initial installation provides 20 percent more light than the minimum target, ensuring the room remains adequately lit throughout the maintenance cycle. For example, a 5m by 4m kitchen: area equals 20 square metres, target lux equals 300. Required lumens equals 300 multiplied by 20, divided by 0.5 multiplied by 0.8, which equals 15,000 lumens. With 800-lumen LED downlights, the number of fittings equals 15,000 divided by 800, which equals 18.75, rounded up to 19 fittings. Annual cost is estimated as total watts divided by 1000, multiplied by 4 hours per day, multiplied by 365 days, multiplied by the electricity unit rate. For 19 fittings at 8 watts each (152W total), the annual cost is approximately 75 pounds at 34 pence per kWh.
CIBSE publishes detailed lighting guides including the Lighting Guide 7 (LG7) for offices and the Society of Light and Lighting (SLL) Code for Lighting, which covers all building types. For domestic properties, Part L of the Building Regulations (Conservation of fuel and power) requires minimum lighting efficacy standards for new installations. LED fittings now comfortably exceed these requirements, typically achieving 100 lumens per watt or better. When choosing LED fittings, consider the colour temperature: 2700K (warm white) is preferred for living rooms and bedrooms, 4000K (cool white) suits kitchens and bathrooms, and 5000K (daylight) is ideal for garages and utility rooms. All lighting circuits should be protected by an appropriate MCB (typically 6A) and an RCD where required by BS 7671.