How to use this calculator
- Enter the room length and width and the mounting height of the luminaires above the working plane (for example a 2.8 m ceiling and 0.8 m desks give 2.0 m).
- Enter the target maintained illuminance in lux for the task. For example, offices are commonly designed around 500 lx at the task; take the value for your task from EN 12464-1 or ISO 8995-1.
- Enter the luminous flux per lamp and the number of lamps per luminaire (for LED luminaires, the luminaire flux and 1).
- Enter the utilisation factor from the maker's UF table for your room index and reflectances, and a maintenance factor.
- Optionally enter the power of one luminaire to get the installed power and W/m².
How it works: the formula
The lamps produce a total flux of N × n × Φ lumens. Only part of it reaches the working plane: the utilisation factor UF is that fraction, and it depends on the luminaire's light distribution, the room proportions (summarised by the room index K) and the reflectances of the ceiling, walls and floor. The maintenance factor MF allows for the fall in light output as the lamps age and the luminaires and room get dirty. Dividing the flux that arrives by the floor area gives the average illuminance, and turning the relation round gives the number of luminaires.
Variables and units
| Symbol | Meaning | Unit |
|---|---|---|
| L, W | Room length and width | m |
| Hm | Mounting height above the working plane | m |
| K | Room index | — |
| E | Maintained average illuminance on the working plane | lx |
| A | Room area L × W | m² |
| Φ | Luminous flux of one lamp (or of an LED luminaire) | lm |
| n | Lamps per luminaire | — |
| UF | Utilisation factor | — |
| MF | Maintenance factor | — |
| N | Number of luminaires | — |
Worked example
An office 10 m × 6 m, luminaires 2.0 m above the desks, target 500 lx, LED luminaires of 4000 lm and 36 W, UF 0.6 and MF 0.8 (example values):
Assumptions
- Average maintained illuminance over the whole working plane of a rectangular room.
- Luminaires are evenly spaced, with the outer ones about half a spacing from the walls.
- The UF and MF entered apply to this luminaire, room index, reflectances and maintenance plan.
Limitations
- The UF must come from the luminaire maker's utilisation factor table for the room index and reflectances; the default 0.6 is only an example.
- The default MF of 0.8 is an example for LED luminaires in a clean environment; dirtier rooms, longer cleaning intervals or light sources with faster lumen depreciation need a lower value.
- Uniformity, glare (UGR), cylindrical illuminance, daylight and emergency lighting are not covered; use a point-by-point calculation, for example in DIALux or Relux.
- Spacing must stay within the maker's maximum spacing-to-height ratio, or the light will be uneven.
- Illuminance requirements for specific tasks should be taken from the applicable standard, not from this page.
References
Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.
- EN 12464-1, Light and lighting — Lighting of work places — Part 1: Indoor work places (maintained illuminance, uniformity and glare requirements).
- ISO 8995-1, Lighting of work places — Part 1: Indoor.
- CIBSE / SLL, The SLL Code for Lighting (lumen method, room index, utilisation and maintenance factors).
- CIE 97, Guide on the maintenance of indoor electric lighting systems (maintenance factor).
Frequently asked questions
What illuminance should I design for?
It depends on the task and is set by standards such as EN 12464-1 and ISO 8995-1. For example, offices are commonly designed around 500 lx at the task. For other tasks look up the value in the standard that applies to your project rather than relying on a general list.
Where does the utilisation factor come from?
From the luminaire maker's photometric data. The UF table lists values against the room index K and the reflectances of the ceiling, walls and floor. A wide, low room with light-coloured surfaces has a high UF; a narrow, tall or dark room has a low one.
What does the maintenance factor allow for?
The light falls over time because the light source loses output and the luminaire and room surfaces collect dirt. The maintenance factor makes sure the target is still met at the end of the maintenance cycle. CIE 97 gives the method for working it out from its components.
Why is the achieved illuminance higher than the target?
Because the number of luminaires is rounded up to a whole number, and the layout may need a few more to form a regular grid. A small excess is normal; if it is large, a luminaire with a different flux may fit better.
What is the room index?
K = L × W / (Hm × (L + W)) compares the floor area with the wall area between the working plane and the luminaires. It summarises the room's shape so that the UF can be read from a table.