Lighting levels and fixture count
How many fittings does a room need? The lumen method answers it from the room geometry, the surface reflectances and the maintenance regime. This computes the cavity ratio, the coefficient of utilisation and the fixture count, then reports the illuminance actually delivered and the power density it costs.
| Quantity | Value | Note |
|---|---|---|
| Room | 10.0 × 8.0 m | 80.0 m² |
| Mounting height above work plane | 2.05 m | ceiling 2.80 m, work plane 0.75 m |
| Room cavity ratio | 2.31 | 5 h (L + W) / (L × W) |
| Coefficient of utilisation | 0.644 | ceiling 70%, walls 50% |
| Light loss factor | 0.80 | lamp lumen depreciation and dirt |
| Lumens per luminaire | 4000 | rated output |
| Delivered per luminaire | 2061 | lumens reaching the work plane |
| Target illuminance | 400 lux | 37 fc |
| Luminaires calculated | 15.52 | before rounding up |
| Luminaires specified | 16 | rounded up to a whole fitting |
| Delivered illuminance | 412 lux | 38.3 fc |
| Spacing | 2.50 × 2.00 m | roughly square grid |
| Installed load | 512 W | 6.40 W/m² |
| Efficacy | 125 lm/W | luminaire, including driver loss |
| Annual energy | 1280 kWh | 2500 hours |
The light loss factor is not a fudge — it is the difference between a new installation and the same one at the end of its cleaning cycle. A 0.8 factor means you install 25% more light than the room needs on day one.
RCR = 5h(L + W)/(L·W) · h is the mounting height above the work plane.
N = E·A / (Φ·CU·LLF) · E is the target illuminance, Φ the lumens per fitting.
E = N·Φ·CU·LLF / A · What the room actually gets after rounding.
LPD = N·W / A · The number energy codes limit.
A clear path from inputs to results.
How many fittings does a room need? The lumen method answers it from the room geometry, the surface reflectances and the maintenance regime. This computes the room cavity ratio, the coefficient of utilisation and the fixture count, then reports the illuminance actually delivered and the power density it costs.
How to use
- Enter the room length, width and ceiling height.
- Set the work plane height — usually 0.75 m for a desk or 0 m for a floor.
- Enter the lumens and watts per fitting.
- Choose the target illuminance and the light loss factor.
- Read the fixture count, the delivered lux and the lighting power density.
The formula
Reference: IES Lighting Handbook, lumen method; the cavity-ratio definition and the CU tables are in IES LM-57 and the IES Fundamentals volume.
Worked example
A 10 × 8 m office at 400 lux
- Room 10 × 8 m, ceiling at 2.8 m, work plane at 0.75 m.
- 4000 lumens and 32 W per fitting.
- Target 400 lux, light loss factor 0.8.
- Ceiling reflectance 70%, walls 50%.
- 1
The mounting height above the work plane is 2.05 m, so the cavity ratio is 5 × 2.05 × 18 / 80 = 2.31.
- 2
At that cavity ratio and those reflectances the coefficient of utilisation is 0.644.
- 3
Each fitting delivers 4000 × 0.644 × 0.8 = 2,061 lumens to the work plane.
- 4
The room needs 400 × 80 = 32,000 lumens, so 15.5 fittings — rounded up to 16.
- 5
Sixteen fittings deliver 412 lux on a 2.50 × 2.00 m grid.
- 6
That is 512 W installed, or 6.40 W/m², and 1,280 kWh a year at 2,500 hours.
16 fittings · 412 lux · 6.40 W/m² · 1,280 kWh a year.
Interpreting the result
The delivered figure is above the target because you cannot install two thirds of a fitting; the difference is the rounding. The number worth arguing about is the light loss factor: at 0.8 you install 25% more light than the room needs on day one, and if the fixtures are never cleaned that margin is what keeps the room usable in year five.
Assumptions & limitations
- A single uniform luminaire type, mounted horizontally, with a roughly square layout.
- The coefficient of utilisation is a smooth fit to the direct-luminaire tables, accurate to about 0.05.
- The light loss factor covers lamp lumen depreciation and dirt, not a failed lamp.
- The lumen method gives average maintained illuminance on a horizontal plane; it says nothing about glare or uniformity.
- No daylight contribution and no obstructions are modelled.
Frequently asked questions
What illuminance do I actually need?
A general office is 300–500 lux, a corridor 100, a classroom 300, and detailed inspection work 750 or more. The target is set by the task, not by the fixture — specifying 750 lux for a corridor wastes half the energy for no benefit.
What is a realistic light loss factor?
0.8 for a clean, well-maintained space with modern LED fittings; 0.7 for an industrial space with poor maintenance; 0.9 only where fixtures are cleaned regularly and the lamps are new. It is the single most commonly overstated input in a lighting design.
Can this design a room with daylight?
No. The lumen method is for electric lighting only. Daylight needs a daylight factor or climate-based simulation, and the electric design should then be dimmed or zoned against the daylighted perimeter.
Why does the cavity ratio matter?
It describes the shape of the room as the luminaire sees it. A tall room in a small footprint throws light onto the walls instead of the work plane, so the utilisation falls and you need more fittings for the same illuminance.
What is lighting power density and why does it matter?
It is the installed watts per square metre, and it is what energy codes such as ASHRAE 90.1 limit. A typical office target is 8–10 W/m²; the 6.4 W/m² here reflects efficient LED fittings.
Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.