Lighting Level and Fixture Calculator
Lumen method fixture count, achieved lux and power density.
Quick answer
With the default inputs: Room area 50 m² · Target illuminance 300 lx · Lumens per fixture 3000 lm etc.Fixtures required 9 pcs
Edit the inputs below to recalculate.Inputs
Result
Fixtures required
9pcs
| Calculated (unrounded) | 8.93pcs |
| Total lumens required | 15000lm |
| Effective lumens each | 1680lm |
| Achieved illuminance | 302lx |
| Illuminance margin | 0.8% |
| Total installed power | 270W |
| Power density (LPD) | 5.40W/m² |
| Area per fixture | 5.56m² |
| Spacing in a square grid | 2.36m |
The lumen method suits uniform general lighting and gives average maintained illuminance. Utilisation depends on room index, surface reflectances and luminaire distribution, so exact values come from the manufacturer; the defaults here are for scheme-stage estimates. Power density is a mandatory efficiency metric: offices typically under 9 W/m², retail 10-12, corridors 5.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Fixture count | N = E × A ÷ (Φ × CU × MF) |
| Achieved level | N × Φ × CU × MF ÷ A |
| Power density | N × watts ÷ A |
| Area per fixture | A ÷ N |
| Square spacing | √(A ÷ N) |
| Symbols | E lux, A area, Φ lumens, CU utilisation, MF maintenance |
50 m² at 300 lx with 3000 lm fixtures, 0.7 utilisation, 0.8 maintenance and 30 W: 8.93 calculated, 9 installed, 302 lx achieved, 270 W total and 5.40 W/m².
Illuminance standards and power density limits
| Space | Illuminance lx | Power density W/m² | Note |
|---|---|---|---|
| Corridors and stairs | 100 | 5 | Emergency lighting at least 5 lx |
| Bedroom | 75 | 6 | |
| Kitchen | 100 | 6 | 300 lx at the worktop |
| Office | 300 | 9 | General office work |
| Meeting room | 300 | 9 | |
| Classroom | 300 | 9 | 500 lx on the blackboard |
| Retail floor | 300 | 12 | Good colour rendering needed |
| Fine machining | 500 | — | Set by the process |
| Operating theatre | 750 | — | Theatre lamp separate |
How to work it out
- Set the illuminance target first (GB 50034-2013): 100 lx for a living room, 75 lx bedroom, 150 lx dining, 100 lx kitchen and bathroom; 300 lx for offices and classrooms, 500 lx for drafting rooms. These are maintained values on the working plane, not peaks directly under a fitting.
- Establish the luminous flux per fitting. LED efficacy is 100-140 lm/W, so a 30 W luminaire is nominally 3000-4200 lm — but use the net output after the diffuser, which loses 10-20%.
- Choose the utilisation factor CU: 0.7-0.8 for a light-coloured office, 0.6-0.7 for ordinary finishes, 0.5-0.6 for dark finishes or industrial space. Higher ceilings push it down.
- Choose the maintenance factor MF: 0.8 for clean interiors, 0.7 for general, 0.6 for dusty workshops and kitchens.
- N = E × A ÷ (Φ × CU × MF), rounded up. A 50 m² office at 300 lx with 3000 lm, CU 0.7 and MF 0.8 gives 8.9, so nine fittings.
- Lay them out on a square grid at s = √(A ÷ N) = √(50 ÷ 9) = 2.36 m. The spacing-to-mounting-height ratio should stay between 1.0 and 1.5; beyond that you get visible banding.
Where it goes wrong
- Assuming 1 W of LED equals 100 lm and stopping there. Diffuser loss (10-20%), utilisation and maintenance factors all take a cut, so a design at 300 lx often measures 220 lx once installed.
- Using 0.9 for the utilisation factor. That figure belongs to a white-walled laboratory; a real office ceiling with dark carpet gives 0.6-0.7, and 30% is two or three fittings.
- Checking illuminance but not power density. Meeting the lux target while exceeding the LPD limit fails the energy review — GB 55015 is mandatory.
- Forgetting the maintenance factor. Dust on the diffuser cuts output 20-30% in a year, so a design based on new-lamp output is adequate in year one and short in year two. Users report it as the lights getting dimmer.
Material and cost
30 W LED panels run 60-150 CNY each at project pricing, downlights 40-100, high-bay fittings 150-400, magnetic track heads 200-500. Installation labour is 15-30 CNY per fitting. Designing in 10% spare fittings is much cheaper than retrofitting, which means re-cutting ceilings and re-running cable.
On site
- In workshops and warehouses above 4 m, use narrow-beam fittings or reflectors. Wide distribution just washes the walls and never reaches the working plane.
- In classrooms and reading rooms, run luminaire axes perpendicular to the blackboard or shelving to avoid shadows and veiling reflections.
- Flicker and glare generate more complaints than illuminance levels. Keep UGR below 19 in offices and 22 in industrial space, using louvers or anti-glare shields.
- Measure illuminance at night with no daylight. Daytime readings are mostly sunlight and tell you nothing.
Basis
- GB 50034-2013 Standard for lighting design of buildings, illuminance values and lighting power density limits
- GB 55015-2021 General code for energy efficiency and renewable energy utilisation in buildings
- GB/T 5700-2008 Measurement methods for lighting
FAQ
Why does the measured level fall short of design?
Because design values are initial, while luminaires gather dirt and sources age. LEDs lose 20-30% of output in three years and dirty housings another 10-20%, which is what the maintenance factor is for. Plenty of installations pass at handover and measure 60% two years later. Dusty workshops should use 0.6 or even 0.5.
Is more light always better?
No. Beyond what the task needs you get glare, eye strain and wasted energy. Standards grade by task precision: 100 lx in corridors, 300 in offices and classrooms, 500 for fine machining, 750-1000 in operating theatres. Uniformity matters too — the minimum over average should not fall below 0.7, or patchiness proves more tiring than a slightly dim room.
How is power density controlled?
Through efficacy. The same 300 lx needs 20 W/m² with incandescent, 9 with fluorescent and only 5-6 with LED. Codes cap LPD by room type — 9 W/m² for offices, 12 for retail — and exceed it and the job fails sign-off. The right way to cut it is better efficacy and utilisation, through light finishes and good optics, not fewer fixtures and dimmer rooms.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Lighting Level and Fixture Calculator. 2026-10-25. https://jianzhujisuan.com/en/lighting-lux
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.