Watts to Lux Calculator
Convert watts to lux for a room: lumens = watts × efficacy, then lux = lumens × utilisation × maintenance / area. Formula, worked example and a table.
Try your own numbers
Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
Formula
Worked example
| Input | Value |
|---|---|
| Electrical power | 100 W |
| Luminous efficacy (lm/W) | 100 |
| Area | 20 m² |
| Utilisation factor | 0.6 |
| Maintenance factor | 0.8 |
- Φ = P × η = 100 × 100 = 10,000 lm
- E = Φ × UF × MF / A = 10,000 × 0.6 × 0.8 / 20 = 240 lux
- Power density = 100 / 20 = 5 W/m²
Typical values
| Electrical power (W) | Average illuminance (lux) |
|---|---|
| 10 | 24 |
| 50 | 120 |
| 100 | 240 |
| 250 | 600 |
| 500 | 1,200 |
Other inputs are the same as in the worked example above.
Assumptions
- Uniform illuminance across the area
- Factors apply to the whole installation
- Power is lamp or luminaire input power as stated for the efficacy
This is the reverse of Lighting Power for a Target Lux. Use the lumen method calculator for a luminaire count and the point calculation for uneven layouts.
Frequently asked questions
How do I convert Watts to Lux?
Φ = P × η; E = Φ × UF × MF / A. Worked example: Electrical power 100 W, luminous efficacy (lm/W) 100, area 20 m², utilisation factor 0.6 gives 240 lux.
What do I need to enter?
Enter electrical power, luminous efficacy (lm/W), area, utilisation factor and maintenance factor. Change the unit of a value with its selector and the result stays the same.
Why are there two factors, utilisation and maintenance?
Not every lamp lumen reaches the work plane, which the utilisation factor covers, and lamps and luminaires lose output with age and dirt, which the maintenance factor covers. Typical room values are about 0.5 to 0.7 and 0.7 to 0.85, but take them from your luminaire data and design standard.
Is there a fixed lux to watts factor?
No. Lux depends on the area, the lamp efficacy in lm/W and the room factors, so a lux figure cannot be turned into watts without them. For a precise layout use the lumen method or a point-by-point calculation.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. Check design values against the standard and the equipment data you work to.
Related
Next steps in a design: Lux to WattsLighting LevelLumens to Lux
Opposite direction: Lux to Watts
Also see: Amps to Watts Watts to Amps kVA to Watts Watts to kVA
Formula guides: Lighting Formulas: Lux, Lumens and CandelaPoint Illuminance Formulas
All 78 conversionsIlluminance from Lamp Power (watts to lux) in Elecmetric