Watts to Lux Calculator
Enter the power of your light source, choose the bulb type (or enter a custom efficacy), and specify the surface area the light falls on. The calculator converts watts to lux (illuminance) using the standard photometric formula. Switch to reverse mode to solve for watts from a target lux level. Results update as you type.
Formula
Worked example
A 60 W LED bulb with 100 lm/W efficacy illuminates a 10 m² floor: lumens = 60 × 100 = 6000 lm; lux = 6000 ÷ 10 = 600 lx. Reverse: to hit 500 lx on a 12 m² table with a CFL at 60 lm/W: lumens needed = 500 × 12 = 6000 lm; power = 6000 ÷ 60 = 100 W.
How the watts-to-lux conversion works
Watts measure electrical power - how fast energy is consumed. Lux (lx) measures illuminance - how much luminous flux falls on a surface per square metre. The link between them is luminous efficacy (eta, in lm/W), which describes how efficiently the lamp converts electrical power into visible light. The formula is: Lux = (Watts x Efficacy) / Area. First the wattage is multiplied by the efficacy to get the total luminous flux in lumens. That total is then divided by the illuminated area in square metres. For an omnidirectional point source the illuminated area is the surface of a sphere: 4 x pi x radius squared. The reverse formula solves for watts: Watts = (Target lux x Area) / Efficacy. Multiply the desired illuminance by the area to get the total lumens required, then divide by the efficacy of your chosen lamp to find the wattage you need.
Luminous efficacy by light source type
Luminous efficacy is the single biggest variable in the conversion, and it varies enormously across technology types. A standard incandescent bulb converts only about 12-18 lm per watt because most of its energy leaves as infrared heat. A compact fluorescent (CFL) does roughly 45-75 lm/W. A modern LED bulb achieves 80-120 lm/W and the best LED chip-on-board products exceed 200 lm/W in laboratory conditions. High-pressure sodium lamps used in street lighting typically reach 85-150 lm/W, and low-pressure sodium - the most efficient conventional source - can hit 100-190 lm/W but produces a monochromatic yellow light unsuitable for most indoor uses. The presets in this calculator use representative mid-range values. Always check the photometric datasheet or luminaire specification sheet for the exact efficacy of the product you are sizing.
What lux levels mean in practice
Lux describes how bright a surface appears independently of the source power or distance. The same 1000 lm torch illuminating a 1 m² patch gives 1000 lx (very bright), but spread over a 10 m² room it drops to 100 lx (dim corridor level). The European standard EN 12464-1 and the Illuminating Engineering Society of North America (IESNA) publish recommended maintained illuminance values for hundreds of task types. Key benchmarks: 50-100 lx for corridors and storage; 300-500 lx for general offices; 500 lx sustained for comfortable reading; 1000 lx or more for surgical tables or fine inspection work. Natural daylight on a bright overcast day is around 10 000 lx; full direct sunlight is roughly 100 000 lx.
Common pitfalls and practical tips
Lamp depreciation: luminous output falls over the lifetime of the lamp, typically by 20-30%. Standards require calculations to be based on the maintained illuminance (average over the useful life), so add a maintenance factor (usually 0.7-0.8) by dividing the initial lux by that factor. Reflectance and room geometry: lux calculated by this tool represents the direct illuminance from one source with perfect uniform distribution. Real rooms have walls, ceilings, and furniture that reflect and absorb light, so the actual illuminance can be higher (from inter-reflections) or lower (from obstructions). Use a proper lighting design tool such as DIALux or RELUX for accurate room calculations. Distance matters: for point sources the lux falls with the square of the distance (the inverse square law). Doubling the distance from a lamp quarters the lux on the surface. The spherical-radius mode in this calculator applies that relationship automatically.
Recommended illuminance levels by task (EN 12464-1 / IESNA)
| Application | Recommended lux | Notes |
|---|---|---|
| Storage, passageways | 50-100 | Minimal visual task |
| Stairways, corridors | 100-150 | Movement without fine work |
| Lounges, living rooms | 150-300 | Relaxed ambient light |
| General office work | 300-500 | EN 12464-1 maintained minimum |
| Reading, writing | 500 | Comfortable sustained reading |
| Detailed desk work | 500-750 | CAD, accounting |
| Fine assembly, labs | 750-1000 | High-precision tasks |
| Surgery, colour grading | 1000-2000 | Critical colour or precision |
| Bright overcast daylight | 10 000 | Reference: outdoor diffuse sky |
| Full direct sunlight | 100 000 | Reference: clear sky at noon |
Typical maintained illuminance values from the European standard EN 12464-1 and the Illuminating Engineering Society of North America.
Frequently asked questions
Can I convert watts to lux without knowing the area?
No. Watts describe the power consumed by the lamp; lux describes how that light is spread over a surface. The same 1000 lm lamp gives very different lux levels on a 1 m² worktop versus a 100 m² warehouse floor. You always need the illuminated area (or the distance to the surface for a point source) to complete the conversion.
What luminous efficacy should I use for an LED bulb?
Most modern quality LED bulbs fall in the range of 80-120 lm/W. Budget or older LEDs can be as low as 35 lm/W, while premium LEDs and LED tubes approach 130-160 lm/W. The most accurate approach is to find the lumens output and the wattage on the packaging, then calculate efficacy = lumens / watts yourself. The presets in this tool use representative mid-range values for quick estimates.
How many lux do I need for a home office?
EN 12464-1 recommends a maintained illuminance of 500 lx for reading and writing tasks at a desk, and 300 lx for general office areas. Screens and video calls are generally comfortable at 300-500 lx, though supplemental task lighting on paper documents may need 750 lx. Ambient background lighting of 100-200 lx helps avoid uncomfortable contrast between the bright screen and a dark surrounding.
What is the difference between lumens and lux?
Lumens (lm) measure the total luminous flux emitted by a source in all directions - how much light the lamp produces in total. Lux (lx) measure the luminous flux per square metre arriving at a specific surface - how bright that surface is. One lux equals one lumen spread over one square metre. A 1000 lm source can give 1000 lx if all its light hits a 1 m² surface, or just 10 lx if the same light is spread over 100 m².
How does distance affect lux from a point source?
For an idealized omnidirectional point source the illuminance follows the inverse square law: lux = lumens / (4 x pi x distance²). Doubling the distance from the lamp reduces the lux to one quarter. This is why a reading lamp positioned 0.5 m above a page is far brighter per square metre than a ceiling fixture 2.5 m away, even if both have the same lumen output. The spherical-radius mode in this calculator implements this relationship.
Why is my LED bulb brighter than my old incandescent at the same wattage?
An incandescent bulb converts most of its electrical energy into heat (infrared radiation) rather than visible light, achieving only about 12-18 lm/W. A modern LED converts much more energy into visible light and reaches 80-120 lm/W. So a 10 W LED can easily match or exceed the 800 lm output of a 60 W incandescent, producing the same lux level for about one sixth of the power.