About the Watts to Lux Calculator
Watts measure how much electricity a lamp uses, not how bright it is, so converting watts to lux needs two more facts: how efficient the lamp is and how big the area is. This calculator multiplies the watts by the luminous efficacy of the lamp type to get lumens, then divides by the area to give lux. Pick LED, CFL, halogen or incandescent, or type your own lumens-per-watt figure, then enter the area in square metres or square feet.
Updated October 2026
How to use the Watts to Lux Calculator
- 1Enter the total power of the lamps lighting the area.
- 2Pick the type of lamp, or choose to type the efficacy yourself.
- 3Enter the size of the area.
- 4Choose square metres or square feet.
- 5Read the average lux and the foot-candles.
Why watts alone don't tell you brightness
An old 60 W incandescent bulb gives about 800 lumens, but an LED gives the same light from about 8 W. That ratio, lumens per watt, is the luminous efficacy. Once you know it, watts become lumens, and lumens spread over an area become lux.
- LED: about 80–120 lm/W
- Fluorescent and CFL: about 50–70 lm/W
- Halogen: about 16–24 lm/W
- Incandescent: about 10–17 lm/W
Planning room lighting
Work backwards from the lux you want. Multiply the target lux by the floor area to get lumens, add about 30 to 50% for losses, then divide by the efficacy to get watts. For a 15 m² kitchen at 300 lux that is 4,500 lm before losses, or roughly 60 W of LED lighting after allowing for them.
Limits of the estimate
The result is an average over the whole area, not the level under each lamp. Spotlights put more light in one place and less elsewhere, and dark walls reflect less than white ones. For exact lighting design, professionals use photometric files and software, but this estimate is close enough to compare options.
Comparing bulbs before you buy
Because efficacy differs so much, two lamps with the same wattage can give very different light levels. Enter the same watts with LED and halogen selected and you will see the LED gives about five times the lux. That is the quickest way to see how much a switch to LED saves: the same lux needs far fewer watts, which means lower bills and less heat in the room.
Phone and computer
The calculator works the same on any device and updates as you type. Nothing is stored or uploaded. Lux depends on the type of lamp as well as watts. Reverse the sum with the Lux to Watts Calculator, find light output with the Watts to Lumens Calculator, and compare illumination with the Lumens to Lux Calculator.
Features
- Watts to lux, lumens and foot-candles
- Efficacy presets for LED, CFL, halogen and incandescent
- Custom lumens-per-watt option
- Area in m² or ft²
- Copy result
- Runs in your browser
Frequently asked questions
How do I convert watts to lux?
Lux = watts × lumens per watt ÷ area in m². A 60 W LED panel at 100 lm/W gives 6,000 lm; over 12 m² that is 500 lux.
How many lux is 1 watt?
It depends on the lamp and the area. 1 W of LED light (about 100 lm) spread over 1 m² gives about 100 lux; over 10 m² only 10 lux.
Why is my measured lux lower?
The calculation assumes every lumen lands on the area. In real rooms walls, ceilings and light fittings absorb some light, so measured levels are often 40 to 70% of this figure.
What efficacy should I use for LEDs?
Modern LED bulbs give about 90 to 120 lumens per watt. The packaging usually shows lumens and watts, so divide one by the other for an exact figure.
How many lux does a room need?
About 100–150 lux for living rooms and corridors, 300 for kitchens, 500 for offices and study desks, and 750 or more for detailed work.
Can I use foot-candles instead?
Yes. Choose square feet for the area and read the foot-candles row; 1 fc equals 10.764 lux.