About the Lux to Watts Calculator
Lux measures how brightly a surface is lit, and it equals the lumens falling on each square metre. To light a room to a chosen lux level you therefore need a number of lumens equal to the lux times the area, increased to allow for light that is absorbed by walls, shades and fittings. This calculator takes your target lux, the area of the room, the efficacy of the lamps in lumens per watt and the share of the light that reaches the work surface, and gives the total lumens and the total watts. It is a quick way to plan lighting for a kitchen, office, workshop or garage.
Updated October 2026
How to use the Lux to Watts Calculator
- 1Enter the light level you want in lux. Hints show typical levels for different rooms.
- 2Enter the floor area in square metres.
- 3Enter the efficacy of your lamps in lumens per watt.
- 4Adjust the utilisation, the share of light that reaches the surface.
- 5Read the total lumens and total watts, and the watts per square metre.
A worked example
A home office measures 3 by 4 metres, so the area is 12 square metres. You want 500 lux on the desk. With utilisation 0.6, the lamps must produce 500 × 12 ÷ 0.6 = 10,000 lumens. LED lamps at 100 lumens per watt need 100 watts in total. A single 100 watt-equivalent ceiling light would be far too weak, so use several fittings plus a task lamp on the desk.
Colour rendering and glare matter as much as raw lux. A bright but harsh light can tire the eyes, while a slightly lower level with good colour rendering feels comfortable and shows colours truthfully. Look for a colour rendering index of 80 or more for homes and 90 or more for craft and makeup areas. Position lights to avoid reflections on screens and shiny surfaces, and use shades or diffusers to soften direct glare.
Planning tips
- Use a mix of ceiling light, task lights and lamps.
- Choose a warm colour for living areas and neutral for work areas.
- Put light where you need it instead of lighting the whole room strongly.
- Dimmable fittings let you vary the level.
- Clean fittings regularly, as dust can cut output by 10 percent.
Related tools
Use the Lumens to Watts Calculator for single bulbs, the Candela to Lumens Calculator for spotlights, the Lumens and Lux Calculator for the reverse sum and the Energy Cost Calculator to see what the lighting will cost to run. To pick a bulb for the result, check the Lumens to Watts Calculator, and measure the floor first with the Area Calculator.
Features
- Total lumens and watts for a room
- Typical lux levels listed
- Utilisation factor included
- Watts per square metre shown
- Works for any lamp type
- Free and private
Frequently asked questions
How do I convert lux to watts?
Lux is lumens per square metre, so multiply lux by the area to get lumens, divide by the efficacy to get watts, and allow for losses. For 300 lux over 12 square metres with LED lamps of 100 lumens per watt and a utilisation of 0.6, you need 6,000 lumens, or 60 watts.
Why can't lux be converted to watts directly?
Lux says how bright a surface looks, while watts say how much power a lamp uses. The link depends on the area lit and how efficient the lamp is. The calculator asks for both.
What lux level do I need?
Corridors need about 100 lux, living rooms 150, kitchens 300, offices 500 and detailed work such as sewing or drawing 750 or more. Older people usually need higher levels, and standards differ by country.
What is utilisation?
It is the fraction of the lamp's light that actually reaches the work surface. Some is absorbed by shades, dark walls and ceilings, and some lights the walls rather than the floor. Light rooms with good fittings reach 0.6 to 0.7, and dark rooms 0.4 or less.
How do I turn the total into the number of lamps?
Divide the total lumens by the lumens per lamp. If you need 6,000 lumens and each downlight gives 600, then ten downlights will do. Spread them evenly to avoid dark patches.
Does this replace a lighting design?
No. It is an estimate for homes and small spaces. Workplaces, shops and sports facilities have specific standards and need proper design, which also covers glare, colour and emergency lighting.