Lumen, lux and watt: how much light do you need
Watts once told us how bright a bulb would be. With LEDs, lumen and lux have taken over. What the three words mean, and how to work out the right amount of light for each room.
For most of the twentieth century, buying a bulb was simple: 40 watts for the bedside lamp, 60 for the ceiling, 100 for reading. Watts measured how much electricity a bulb used, and because almost all bulbs worked the same way, the number also hinted at brightness. LEDs have broken that link. Today a lamp of well under ten watts can match the old 60 watt bulb, so you need a different vocabulary.
Three words, three meanings
- (lm) measures the total amount of visible light a source gives off in all directions. It is the true measure of a lamp’s output, and in the EU it must be printed on the packaging.
- (lx) measures how much of that light actually arrives on a surface. One lux is one lumen spread over one square metre.
- Watt (W) measures electrical power, in other words how much energy the lamp uses, not how much light it makes.
A simple image helps. Picture a garden sprinkler: the lumens are the water leaving the nozzle, the lux is how wet the lawn gets in one spot, and the watts are the water bill. A lamp of 1,000 lumens lighting a single square metre gives 1,000 lux; spread over ten square metres, the same lamp gives only 100 lux.
For spotlights you will also meet the , the intensity of light in one particular direction, and the beam angle. A narrow beam concentrates the same lumens into a smaller, brighter pool, which is why a spotlight can make a painting glow with modest power.
From watts to lumens: the old bulbs translated
EU rules define how manufacturers may compare a new lamp with an old incandescent bulb. A lamp sold as equal to a 60 watt bulb must give at least 806 lumens. The table shows the official reference values and the power a modern LED typically needs to reach them.
| Former incandescent bulb | Light output | Typical LED power |
|---|---|---|
| 40 W | 470 lm | about 4 to 6 W |
| 60 W | 806 lm | about 7 to 9 W |
| 75 W | 1,055 lm | about 9 to 11 W |
| 100 W | 1,521 lm | about 13 to 16 W |
| 150 W | 2,452 lm | about 20 to 25 W |
Lumen values from the EU equivalence table for non-directional lamps. LED wattages are typical and vary with efficiency and colour rendering.
The ratio between light and power is called , measured in lumens per watt. Incandescent bulbs managed only about 10 to 17 lumens per watt, because most of their energy left as heat. Good LED lamps today reach around 100 lumens per watt or more, and the most efficient exceed 200. That is why the watt figure on the box no longer tells you much about brightness.
How much light each room needs
Lux is the figure lighting designers plan with. There is no single legal standard for private homes, but practitioners work with fairly consistent guide values. Offices, for comparison, are usually planned with around 500 lux on the desk; a home needs far less light in general and more only where you read, cook or work.
| Room or task | Typical illuminance |
|---|---|
| Hallway and stairs | 100 to 150 lux |
| Living room, general light | 100 to 200 lux |
| Reading in an armchair | 300 to 500 lux |
| Dining table | 150 to 300 lux |
| Kitchen, general light | 200 to 300 lux |
| Kitchen worktop | 300 to 500 lux |
| Bathroom mirror | 300 to 500 lux |
| Bedroom, general light | 50 to 150 lux |
| Home office desk | 300 to 500 lux |
Guide values for orientation. Older eyes often need considerably more light for reading and detailed work.
Estimating lumens for a room
A rough estimate needs only simple arithmetic. It will not replace a professional lighting plan, but it shows quickly whether a room is under-lit or over-lit.
- Measure the floor area in square metres, for example a living room of 20 m² (about 215 square feet).
- Choose a target from the table, say 150 lux for general light.
- Multiply the two: 20 m² × 150 lux = 3,000 lumens reaching the room.
- Allow for losses. Shades and diffusers absorb light, dark walls and floors swallow more, and high ceilings put sources further away, so plan around one and a half to two times the figure, roughly 4,500 to 6,000 lumens here.
- Divide the total between several sources rather than one ceiling light.
Spreading those lumens over a ceiling or cove light, two table lamps, a floor lamp and a picture light gives a softer and more interesting room than one powerful fitting. It also lets you switch layers on and off through the day. Light, reflective rooms need less; rooms with dark panelling or deep fabric shades need more.
Brightness is not everything
Lumens tell you how much light a lamp gives, not whether it is pleasant. from an exposed bright source can make a room feel harsh even when the lux level is modest, while a shaded, well-placed lamp often feels brighter than its numbers suggest. Colour temperature and colour rendering also shape how bright and comfortable a room appears.
Dimmers solve much of the dilemma. Plan enough lumens for the brightest moment, such as clearing up after a dinner party or a long reading session, and dim down for the rest of the evening. Fitting wall dimmers is a job for a qualified electrician.
Lumens tell you how much light a lamp gives, not whether it is pleasant.
glaminia is independent and does not sell lamps or recommend dealers. Images are AI generated illustrations, not photographs of specific products. Have fixed electrical work done by a qualified electrician.
