A lumen is the unit used to measure the total amount of visible light produced by a light source. In plain English, lumens tell you how bright a light fitting is. The higher the lumen figure, the brighter the light.
The official definition is a little heavier going. A lumen is a unit of luminous flux in the International System of Units, equal to the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions. After 20 years of specifying lighting, we prefer the simple version: lumens are light, watts are energy.
That last point catches people out constantly. Watts measure how much electricity a fitting uses, not how much light it gives off. A 6W LED fitting produces a similar amount of light to a 40W incandescent lamp, because LEDs convert far more of the energy they draw into light rather than heat.
Every Lightmaster light fitting states its approximate lumen output on the spec sheet, so you can always work out how bright your space will be. Our fittings are tested in the factory to make sure there is no lumen loss, and we carry out random testing to confirm that fittings are as bright as they promise to be.
Key points
- A lumen measures the total visible light a fitting produces, so more lumens means a brighter light.
- Watts measure energy use, not brightness. A 6W LED gives about the same light as a 40W incandescent lamp.
- Lux is light landing on a surface. One lux equals one lumen spread over one square metre.
- Multiply your room area by the lux level you need to work out the total lumens required.
Lumens vs Watts
Before LEDs arrived, wattage was a reliable shortcut for brightness because every incandescent bulb used the same technology. A 100W bulb was always brighter than a 60W bulb. That logic no longer holds. Two fittings can draw identical power and give completely different light output.
Here is a rough guide to how old wattages translate into lumens, and what LED wattage delivers the same brightness today.
| Old incandescent bulb | Approximate lumens | Equivalent LED |
|---|---|---|
| 25W | 220 lm | 3W |
| 40W | 470 lm | 5W |
| 60W | 800 lm | 8W |
| 75W | 1,050 lm | 10W |
| 100W | 1,520 lm | 14W |
| 50W halogen GU10 | 400 to 500 lm | 5W to 6W |
A useful figure to know is efficacy, which is measured in lumens per watt. A good LED produces somewhere between 80 and 120 lumens per watt. A traditional incandescent bulb managed around 15. That gap is why your electricity bill drops when you switch, and why wattage is now a poor guide to brightness.
What is Lux?
Lux measures light that has landed somewhere. One lux equals one lumen spread over one square metre. So while lumens describe the amount of light coming out of a fitting, lux describes the amount of light hitting a surface.
The distinction matters. Take a single fitting producing 1,000 lumens. Hang it over a small desk and the surface below is brightly lit. Hang the same fitting in a large open room and the light spreads thinly across the floor, so the lux level drops sharply even though the lumen output has not changed. Height, spacing, beam angle and wall colours all affect the result.
Lux is the measurement that matters most in offices and work areas. A general office space needs 300 to 500 lux at desk level, while corridors and stairwells only need 80 to 100 lux. We use a lux meter to measure light levels on a surface, though there are other ways of calculating lux at the design stage.
Typical lux levels
| Setting | Lux |
|---|---|
| Full moon on a clear night | 0.05 to 0.3 |
| Dining, general | 50 |
| Office, general | 80 |
| Kitchen, general | 110 |
| Office, task | 450 |
| Kitchen, task | 500 |
| Direct sunlight | 32,000 to 100,000 |
Lumens, Lux, Watts and Candela at a Glance
| Unit | What it measures | Everyday meaning |
|---|---|---|
| Lumen (lm) | Total visible light leaving a source | How bright the fitting is |
| Lux (lx) | Lumens per square metre | How bright the surface is |
| Watt (W) | Electrical power drawn | How much energy it uses |
| Candela (cd) | Light intensity in one direction | How focused the beam is |
| Kelvin (K) | Colour temperature | How warm or cool the light looks |
How Many Lumens Do You Need?
There is no single answer, because room size, ceiling height, decor and the job the light is doing all change the figure. As a starting point, these are the lumens per square metre we would generally work to for general lighting in a home.
| Room | Lumens per m² |
|---|---|
| Living room | 100 to 150 |
| Bedroom | 100 to 150 |
| Hallway and landing | 100 to 200 |
| Bathroom | 150 to 250 |
| Kitchen, general | 200 to 300 |
| Kitchen worktop, task | 500 to 700 |
| Home office | 300 to 400 |
| Garage and utility | 200 to 300 |
Most rooms need a mix of general lighting and task lighting rather than one bright source in the middle of the ceiling, which is where lighting design earns its keep.
Worked Example: Lighting a Kitchen Worktop
Multiply the area of the space by the lux level you want, and you get the lumens you need.
A kitchen worktop measuring 4m by 0.6m gives you 2.4m². Kitchen task areas want around 500 lux, so:
2.4m² x 500 lux = 1,200 lumens needed
The Lightmaster Dalek fixed downlight produces 486 lumens. Divide the lumens you need by the lumens each fitting produces:
1,200 ÷ 486 = 2.5, so three downlights over the worktop
Bear in mind that this is a quick calculation rather than a full design. In practice a designer also allows for the beam angle of the fitting, the mounting height, how much light the walls and ceiling absorb, and lumen depreciation over the life of the fitting. Those factors usually push the real figure a little higher, which is why professional schemes often specify more light than the simple sum suggests.
Three Mistakes People Make With Lumens
Buying on watts. Wattage tells you running cost, not brightness. Always compare lumen figures when you are choosing between fittings.
Ignoring beam angle. Two downlights with identical lumen ratings will light a room very differently if one throws a narrow 24 degree beam and the other a wide 60 degree flood.
Forgetting lumen maintenance. LEDs dim gradually over their lifetime. An L70 rating of 50,000 hours means the fitting still produces 70% of its original lumens after 50,000 hours of use. Good schemes account for this from the start.
Frequently Asked Questions
How many lumens is bright? For a domestic room, 1,000 to 3,000 lumens in total is usually plenty, depending on the size of the space. For a single fitting, anything above 800 lumens reads as bright indoors.
How many lumens is a 60 watt bulb? Roughly 800 lumens. An LED producing the same amount of light draws around 8W.
What does lm mean on a light bulb? Lm is the abbreviation for lumens, the measure of how much visible light the bulb produces.
What is the difference between lumens and lux? Lumens measure the light coming out of the fitting. Lux measures the light landing on a surface. One lux is one lumen per square metre.
Do more lumens always mean better lighting? No. Quality of light matters as much as quantity. Colour temperature, colour rendering, beam angle, glare control and dimming all shape how a room feels, and an overlit space is just as uncomfortable as an underlit one.
Get the Lighting Right First Time
Lumens are only one part of a good lighting scheme. If you would like help working out what your space actually needs, our team has been designing lighting for homes and commercial projects for over 20 years.