Your Ultimate Guide to Lux, Lumens and Watts

Understanding lux vs lumens is useful when choosing lamps and planning room lighting. The terms describe different things: lumens measure the light a lamp emits, while lux measures how much light reaches a surface. Watts tell you how much electrical power the lamp uses. Knowing which figure answers which question helps you compare LED bulbs without mistaking a high wattage for high brightness.

This guide explains lux, lumens, watts and luminous efficacy, then shows how to make a rough lighting estimate. For practical examples of task-based lux levels, see our companion guide.

What does lux measure?

Lux (lx) is the unit of illuminance: the amount of light arriving at a surface. One lux equals one lumen per square metre: 1 lx = 1 lm/m². If an evenly lit 10 m² surface receives 10 lux, about 100 lumens are reaching that surface in total. That does not mean a 100-lumen lamp will produce 10 lux in the room; some of its light may miss the surface or be absorbed by the fitting and surroundings.

Lux is measured at a position, such as a desk, floor or worktop. The reading changes with distance, beam spread, fitting optics, mounting height and room surfaces. A single measurement also cannot describe every point in a room.

What do lumens measure?

Lumens (lm) measure luminous flux: the total visible light output from a lamp or luminaire. When comparing bulbs for general room lighting, lumens are a more useful brightness specification than watts. Check the product’s stated lumen output, and remember that a directional spotlight sends its light into a beam while an omnidirectional lamp spreads it more widely.

What do watts tell you?

Watts (W) measure electrical power drawn by the lamp. A watt figure is not a brightness rating. Two LED bulbs can use different wattages and emit similar lumens, depending on their design. If a product emits more lumens for the same wattage, it has higher luminous efficacy.

What is luminous efficacy?

Luminous efficacy is the amount of light output per unit of electrical power, expressed in lumens per watt (lm/W). To compare efficiency, divide the lamp’s lumen output by its wattage. For example, a 1,000-lumen lamp using 10W has an efficacy of 100 lm/W. This figure is useful for comparing light output against power use, but it does not tell you about colour quality, dimmer compatibility, beam shape or service life.

How are lux and lumens related?

Lumens describe the light leaving the source; lux describes the light arriving on an area. A simplified estimate of average illuminance is:

Average lux ≈ total lamp lumens × utilisation factor × maintenance factor ÷ area (m²)

The utilisation factor estimates how much light reaches the area of interest. The maintenance factor allows for reduced output over time, for example from dirt or light-output depreciation. As an illustration, 2,000 lumens × 0.60 utilisation × 0.80 maintenance ÷ 10 m² gives an estimated average of 96 lux. The factors are assumptions, not specifications for a particular bulb. This estimate cannot show uniformity or the lux at an exact point; a detailed layout needs the luminaire’s photometric data and room information.

How much lux do you need?

The useful illuminance depends on the task and setting. A circulation route, a kitchen counter and detailed inspection work do not need the same lighting. HSE guidance gives examples such as about 50 lux for a corridor or walkway, 300 lux for a process control room and 750 lux for studying an engineering drawing. These figures illustrate the difference between tasks; they are not universal targets for every room or a replacement for a current lighting assessment.

For homes, combine general lighting with focused task lighting where useful, and consider glare, shadows and dimming as well as a lux reading. For workplaces, follow the applicable standards and assess the work actually being carried out. Our HSE lighting guidance discusses task detail and visual conditions.

Which figures should you compare when buying a bulb?

If you want to know…	Check…
How much light the lamp emits	Lumens (lm)
How much light reaches a work surface	Lux (lx), measured or calculated for that position
How much electrical power it uses	Watts (W)
How much light it produces per watt	Luminous efficacy (lm/W)

When replacing a bulb, also check the cap fitting, dimensions, colour temperature, beam angle and whether the product is suitable for your dimmer or enclosed fitting. The best comparison is the one that matches the job: lumens for total output, lux for light at the task, and watts or lm/W for power and efficacy.

Common questions about lux, lumens and watts

Does one lux equal one lumen?

One lux equals one lumen per square metre. Lux and lumens are related, but they are not interchangeable because lux includes the area over which light falls.

Does higher wattage mean a brighter LED bulb?

No. Watts measure electrical input. Compare the lumen output for total light, and use a lux reading or lighting calculation if you need to know how much light reaches a specific surface.

Can I calculate lux from a bulb’s lumens alone?

Not reliably. You also need the area, fitting and beam distribution, mounting position, room surfaces and allowances for maintenance. A simple formula can give an initial average estimate, while a lux meter or photometric design can give more useful project information.