Watts to Lux Calculator (lamp power × lm/W → lumens → lux on an area or at a distance)

Understanding the watts to lux calculator — concept and principle

Watts measure the electrical power a lamp draws, while lux measures how much light lands on a surface. This calculator converts between them in two steps: first lamp power times luminous efficacy gives lumens, then lumens are spread over an area or over a beam at a distance to give lux. It also shows foot-candles and, for a beam, the luminous intensity in candela.

Watts become lumens through efficacy, and lumens per m² are lux.

How it is calculated

Φ (lm) = P (W) × η (lm/W)

Even spread: E (lx) = Φ ÷ A

Beam: Ω = 2π(1 − cos(β/2)); I = Φ ÷ Ω (cd); E = I ÷ d²

1 foot-candle = 10.764 lx (lx × 0.09290304)

P lamp power, η luminous efficacy, A area in m², β beam angle, d distance in m.
Efficacy is limited to 683 lm/W, the theoretical maximum fixed by the SI definition of the candela.

The principle

Not every watt becomes visible light, and the eye is more sensitive to some colours than others. Luminous efficacy captures both, turning electrical watts into lumens, the total visible light output. Lux is lumens per square metre, so if all the light lands evenly on an area, you simply divide. For a lamp aimed at a surface, the light spreads into a cone. The cone's solid angle tells you how concentrated the beam is; dividing lumens by it gives candela. Moving twice as far spreads the same beam over four times the area, so illuminance falls with the square of distance.

Worked example

Default inputs: 10 W, 100 lm/W, spread evenly over 4 m².

Φ = 10 × 100 = 1,000 lm.
E = 1,000 ÷ 4 = 250 lux.
In foot-candles: 250 × 0.0929 = 23.2 fc.
Candela is not computed in the even-spread mode (—); the lit area is 4 m².

Each point is the calculator re-run with only “Area lit (m²)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Area lit (m²)	Illuminance (lux)
1	1,000
2	500
3	333
4	250
5	200
6	167
8	125

How to read the result

Lighting guidelines give target illuminances by task: corridors need far less light than offices or detailed work. Typical office desk targets are commonly around 300–500 lux, and fine detailed tasks need more; check the standard that applies to you. Modern LED lamps often reach roughly 80–150 lm/W, while older incandescent lamps were around 10–15 lm/W.

Common mistakes

Using a lamp's "equivalent" wattage instead of its actual power.
Ignoring light lost outside the target area or absorbed by fixtures.
Assuming the even-spread result applies at every point; real illuminance is higher under the lamp.
Confusing lumens (total output) with lux (light per area).

Keep in mind: Results ignore reflections from walls and ceilings and losses in fittings. For lighting design use manufacturer photometric data and recognised design methods or software.

Frequently asked questions

How many lux is 1 watt?

There is no fixed conversion; it depends on the lamp's efficacy and how the light is spread.

What is 683 lm/W?

The SI-defined efficacy of monochromatic green light at 540 THz, the upper limit for any light source.

Why does lux drop so quickly with distance?

The same light spreads over an area that grows with the square of the distance.

What is a foot-candle?

One lumen per square foot, about 10.76 lux.

Sources: BIPM, The International System of Units (SI Brochure), definition of the candela

Checked 9 Nov 2026. BIPM, SI base unit “candela” (luminous efficacy of 540 × 10¹² Hz radiation fixed at K_cd = 683 lm/W); lux = lm/m², inverse-square law E = I/d² and solid angle of a cone 2π(1 − cos α) (standard photometric formulas); foot from its exact SI definition.

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