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Lumens to Lux Calculator

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Lumens describe a lamp and lux describe a place, so no unit converter can turn one into the other — you need an area or a distance in between. This does that conversion in either direction, and compares the answer against the levels different tasks actually need.

What are you working out?

On the box — it replaced the wattage figure

Floor area of the room, or of the surface

How the light is spread. A bare bulb is 360°.

Used to show how wide the beam is where it lands

How to use the lumens to lux calculator

  1. 1Pick a direction: lumens to lux, a target lux level to the lumens needed, or candela at a distance.
  2. 2Enter the lamp output and the floor area, or the intensity and the distance.
  3. 3Set the beam angle if the fitting is directional — a bare bulb is 360°.
  4. 4Compare the result against the recommended levels table before deciding.

Examples

One bulb in a small room

Input
800 lumens over 12 m²
Result
66.7 lux, or 6.19 foot-candles — about corridor level, and well short of the 300 lux a kitchen worktop wants.

Ignores light bouncing off walls and ceiling, which in a small pale room adds meaningfully to the measured figure.

Working backwards from a target

Input
300 lux across the same 12 m²
Result
3,600 lumens needed — roughly four to five typical bulbs, which is why one ceiling fitting is rarely enough.

A spotlight

Input
500 candela at 2 m with a 24° beam
Result
125 lux on the surface, lighting a circle 0.85 m across.

About the lumens to lux calculator

Four quantities, easily confused

Lighting has four related measures and mixing them up is the usual source of error. Luminous flux, in lumens, is everything a source emits. Luminous intensity, in candela, is flux per unit solid angle — how concentrated it is in a particular direction. Illuminance, in lux, is flux per square metre arriving on a surface. Luminance, which this page does not calculate, is how bright a surface looks when you look at it.

The lamp box quotes lumens, the spotlight datasheet quotes candela, the building standard specifies lux, and the photographer's meter reads luminance. Each is the right unit for its job, and none converts to another without knowing something about the geometry.

Why watts stopped meaning anything

For most of a century a bulb's wattage was a usable proxy for its brightness, because nearly every domestic bulb was an incandescent with roughly the same efficiency. A 60 W bulb produced about 800 lumens and everyone knew what a 60 W room looked like.

LEDs broke the association completely. An LED producing that same 800 lumens draws somewhere between 6 and 10 watts depending on its efficiency, and the figure keeps improving, so wattage now says something about running cost and nothing reliable about light output. The lumen figure on the box is the one that carries the meaning — and this page is about what happens to those lumens once they leave the lamp.

Frequently asked questions

Why can lumens not simply be converted to lux?
Because they measure different things. A lumen counts the total light a source emits in every direction; a lux counts how much light lands on a square metre of surface. The same 800-lumen bulb produces 800 lux over one square metre and 8 lux over a hundred, so the area is not a conversion factor you can look up — it is part of the question.
What does the beam angle change?
How concentrated the light is. A narrow beam puts the same lumens onto a smaller patch, so the lux figure rises sharply while the area lit shrinks. The tool converts the angle into a solid angle in steradians, which is what candela — lumens per steradian — is defined against, and reports the diameter of the circle the beam lights at your chosen distance.
How accurate is this for a real room?
It gives the light arriving directly from the source and nothing else. Real rooms bounce light off walls, ceiling and floor, so a small room with pale surfaces measures noticeably brighter than the arithmetic, and a large one with dark surfaces can measure lower. Lighting designers handle this with a utilisation factor drawn from tables for the specific room and fitting. Treat this as a sound first estimate rather than a design calculation.
Does the inverse square law always hold?
Only for a point source in the open. Double the distance and the light falls to a quarter, which works well for a spotlight several metres away. Close to a large panel fitting it breaks down badly — a wide flat source behaves more like a plane, where light falls off far more slowly, until you are several times its own width away from it.
What are foot-candles?
The same quantity as lux in imperial units: one lumen per square foot rather than per square metre. One foot-candle is about 10.76 lux, since that is how many square feet fit in a square metre. American lighting specifications are usually written in foot-candles and European ones in lux, so both are shown to save a second conversion.

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