Lux and Lumens: The Difference, and How to Convert Between Them

Four units, and only one of them is on the box

The lumen measures total light output: everything a lamp radiates, in every direction, weighted for how the human eye responds to each wavelength. It says nothing about where that light goes. The candela measures light in one direction — luminous intensity, the SI base unit of the four, from which the others are derived. The lux measures what arrives on a surface: one lumen per square metre. And the nit, or candela per square metre, measures what a surface sends back at your eye, which is why it is the unit on a screen specification and not on a light bulb.

Only the lumen is printed on the packaging, and that is the source of nearly every mistake in this subject. A lamp maker can tell you what leaves the lamp; nobody can tell you what arrives on your desk without knowing your desk. Once that sinks in, the fact that a conversion needs an area stops feeling like a technicality and starts feeling like the definition, which is what it is.

Inverse square, and why a bulb's lumen figure tells you nothing on its own

Light from a small source spreads over a sphere, and a sphere's area grows with the square of its radius. So illuminance falls with the square of the distance: a source of 1,000 candela puts 4,000 lux on a surface half a metre away, 1,000 lux at one metre, 250 lux at two and 111 lux at three. Move a desk lamp twice as far away and you have quartered the light on the page, not halved it. This is the single most useful piece of arithmetic in the whole subject, and it is the reason lowering a pendant over a table works so much better than buying a brighter bulb.

Beam angle is the other half. A lamp that concentrates its output into a narrow cone puts far more lux under it than the same lumens spread wide. Take a 600-lumen spot. In a 24-degree beam it works out at about 4,370 candela, which is 1,092 lux at two metres. The same 600 lumens in a 36-degree beam gives 488 lux at the same distance; in a 60-degree beam, 178 lux; in a 120-degree flood, 48 lux. One lamp, one lumen figure, and a twenty-fold range of results depending only on how tightly the light is aimed.

Run the same arithmetic on an ordinary bare bulb and the result is sobering. An 800-lumen lamp radiating into the whole sphere is about 64 candela, which is 64 lux at one metre and 16 lux at two. A ceiling fitting two and a bit metres above a table therefore contributes something in the low tens of lux to that table by direct light alone. A lit room reaches 300 lux because there are several fittings, because the walls and ceiling bounce a large share of the output back down, and because the shade redirects rather than absorbs. That bouncing is exactly what the utilisation factor stands for, and it is why the room method exists at all.

How much light real tasks actually need

In the United States the free, primary figures are OSHA's construction minimums, which are stated in foot-candles: 5 for general construction area lighting, 3 for concrete placement, excavation, waste areas, access ways, active storage areas, loading platforms, refuelling and field maintenance, 5 indoors for warehouses, corridors, hallways and exitways, 10 for construction plant and shops, and 30 for first aid stations, infirmaries and offices. One foot-candle is 10.76 lux, so the office figure is about 323 lux. Note what these are: legal minimums for a job site, not comfort targets for a place you spend eight hours a day.

In Europe the reference is EN 12464-1, which sets maintained illuminance for hundreds of named tasks. It is a paid standard, so its exact values could not be checked against the text itself for this article and are not quoted here as if they had been. What can be said honestly is the shape of the ladder that every published version of these recommendations agrees on: circulation and corridors at the bottom around 100 lux, general storage around 150, an entrance hall around 200, general office and kitchen work around 300 to 500, sustained reading, writing and screen work around 500, technical drawing and fine assembly around 750, and inspection or very fine work at 1,000 and above.

Two things make those numbers less absolute than they look. Older eyes need substantially more light for the same task, which is why the recommendations are written for a working population rather than for you. And a target illuminance is a maintained value — the level the installation must still deliver at the end of its cleaning and replacement cycle, not the level it gives on the day it is switched on. A new installation should therefore overshoot, which is what a maintenance factor is for and which this calculator does not apply.

What the calculator does, exactly, and where it stops

Six inputs, three outputs, one line of arithmetic. It multiplies your length by your width, converting feet to metres if you asked for feet; multiplies that area by your target lux; divides by the utilisation factor; and divides the result by the lumens per bulb, rounding up. Nothing else happens. The defaults give 7,500 lumens and ten bulbs, and you can check it in your head: 5 times 4 is 20, times 300 is 6,000, divided by 0.8 is 7,500, divided by 800 is 9.375, rounded up to 10.

The utilisation factor is the one judgement call it asks of you, and it moves the answer a lot. At 100 %, an unreachable ideal, the same room asks for 6,000 lumens and eight bulbs. At 80 %, the typical-room default, 7,500 and ten. At 60 %, for a large room or dark walls, 10,000 and thirteen. That is a swing of five bulbs on one dropdown, and it is the input worth thinking about hardest, because white walls and a pale ceiling genuinely do a large share of the work.

Where it stops matters as much as what it does. There is no distance field, no beam angle, no mounting height and no maintenance factor. It answers "how much lamp output does this room need in total", not "how bright will it be on my desk", and it cannot be coaxed into answering the second question. It also has no validation: type nothing into length and width and it reports zero lumens and zero bulbs as if that were an answer. And one detail that surprises anyone working in feet: the floor-area result is always shown in square metres. That is correct — the calculation is metric throughout, and the conversion is done properly — but a room entered as 14 by 12 feet comes back as 15.61, not 168.

Frequently asked questions

How many lumens is 500 lux?

The question is incomplete, and that is not pedantry. 500 lux over one square metre is 500 lumens. Over the 20-square-metre room used throughout this article it is 10,000 lumens landing on the floor, or 12,500 of lamp output once you allow 80 % utilisation. Over a 168-square-foot room, 15.61 square metres, it is 7,805 on the floor and 9,755 of output. Same target, three answers, and the only thing that changed was the area. Any page that gives you a single multiplier for lux to lumens has silently assumed a room size and not told you which.

Why does my room feel dark when the bulbs add up to the right number?

Because total lumens says nothing about where the light lands. A single fitting of 7,500 lumens in the middle of a ceiling produces a bright patch under it and gloom at the edges, and the eye judges the room by the gloom. Spread the same output over several fittings and the room reads as evenly lit. Two more common causes: dark walls, which absorb the light that would otherwise be bounced back into the room — that is the utilisation factor doing its work in reverse — and shades that send most of the output upward or downward instead of outward.

Can this calculator tell me the lux on my desk?

No, and it should not pretend to. It has no field for the distance between the lamp and the surface, none for the beam angle and none for the mounting height, so it has no way to know. What it answers is the room-level question: how much total lamp output does a space of this size need to reach an average illuminance of this level. For the desk question you need the point-source arithmetic instead — intensity in candela divided by the square of the distance in metres — and the intensity is the lamp's lumens divided by the solid angle of its beam. The section on inverse square above works two examples through.

What utilisation factor should I pick?

Eighty per cent for an ordinary room with light walls and a white ceiling, which is the default and the right starting point for most homes. Sixty per cent for a large room, a room with dark or wood-panelled walls, a high ceiling, or fittings that throw most of their output at the ceiling. A hundred per cent is not a real room: it stands for a perfect enclosure that loses nothing, and choosing it will always leave you short. If in doubt, pick the lower one — a room that is slightly too bright can be dimmed, and one that is too dark cannot be un-wired.

Do lux and lumens work the same way for daylight?

The units do; the arithmetic in this calculator does not. Illuminance from the sky is measured in lux exactly as artificial illuminance is — an overcast day gives something in the low thousands on a horizontal surface outdoors, direct summer sun tens of thousands, and a room a few metres from a window a small fraction of either. What breaks is the room method: daylight arrives through an opening in one wall rather than from fittings distributed across the ceiling, so it falls off sharply with distance from the window and cannot be modelled as an average over the floor. Daylight is assessed with its own measure, the daylight factor, which this tool does not compute.