Lighting Converter: Lumens, Lux, Candela and Footcandles

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How to use this tool

Pick the pair you actually have. Lux and footcandles convert straight across with a fixed factor and need nothing else from you. The other three pairs need one piece of geometry, and the tool asks for it as soon as you select the mode, because without it the conversion has no answer at all.

The input people get wrong is the area in lumens to lux mode. It is the area the light actually lands on, not the floor area of the room and not the footprint of the fixture. A 800 lumen bulb in a 12 square metre room does not put 800 lumens on the floor, because a good share of it goes into the ceiling, the walls and the shade. Treat the answer as a ceiling on what you will measure, then expect the real reading to come in lower.

What the conversion is actually doing

Four quantities, and only one of them belongs to the lamp by itself. Luminous flux in lumens is the total light output in every direction. Luminous intensity in candela is how much of that flux is heading down one particular direction, per unit of solid angle. Illuminance in lux is what arrives at a surface, one lumen spread over one square metre. Footcandles are the same idea in imperial units, one lumen over one square foot.

That last pair is why the conversion factor looks arbitrary but is not. A foot is 0.3048 metres exactly, by international agreement since 1959. Square that and you get 0.09290304 square metres in a square foot. Because a footcandle packs its lumen into a smaller patch, one footcandle is a brighter reading than one lux, by a factor of 10.763910416709722. Nothing is measured or approximated in that number, it falls out of the definition of the foot.

The candela relationships are geometric. Illuminance from a point source is intensity divided by distance squared, which is the inverse square law. Flux is intensity multiplied by the solid angle it covers, and the solid angle of a cone with full apex angle A is two pi times one minus the cosine of half A. Put 360 degrees into that and you get four pi steradians, the whole sphere, which is why a perfectly isotropic one candela source emits 12.566 lumens.

Why a lumen figure on a box tells you less than you think

Retail packaging quotes lumens because it is the one number that lets you compare two bulbs fairly. It is also the number that tells you least about whether a space will be pleasant to work in, because it says nothing about where the light goes.

Take two lamps rated at 1000 candela. One is a 10 degree spot, the other a 60 degree flood. The spot puts out about 24 lumens in total. The flood puts out about 842. Same intensity on axis, same reading on a meter held straight in front of them, and a thirty five fold difference in how much light they are actually producing. If you buy on candela you are buying a beam. If you buy on lumens you are buying output and you still have no idea what shape it arrives in.

This is why lighting designers work in lux at the working plane rather than in lumens at the fixture. A drafting desk wants somewhere around 500 lux, a corridor is fine at 100, and detailed inspection work climbs past 1000. Those are surface numbers. Getting from a lamp specification to a surface number is exactly the conversion this tool does, and the geometry you feed it is doing most of the work.

A worked check you can repeat on paper

Use the tool defaults. Take 1000 candela measured at 2 metres, in candela to lux mode.

Illuminance is intensity over distance squared. Two metres squared is 4, so 1000 divided by 4 gives 250 lux. Convert that to footcandles by multiplying by 0.09290304 and you get 23.226 fc. Move the meter to 4 metres and the distance squared becomes 16, so the reading drops to 62.5 lux. Doubling the distance quartered the illuminance, which is the inverse square law doing its only trick.

Now switch to candela to lumens with the same 1000 cd and the 36 degree flood preset. Half the apex angle is 18 degrees, whose cosine is 0.951057. One minus that is 0.048943. Multiply by two pi and the solid angle is 0.307519 steradians. Times 1000 candela gives 307.52 lumens. That is the entire output of the lamp, which is a useful sanity check against the box: if the packaging claims 1200 lumens for a 36 degree beam at 1000 candela, either the beam is wider than stated or the candela figure is a peak reading rather than an average across the cone.

One more, in lumens to lux. A standard 800 lumen bulb, the modern replacement for a 60 watt incandescent, spread over 10 square metres gives 80 lux. That is dim for a kitchen and about right for a hallway, which matches the everyday experience of putting one bare bulb in a room and finding it not quite enough.

Popular conversions

Each link opens the tool with that value already loaded, so you can check the answer and then keep going with your own numbers.

Lux to footcandles. 1 lux to fc · 5 lux to fc · 10 lux to fc · 20 lux to fc · 25 lux to fc · 30 lux to fc · 50 lux to fc · 75 lux to fc · 100 lux to fc · 120 lux to fc · 150 lux to fc · 200 lux to fc · 250 lux to fc · 300 lux to fc · 320 lux to fc · 400 lux to fc · 500 lux to fc · 600 lux to fc · 750 lux to fc · 800 lux to fc · 1000 lux to fc · 1200 lux to fc · 1500 lux to fc · 2000 lux to fc · 3000 lux to fc · 5000 lux to fc · 10000 lux to fc · 20000 lux to fc · 50000 lux to fc · 100000 lux to fc

Footcandles to lux. 1 fc to lux · 2 fc to lux · 3 fc to lux · 5 fc to lux · 7.5 fc to lux · 10 fc to lux · 15 fc to lux · 20 fc to lux · 25 fc to lux · 30 fc to lux · 40 fc to lux · 50 fc to lux · 60 fc to lux · 75 fc to lux · 100 fc to lux · 150 fc to lux · 200 fc to lux · 250 fc to lux · 300 fc to lux · 500 fc to lux · 750 fc to lux · 1000 fc to lux · 2000 fc to lux · 5000 fc to lux

Lumens to lux, over the default 10 square metres. 100 lumens to lux · 200 lumens to lux · 250 lumens to lux · 300 lumens to lux · 400 lumens to lux · 450 lumens to lux · 500 lumens to lux · 600 lumens to lux · 700 lumens to lux · 800 lumens to lux · 900 lumens to lux · 1000 lumens to lux · 1100 lumens to lux · 1200 lumens to lux · 1300 lumens to lux · 1500 lumens to lux · 1600 lumens to lux · 1800 lumens to lux · 2000 lumens to lux · 2200 lumens to lux · 2500 lumens to lux · 3000 lumens to lux · 3500 lumens to lux · 4000 lumens to lux · 5000 lumens to lux · 6000 lumens to lux · 8000 lumens to lux · 10000 lumens to lux

Candela to lux, measured at the default 2 metres. 1 cd to lux · 5 cd to lux · 10 cd to lux · 25 cd to lux · 50 cd to lux · 75 cd to lux · 100 cd to lux · 150 cd to lux · 200 cd to lux · 250 cd to lux · 300 cd to lux · 500 cd to lux · 750 cd to lux · 1000 cd to lux · 1500 cd to lux · 2000 cd to lux · 2500 cd to lux · 3000 cd to lux · 4000 cd to lux · 5000 cd to lux · 7500 cd to lux · 10000 cd to lux · 15000 cd to lux · 20000 cd to lux · 50000 cd to lux

Mistakes that catch people out

Converting lumens to lux with no area. There is no fixed factor between them and never has been. Any site that offers a one box lumens to lux converter is quietly assuming an area, usually one square metre, which makes the two numbers identical and the conversion pointless. If you do not know the area, you do not have an answer yet.

Using the fixture footprint instead of the lit area. A downlight 2.4 metres above a desk lights a pool far wider than the fitting. Measure or estimate the patch that actually receives light, and if the beam is a cone, the lit diameter is roughly twice the distance times the tangent of half the beam angle.

Treating candela as a brightness rating. Candela is intensity in one direction only, usually the peak on the beam axis. Two lamps with identical candela figures can differ enormously in total output, and a torch marketed on candela is being marketed on how tightly it focuses rather than how much light it makes.

Forgetting the inverse square law applies to point sources. It holds well once you are more than about five beam diameters from the source. Close to a large panel or a linear fitting the falloff is much gentler, closer to inverse first power for a long tube, and using distance squared there will underestimate the reading badly.

Rounding the footcandle factor to 10.76 and then chaining conversions. One step is fine, the error is under a hundredth of a percent. Convert back and forth several times inside a spreadsheet and it accumulates. The exact value is 10.763910416709722 and the tool carries it in full.

How this was checked

The engine was written twice, once as a plain reference script and once as the browser tool, and the two were compared on the same cases. Four benchmarks anchor it. One candela at one metre gives exactly one lux, which is the definition of the lux. A full sphere solid angle comes out at 12.566371 steradians against four pi. A 180 degree cone gives 6.283185 steradians against two pi. One footcandle converts to 10.763910 lux, matching the NIST value.

Round trips were run at 0.001, 1, 37.5, 800 and 125000 in every mode and every direction, and all returned to the starting value within one part in a billion. The chart is drawn from the same functions the readout uses, so the curve cannot drift away from the numbers.

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