Lux to Foot-Candles: A Guide to Measuring Light Intensity

From office lighting to greenhouse management, measuring light intensity is vital. Learn how to convert between Lux and Foot-candles.

Introduction

Lighting is specified in two units that measure exactly the same thing over differently sized patches of floor. The lux is lumens per square metre. The foot-candle is lumens per square foot. Everything else about them is identical, which means the conversion between them is simply the conversion between square metres and square feet.

The Core Mathematics

1 foot-candle = 10.764 lux

That 10.764 is the same number that converts square metres to square feet, and for exactly the same reason: spreading a given quantity of light over a square metre gives a lower reading than spreading it over the much smaller square foot. For rough work, dividing lux by ten gives foot-candles to within 8 percent.

Lumens, Lux and Candelas Are Not the Same

Three units appear on lighting products and they answer different questions.

Lumens measure the total light a source emits in all directions. It is a property of the bulb, and it is what has replaced watts on packaging now that efficiency varies so much between technologies.

Lux measures how much of that light lands on a surface. It depends on the bulb, the distance, and how the light is directed. The same 800 lumen bulb produces very different lux readings on a desk directly beneath it and on one across the room.

Candelas measure intensity in a particular direction, which is why they appear on spotlights and torches where the beam is deliberately narrow.

The practical consequence: a bulb's lumen rating tells you nothing on its own about whether a desk will be well lit. That depends on placement, and placement is measured in lux.

Recommended Light Levels

Typical targets for interior spaces, in both units:

For comparison, natural light dwarfs all of it. An overcast day gives roughly 1,000 lux outdoors, and direct sunlight reaches 100,000 lux. Full moonlight, by contrast, is around 0.25 lux. That range is why the human eye adapts rather than measures, and why a room that feels bright can still measure poorly against a working standard.

Light Falls Off With the Square of Distance

The single most useful physical fact in lighting design: for a small source, illuminance falls with the square of distance. Doubling the distance from a lamp to a surface leaves that surface with a quarter of the light, not half.

This is why a desk lamp moved slightly closer makes such a dramatic difference, and why ceiling lighting alone rarely achieves the levels recommended for detailed work. Raising a fixture from two metres to three above a bench cuts the light there to under half.

Measuring Light Yourself

Dedicated light meters read in either unit, and most phone apps using the ambient light sensor report lux. Two practical points make a reading meaningful. Measure at the working surface rather than at eye level, since that is where the light is needed. And measure with the sensor flat and facing up, because tilting it towards the lamp inflates the figure considerably.

Quick Reference Conversion Table

Common Mistakes Worth Avoiding

Buying by lumens alone

Total output says nothing about what reaches the surface you care about. Distance and direction do the rest.

Measuring at the wrong place

A reading taken at the ceiling or at eye level bears little relation to the light on a desk.

Ignoring the inverse square rule

Moving a fixture is often far more effective than buying a brighter one, and considerably cheaper.

Conclusion

Lux and foot-candles measure the same quantity over different areas, so the conversion is the familiar 10.764. What matters more than the unit is where you measure and how far the light has travelled, since illuminance falls with the square of distance.

Try our lux to foot-candles converter for quick and accurate conversions.

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