Light Illumination Converter
Convert light levels between lux, foot-candles, phot, and lumen-based units. Enter an Illumination value, then choose the From unit and To unit.
Convert Light Illumination Between Units
Light Illumination Formula
Where:
- \(E_{\mathrm{from}}\) — the entered illumination value.
- \(E_{\mathrm{to}}\) — the converted value.
- \(f_{\mathrm{from}}\) and \(f_{\mathrm{to}}\) — the selected units’ factors relative to lux.
The converter first expresses the entered value in lux, then divides by the factor for the destination unit. This formula applies to the illuminance units listed below; the candela-per-square-meter option has an important limitation explained later.
Light Illumination – Calculation Example
Example: convert the default value of 500 lux to foot-candles. The calculator uses 10.76391 lux per foot-candle.
\[500\times\frac{1}{10.76391}=46.451522\ \mathrm{fc}\]
Result: 500 lx is approximately 46.451522 fc.
How to use the calculator
- Enter a nonnegative number in Illumination value. Use the light level measured or specified for the surface of interest.
- Choose the unit of that number under From unit. The default is Lux (lx).
- Choose the unit you need under To unit. The default is Foot-candle (fc).
- Read Converted Illumination. The result includes the selected destination unit and is rounded to six decimal places.
For example, leave the defaults unchanged to reproduce the calculation above. You can also select the same unit in both menus to leave the numerical value unchanged. Check the unit beside any value you copy into a lighting plan or measurement record.
What illuminance measures
Illuminance describes the luminous flux arriving at a surface per unit area. Its SI unit is the lux: one lux equals one lumen per square meter. A surface receiving 500 lumens spread uniformly over one square meter therefore has an average illuminance of 500 lx. The measurement describes light falling on the surface, not the total light produced by a lamp or how bright the surface appears to an observer.
Unit conversion helps when a light meter, design document, or lighting recommendation uses a different system. Foot-candles are common in some US lighting work; one foot-candle is one lumen per square foot, approximately 10.76391 lx. Phot is a metric CGS unit equal to 10,000 lx. The menu’s Lumen/m² option is numerically identical to lux, while one lumen per square centimeter equals 10,000 lx.
A converted value represents the same stated illuminance, not a change in the lighting. If a plan calls for a value in foot-candles but a meter reports lux, conversion puts the figures on a comparable scale. It does not establish whether the measured level meets a particular task requirement: that also depends on where the reading was taken, the applicable guidance, and the conditions during measurement.
Units, assumptions, and practical limits
- Lux (lx) and lumen/m²: factor 1; these are two ways to express the same illuminance unit.
- Foot-candle (fc): factor 10.76391 lx per fc, as used by this calculator.
- Phot (ph) and lumen/cm²: factor 10,000 lx per unit.
- Candela/m² (cd/m²): a unit of luminance, not illuminance. Its inclusion in the menu does not make it directly convertible to lux.
When cd/m² is selected, this calculator assigns it a factor of 1, so it displays the same number when converting between cd/m² and lux. Treat that output as a numerical placeholder, not a physical conversion. Luminance describes light emitted or reflected in a particular direction per unit projected area; illuminance describes light arriving at a surface. Relating the two requires additional information about the surface and lighting geometry. For luminance units, use the Light Luminance converter instead.
Real readings can vary across a desk, floor, or wall. Measure at the relevant surface and position, and record the conditions if you need to compare readings later. This converter changes units only: it cannot account for distance from a light source, beam spread, shadows, or reflectance. Likewise, lamp output in lumens does not by itself determine illuminance at a surface. To examine related quantities separately, see the Luminous Intensity converter or the Lumens to Watts calculator.
Frequently Asked Questions
How do I convert foot-candles to lux by hand?
Multiply the foot-candle reading by 10.76391. For example, 30 fc is approximately 323 lx. To check your result, divide the lux value by 10.76391; you should get back about 30 fc. Keep a few extra digits during calculation and round only the final result.
What is a good lux level for a room or workspace?
It depends on the activity, not just the room. Roughly 100–300 lx may suit general indoor lighting, while reading or detailed desk work often calls for around 300–500 lx or more. These are broad examples, not universal requirements. Consider glare, contrast, the age of the occupants, and any applicable workplace or building guidance.
Can I calculate lux from a bulb’s lumen rating and the room’s area?
Dividing lumens by area gives a useful estimate only if you know how much of the bulb’s light reaches that area and can assume it is spread evenly. A 1,000-lumen bulb in a 10 m² room does not necessarily provide 100 lx on a desk: fixtures, walls, distance, and direction all affect the light received there.
Does illuminance fall to one-quarter when the distance doubles?
For a small source in unobstructed space, doubling the distance approximately quarters the illuminance on a surface facing the source. This inverse-square rule is less reliable for large fixtures, focused beams, or rooms where walls reflect light. It predicts a change in actual lighting conditions; it is not a rule for converting between illuminance units.
Are phone light-meter apps accurate enough to measure lux?
Phone apps can help compare brighter and darker spots, but their absolute lux readings may be unreliable. Phones differ in sensor placement, spectral response, and calibration, and a finger or case can shade the sensor. For measurements used to verify a lighting requirement, use a suitable calibrated light meter and follow the relevant measurement procedure.
Why does a small converted value show as zero?
The calculator rounds its displayed result to six decimal places, so a positive result smaller than 0.0000005 in the destination unit may appear as zero. For example, very low lux values converted to phot can be lost in the display rounding. Zero on the screen therefore does not always mean the entered illuminance was zero.
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