Physics · Photometry
Lux to lumens calculator.
Lux is lumens per square metre. Enter any two of lumens, lux and area to get the third, plus foot-candles and the equivalent candela.
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How to convert lux to lumens
Lux and lumens measure different things. Lumens (lm) are the total light a source emits; lux (lx) is how much of it lands on each square metre of a surface. The SI defines 1 lx = 1 lm/m², so lux = lumens ÷ area and lumens = lux × area when the light is spread evenly. There is no direct lux-to-lumen conversion without an area, and no lumen-to-watt conversion without the lamp's efficacy.
Worked example. 800 lumens falling evenly on 4 m² gives 200 lux. One foot-candle is one lumen per square foot, equal to 10.7639 lux, so 200 lux is 18.58 foot-candles. The candela is lumens per steradian: an 800 lm source radiating equally in every direction (4π sr) is 63.7 cd; the same 800 lm squeezed into a 60° cone (2π(1 − cos 30°) = 0.842 sr) is 950 cd.
Frequently asked questions
How do you convert lux to lumens?
Multiply lux by the area in square metres: lumens = lux x m2. Lux alone cannot be converted without the lit area.
Is lux the same as lumens?
No. Lumens measure the total light output of a source; lux measures the light arriving per square metre of a surface.
How many lumens equal one lux?
One lumen spread evenly over one square metre is one lux.
What is a foot-candle in lux?
One foot-candle is one lumen per square foot, which equals 10.7639 lux.
Can I convert lumens to watts?
Not directly. It depends on the luminous efficacy of the light source (lumens per watt), which differs for every lamp type.
Sources
Checked
- NIST SP 330 (2019), Table 4: SI derived units with special names (lumen = cd sr, lux = lm/m2)
- BIPM: The candela (SI base unit of luminous intensity)
NIST SP 330 lists lm = cd sr and lx = lm/m2. The foot-candle factor is 1/0.3048^2 = 10.7639 (international foot). Candela from lumens divides by the solid angle: 4 pi sr for all directions, 2 pi (1 - cos half-angle) for a cone. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Average-value lighting maths. Not a lighting design or workplace compliance tool; check the applicable lighting code for required levels.