Lumen to Lux to Candela Calculator

Lumens, lux, and candela describe different aspects of visible light. A lumen measures luminous flux, or the total amount of light a source emits as perceived by the human eye. Lux measures illuminance, meaning how much of that flux reaches each square metre of a surface. Candela measures luminous intensity in a particular direction. Keeping these distinctions clear is essential when comparing lamps, planning a room, specifying a spotlight, or checking whether a work surface receives enough light. The relationship between lumens and lux depends on area. When luminous flux is distributed uniformly, illuminance in lux equals lumens divided by area in square metres. The inverse calculation multiplies lux by area to obtain lumens. A 1,200-lumen source spread evenly over 12 square metres therefore produces 100 lux. Real installations can differ because beam shape, distance, reflection, absorption, and fixture efficiency make illumination less uniform, but the formula provides a reliable ideal estimate and a useful planning baseline. The relationship between lumens and candela depends on solid angle, measured in steradians. Luminous intensity equals luminous flux divided by the solid angle through which the light is emitted. Conversely, lumens equal candela multiplied by steradians. A narrow beam can have high candela even when its total lumen output is modest because its light is concentrated into a small angular region. A broad lamp can produce many lumens but a lower candela rating in any one direction. Area and solid angle are not interchangeable. Area describes the receiving surface, while solid angle describes the three-dimensional spread of a beam. A complete sphere contains approximately 12.566 steradians, equal to four times pi. Directional fixtures occupy much less than this. Manufacturer beam-angle data can be converted to solid angle for more detailed photometric analysis, but this calculator expects the solid angle directly so that the underlying relationship remains transparent. Use the converter for preliminary lighting layouts, classroom exercises, photography, stage lighting, architectural design, and product comparisons. Enter values expressed in the units shown and verify that the ideal uniform-distribution assumption fits the task. For compliance-critical projects, confirm results with a calibrated lux meter and the fixture manufacturer's photometric file, because mounting height, room geometry, surface reflectance, and optical losses all influence measured illumination.