Lumen Calculator

A lumen calculator converts between the interrelated units of light measurement, lumens, lux, candela, millicandela, and watts, routing every conversion through lumens as the common intermediate unit and applying the physical relationship appropriate to each step. Values accept decimals, fractions, mixed numbers, and scientific notation.

Converting to or from candela or millicandela needs a beam angle, since candela measures intensity within a cone and converting to lumens, a measure of total luminous flux, requires the cone's solid angle: lumens = candela x 2pi x (1 - cos(angle / 2)). Common beam angles are 15, 30, 60, and 120 degrees.

Converting to or from lux needs an area, since lux measures illuminance over a surface: lumens = lux x area.

Converting to or from watts needs a luminous efficacy figure, in lumens per watt, which describes how many lumens a source produces per watt of electrical power. Incandescent bulbs run 12 to 18 lm/W, compact fluorescents 45 to 75, and LEDs 80 to 150.

A source rated at 100 candela with a 120-degree beam angle converts to lumens as: lumens = 100 x 2pi x (1 - cos(60 degrees)) = 100 x 2pi x (1 - 0.5) = 100 x 2pi x 0.5 = 100 x pi, or approximately 314.2 lumens. Halving the beam angle to 60 degrees changes the half-angle to 30 degrees, where cos(30 degrees) is about 0.866, so the same 100 candela source yields only about 100 x 2pi x 0.134, roughly 84.2 lumens, since a narrower beam concentrates the same intensity into a smaller solid angle.

• Swap exchanges the From and To units without re-entering the value.
• Results appear automatically after a brief delay, with a manual convert action available.
• Settings can display a step-by-step breakdown: the symbolic formula, the substituted values, and the final computed result.
• Reset restores all fields and settings to their default values and clears any saved state.

Photometric conversions appear across a wide range of disciplines, from architectural lighting design to consumer product labeling. The relationships between lumens, lux, candela, and watts depend on beam geometry and source efficiency, making a dedicated converter valuable for quick reference and verification.

• Architectural Lighting: a target lux level across a conference room floor, converted to lumens using the room's area, sizes the fixture needed to reach it.
• LED Product Development: a measured candela value from a goniophotometer, converted to lumens with the beam angle set to match the LED optic, produces the figure a datasheet lists.
• Stage and Event Lighting: spotlights rated in candela at different beam angles compare on an apples-to-apples basis once each is converted to lumens.
• Photography: a studio light's lumen rating, converted to lux at a known illuminated area, verifies whether it delivers adequate illuminance at the subject distance.
• Energy Auditing: a lumen target converted to watts using the installed lamp type's luminous efficacy estimates power consumption, and comparing incandescent against LED efficacy quantifies the savings.
• Indicator Electronics: millicandela ratings from LED component datasheets, converted to lumens with the viewing half-angle set, feed directly into optical system modeling.
• Classroom Education: the step-by-step display shows how beam angle and solid angle connect candela to lumens, demonstrating the relationships between photometric units.
• Horticulture: fixture wattage converted to lumens and then to lux verifies that grow lights meet a minimum illuminance threshold at canopy height for a given grow area.