Lumen Calculator (Lumen to Lux to Candela)

Written by Great Medico

Updated September 25, 2026 •2 min read

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For lumen (lumen to lux to candela), a fast calculation is only useful when the setup is easy to audit. In optics, a small change in angle, focal length, wavelength, or refractive index can move the answer enough to matter. The goal here is a result you can reproduce from the visible inputs, not a number that hides its assumptions.

What this calculator does

The Lumen Calculator (Lumen to Lux to Candela) is a focused solver for luminous flux using Lumen, Candela, Lux. Its job is to make the active equation and the quantities feeding it easy to inspect rather than model every possible real-world effect.

How to use it

Begin with Lumen, Candela, Lux, Source radiation angle, Distance from the source. Do not strip the units from those values when copying them from a datasheet or measurement. Once calculated, vary the most influential input slightly and confirm that the response agrees with the equation before using the number elsewhere.

How the calculation works

The source angle is converted to solid angle Ω = 2π[1−cos(θ/2)]. From luminous flux Φ, candela is I = Φ/Ω. Illuminance is E = Φ/A when a surface area is supplied, or E = Φ/(Ωr²) when distance is used. The page can also back-calculate lumens from candela or from lux plus area.

Example

The defaults specify a 60° source angle and 1 m distance but no luminous quantity, so the headline is 0 lm. For a simple check, entering 1000 lm at 60° gives a solid angle of about 0.842 sr; the ideal uniform-cone path corresponds to roughly 1188 cd and about 1188 lx at 1 m when no separate illuminated area is entered.

How to interpret the result

The luminous flux belongs to this calculator’s geometric or wave-optics model. Its meaning comes from the entered wavelength, focal length, angle, distance, or index; a plausible number is not interchangeable with a measured optical specification unless those conditions match.

Limitations and notes

Lumens, candela, and lux describe different photometric quantities. The distance path assumes the source distributes flux uniformly over the entered solid angle, while the area path assumes the entered lumens fall on that surface. Real beam profiles and cosine incidence can change measured illuminance.