Convert lumens to estimated lux using a known lit area, cone beam, or bare bulb spread.
Advanced options
Table of contents
How to use our Lumens to Lux Calculator
- Choose How the light spreads: Known lit area for a measured surface, Cone-shaped beam for a spotlight, or Bare bulb all directions for an open bulb.
- Enter Light output (lumens), then enter only the measurement needed for that method: Lit area (square feet), Distance to surface (feet), and Full beam angle (degrees) when shown.
- Open Advanced options if needed, then set Light reaching the surface (percent), Target to compare (lux, optional), and Number format.
- Select Calculate and read Estimated light on the surface first, then check Area used in the lux math and Assumption used to make sure the method matches your real setup.
- Sanity-check the answer: if the area is huge, the lux should drop; if the distance doubles for a beam or bare bulb, the lux should usually be much lower.
Definitions
Lumens: Total visible light output from a source. More lumens means more total light, but not always more lux on your surface.
Lux: Light level on a surface. One lux is one lumen spread over one square meter [1].
Foot-candles: A US light-level unit equal to lumens per square foot. The calculator also shows Same estimate in foot-candles.
Effective lit area: The square-meter area used in the lux math after the selected spread method is converted.
Full beam angle: The full width of a cone-shaped beam, measured in degrees. It is not the half angle from the center line to one edge.
Light reaching the surface (percent): The percent of the rated lumens assumed to reach the target surface after shade, fixture loss, spill, or blockage.
Common mistakes and quick fixes
Mistake: Picking Known lit area in How the light spreads for a narrow spotlight.
Fix: Use Cone-shaped beam if the light has a Full beam angle (degrees), then check Assumption used.
Mistake: Entering watts instead of lumens in Light output (lumens).
Fix: Use the lumen rating from the bulb, fixture, projector, or lamp label.
Mistake: Using the whole room size for Lit area (square feet) when only a desk or wall is lit.
Fix: Enter the surface that actually receives the light, then compare it with Area used in the lux math.
Mistake: Measuring Distance to surface (feet) from the ceiling or floor instead of from the light source.
Fix: Measure from the bulb, LED, lens, or fixture opening to the surface you care about.
Mistake: Entering a half angle in Full beam angle (degrees).
Fix: Enter the full listed beam angle, such as 60 degrees, not 30 degrees.
Mistake: Treating a negative Amount above or below your target as an error.
Fix: A negative value means Estimated light on the surface is below Target to compare (lux, optional).
Limitations & Key Assumptions / Boundary Conditions
- The Known lit area method assumes the entered Lit area (square feet) is the area that actually receives the light.
- The Cone-shaped beam method assumes an evenly filled circular beam on a surface perpendicular to the beam.
- The Bare bulb all directions method assumes an ideal point source that sends light evenly in every direction.
- Real fixtures can have reflectors, lenses, shades, hot spots, spill light, and room reflections, so measured lux can differ from the estimate.
- Light reaching the surface (percent) is a user estimate. If that percent is wrong, Effective lumens and Estimated light on the surface will be wrong.
- The calculator estimates average lux over the computed area, not the brightest point or the darkest point.
- Target to compare (lux, optional) is only your chosen target. It is not a built-in lighting code or task recommendation.
Methodology
Core idea
The calculator first reduces the entered Light output (lumens) by Light reaching the surface (percent). It then spreads those effective lumens over the area that matches How the light spreads. Lux is lumens per square meter, so the main result is effective lumens divided by effective area.
effective_lumens = lumens * light_reaching_percent / 100
Known lit area
For Known lit area, the calculator converts Lit area (square feet) to square meters, then divides effective lumens by that area.
area_m2 = lit_area_ft2 * 0.09290304
lux = effective_lumens / area_m2
Cone-shaped beam
For Cone-shaped beam, the calculator converts Distance to surface (feet) to meters, uses Full beam angle (degrees) to find the beam radius, and treats the footprint as a circle.
distance_m = distance_ft * 0.3048
radius_m = distance_m * tan(beam_angle_deg * pi / 360)
area_m2 = pi * radius_m^2
lux = effective_lumens / area_m2
Bare bulb all directions
For Bare bulb all directions, the calculator treats the light as if it is spread over the surface area of a sphere at the entered distance.
distance_m = distance_ft * 0.3048
area_m2 = 4 * pi * distance_m^2
lux = effective_lumens / area_m2
Extra outputs
Foot-candles are calculated from the same lux result. The target comparison stays signed so a negative value clearly means the estimate is below your target.
foot_candles = lux / 10.76391
target_gap_lux = lux - target_lux
Mini example
For 1000 lumens over 100 square feet with 100 percent reaching the surface, the effective lumens are 1000 and the converted area is 9.290304 m^2.
lux = 1000 / 9.290304 = 107.64 lx
foot_candles = 107.64 / 10.76391 = 10.00 fc