Lumen, Lux, Candela & Foot-Candle Converter

Introduction to lumen, lux, candela, and foot-candle conversion

Lumen, lux, candela, and foot-candle conversion makes sense only after you separate how much light leaves the source from how strongly that light is aimed and how much of it lands on a surface. Lumens describe total visible-light output. Candela describes intensity in a particular direction. Lux and foot-candles describe illuminance, which means light arriving on an area. Because each unit measures a different part of the same physical story, a trustworthy converter cannot jump between them with a fixed multiplier unless the missing geometry is supplied.

This photometric converter asks for that geometry directly. If you know the illuminated area, the calculator can relate lumens to lux or foot-candles. If you know a beam angle, it can estimate the solid angle of an ideal conical beam and convert lumens to candela or the reverse. If you know a throw distance, it can estimate on-axis illuminance from candela with the inverse-square law. That approach is more honest than a one-click unit swap because it reflects what lighting designers, photographers, theatre technicians, and facility managers actually need to know before a number means anything in the field.

Unit references: the BIPM practical realization of the candela and derived photometric units and the NIST light-unit conversion table. NIST gives 1 foot-candle = 10.76391 lux.

How to use the lumen, lux, candela, and foot-candle converter

This lumen, lux, candela, and foot-candle converter is easiest to use when you first decide what you actually know from the fixture data, measurement, or design brief. A lamp package may list lumens. A spotlight datasheet may list peak candela or a beam angle. A light meter gives lux or foot-candles on the work plane. The route selector in the form is there to keep those starting points separate instead of mixing them into the wrong equation.

For area-based conversions, enter the total lumens or the measured illuminance and then supply the receiving area in square metres or square feet. For beam-based conversions, enter the full beam angle from edge to edge, not the half-angle. For throw calculations, use the perpendicular distance from the source to the target plane and remember that the inverse-square step estimates the centre of the beam under an ideal point-source assumption. If you only need lux to foot-candles or foot-candles to lux, choose the unit-only route because that conversion does not depend on beam shape or distance.

1. Choose the route that matches the data you have, such as lumens plus area, lumens plus beam angle, or candela plus distance.
2. Enter positive values in the units shown. The form accepts decimals, and the calculator converts square feet to square metres and feet to metres internally when needed.
3. Submit the form to see the primary answer, supporting values, and a short explanation of the steps used in that route.
4. Use the copy buttons if you want a clean text summary or a shareable URL with the current inputs embedded.

The result cards are meant to be read as a small decision aid rather than a mystery number. The first card gives the main answer. The additional cards show the area, distance, solid angle, or beam footprint that produced it. The notes below the cards explain the assumption behind the route, which matters because many real luminaires have spill light, centre-beam hot spots, and manufacturer-specific beam definitions that differ from the idealized cone used here.

Which photometric conversion route matches your lighting question?

Photometric conversion routes change with the missing geometry, so the table below shows exactly what extra measurement closes the gap between the units. If a conversion seems impossible without more information, that is not a flaw in the page; it is a sign that the lighting question itself is incomplete.

One practical way to read the table is this: lumens answer “how much light exists,” candela answers “how concentrated the beam is,” and lux or foot-candles answer “how much light arrives here.” The geometry column tells you what is missing between those questions.

Formulas for lumens, lux, candelas, and foot-candles

The formulas for lumens, lux, candelas, and foot-candles come from a short chain of relationships between flux, area, solid angle, and distance. Illuminance is flux per receiving area. One lux is one lumen per square metre, while one foot-candle is one lumen per square foot:

A conical beam with full beam angle θ occupies the solid angle below. Half the full beam angle belongs inside the cosine, which is why narrow and wide beam estimates can differ so much even when total lumens stay fixed:

Under the ideal uniform-beam assumption, luminous intensity follows I = Φ ÷ Ω. At a perpendicular target far enough away for a point-source approximation, centre illuminance follows E = I ÷ d². A circular beam's diameter at that plane is 2d tan(θ/2). Put together, those relationships let this page move from total output to directional intensity and then to estimated light on the target plane. They also explain why the same lamp can feel very different after a reflector, lens, or tighter beam spread changes the solid angle.

Plain-text formulas: lux = lumens / squareMetres; footCandles = lumens / squareFeet; lux = footCandles × 10.76391; steradians = 2π × (1 − cos(beamAngle / 2)); candela = lumens / steradians; luxOnAxis = candela / distanceMetres².

Worked example: a 1,000-lumen spotlight at 5 metres

This worked example for a 1,000-lumen spotlight shows why beam angle changes the answer even when total lumens stay fixed. Start with a 1,000 lm lamp and a 30° full beam angle. The ideal cone occupies about 0.214 steradians, so the average intensity across that cone is 1,000 ÷ 0.214 ≈ 4,671 cd. At 5 m, the on-axis estimate becomes 4,671 ÷ 25 ≈ 187 lux, which is about 17.4 fc.

Now widen the same 1,000 lm to 60°. The conical solid angle grows to about 0.842 sr, so intensity falls to about 1,188 cd and the 5 m centre estimate falls to about 47.5 lux. Nothing about the lamp's total lumen output changed; only the directional concentration changed. The 30° beam also draws a circle roughly 2.68 m across at 5 m. If you divide all 1,000 lumens by that flat circle, you get an ideal average of about 177 lux, which is close to but not identical with the on-axis estimate because a spherical solid angle and a flat target footprint are different geometries. The wider the beam becomes, the larger that difference can grow.

Limitations of ideal lumen-to-lux and candela throw estimates

The limitations of ideal lumen-to-lux and candela throw estimates matter any time a real fixture departs from a perfect cone or a target plane is not square to the beam. This calculator is deliberately useful, but it is also deliberately honest about what it assumes. It does not model off-axis intensity falloff, cosine losses on tilted surfaces, multiple overlapping luminaires, reflectance from walls or ceilings, dirt depreciation, colour shifts, or the manufacturer-specific threshold used to define a beam angle. Those details are exactly why lighting plans often move from quick calculator work to full photometric software and then to meter verification on site.

• Lumens are total flux: they help compare overall output, but they do not tell you how bright one point on a floor, table, or backdrop will be without area or beam information.
• Candela is directional: datasheets often report peak candela or centre-beam candlepower rather than a beam-wide average, so a single candela number may already describe a hot spot.
• Beam angle needs a definition: manufacturers commonly define it at a percentage of peak intensity; field angle and spill can be much wider than the bright core.
• Lux and foot-candles are illuminance only: they measure incident light on a surface, not the surface luminance that the eye or camera finally perceives after reflectance and texture come into play.
• The inverse-square law has conditions: it works best on-axis, at normal incidence, and far enough away that the source behaves like a point compared with the throw distance.
• Real installations lose light: optics, screens, glazing, haze, dust, aiming errors, maintenance factors, and obstructions all move measured results away from the ideal estimate.

Use this converter for first-pass fixture comparison, photography setup, theatre focus sessions, simple work-plane checks, and educational unit conversion. For building-code compliance, sports lighting, roadway design, horticulture, emergency egress, museum conservation, or safety-critical workplace illumination, pair manufacturer IES or LDT files with proper design software and confirm the installed result with a calibrated meter.

Frequently asked questions about lumens, lux, candelas, and foot-candles

These frequently asked questions about lumens, lux, candelas, and foot-candles address the most common points of confusion that appear on fixture labels, spec sheets, and handheld meters.