Use this lumen lux candela calculator to convert between lumens, candela and lux using beam angle, distance and illuminated area.
How to use the lumen, candela and lux calculator
Enter luminous flux (optional): In the “Luminous flux” box, type a value in lumens if you know how much light the source outputs. Leave this empty if you plan to start from candela or lux instead.
Enter luminous intensity (optional): In the “Luminous intensity” box, enter a value in candela if you know the beam strength of the source. You can leave this blank if you are working from lumens or lux.
Enter illuminance (optional): In the “Illuminance” box, type a lux value if you know how bright the light is on a surface. If you do not know this, you can let the calculator derive it from lumens or candela with geometry.
Check your starting values: Make sure at least one of the three main fields, lumens, candela or lux, is filled with a positive number. The calculator needs one of these to solve the others.
Open advanced options (optional): Click “Advanced options” if you want the calculator to use beam angle, distance from the source, or surface area when converting between lumens, candela and lux.
Set beam angle and units (optional): In “Source radiation angle”, enter the full beam angle and choose degrees or radians. This defines the solid angle used to relate lumens and candela for directed beams.
Set distance from source (optional): In “Distance from source”, type how far the surface is from the light and pick the distance unit. When starting from candela, this distance lets the calculator estimate lux on the surface.
Set illuminated area (optional): In “Surface area”, enter the area that the light covers and choose the unit. If you leave this blank, the calculator can estimate area from beam angle and distance, or fall back to a default area when needed.
Run the calculation: Press “Calculate” to let the tool combine your inputs. It will fill in any missing lumens, candela or lux values that can be derived from your entries and show a primary result.
Review the outputs: Read the results box to see the main quantity, the full set of lumens, candela and lux, and a short note about how beam angle, distance and area were used to estimate the illuminated region.
Understanding lumens, candela and lux
Lumens, candela and lux are three related ways of describing visible light. Lumens measure luminous flux, which is the total amount of visible light emitted by a source in all directions. A lamp with a higher lumen rating sends out more light than a lamp with a lower rating, regardless of where that light goes.
Candela measures luminous intensity. It describes how much light is emitted in a particular direction within a unit solid angle. A narrow spotlight can have a high candela value even if its total lumens are modest, because it concentrates most of its light into a small beam.
Lux measures illuminance. It describes how much luminous flux falls on a surface, expressed as lumens per square metre. A lux value tells you how bright a surface is when lit by a given source at a given distance. The same light source produces different lux levels depending on how far away it is and how large an area it illuminates.
These three quantities link together through geometry. Lumens describe the total output of the source. Candela describes output per unit solid angle. Lux describes how that output is distributed over an area at some distance from the source.
How beam angle and distance affect illuminance
Beam angle is the full angular width of the main light beam, usually measured in degrees. A small beam angle creates a tight cone of light with high intensity near the centre. A large beam angle spreads the light across a wider area, reducing the illuminance at any one point if the total lumens stay the same.
Distance from the source also matters. In a simple point source model without lenses or reflectors, illuminance from a given candela value drops with the square of the distance. Doubling the distance from the light reduces lux to about one quarter. This is a direct consequence of the inverse square law for light spreading out in three dimensions.
When you know the beam angle and distance, you can estimate the size of the illuminated patch on a surface. From that area and the total lumens, you can derive the lux. A luminous intensity value in candela combined with distance gives lux directly, since illuminance equals candela divided by distance squared when the surface is perpendicular to the beam.
Real fittings sometimes depart from ideal point source behaviour because of reflectors, lenses and complex beam shapes. Even then, beam angle and distance remain key inputs for practical illuminance estimates.
Using luminous quantities in lighting design
Lighting design for homes, offices and public spaces often starts with target lux ranges for different tasks. Reading, detailed assembly work and inspection need higher illuminance than casual circulation areas or decorative spaces. Once a target lux level is chosen, designers combine room area, beam geometry and fixture layout to decide how many lumens are required from all light sources together.
Lumens are the natural unit for comparing lamps and LED fittings across brands. Given a target lux on a surface and an estimate of area, you can calculate the total lumens needed. From that, you can choose one or several luminaires whose lumen ratings sum to the required total, while also checking their beam angles and distributions.
Candela values are useful when aiming light at specific targets. For example, in stage lighting, museum displays or outdoor floodlighting, designers care about intensity in certain directions. A fitting with a higher candela in the relevant angles produces brighter highlights or better reach at the same power level.
Lux measurements on site, taken with a light meter, provide feedback about how well a design meets its targets. If measured lux is lower than expected, the designer can revisit assumptions about beam angles, distances or surface reflectance and adjust lumens or layout.
Common confusions between lumens, candela and lux
A frequent source of confusion is treating wattage as a proxy for brightness. In older incandescent lamps, power consumption and lumens were loosely linked, but modern LED products can deliver very different lumens per watt. Lumens, not watts, are the direct measure of light output.
Another common confusion is mixing up lumens and lux. Lumens describe what leaves the lamp. Lux describes what arrives on a surface. Changing the distance or the size of the illuminated area changes the lux without changing the lumens of the source.
Candela is sometimes misunderstood as a general brightness rating, but it is directional. A source with high candela in a narrow beam can feel very bright when you look into that beam, while the same source may contribute little to general room lighting. A wide beam with lower candela but higher lumens can create a more even and useful illuminance over a large area.
A calculator that relates lumens, candela and lux using beam angle, distance and area makes these distinctions clearer. It shows how changing one quantity affects the others and helps translate between lamp specifications, design targets and measured light levels in real spaces.
FAQs
What does this lumen lux candela calculator do?
This calculator links luminous flux in lumens, luminous intensity in candela, and illuminance in lux. It uses your inputs and any available geometry, such as beam angle, distance, and illuminated area, to compute the missing values. You can start with lumens, candela, lux, or any combination, then use the advanced options to refine the result.
What is luminous flux in lumens?
Luminous flux measures the total visible light output from a source in all directions, weighted by the sensitivity of the human eye. It is measured in lumens and is often printed on light bulb packaging as the main brightness rating.
What is luminous intensity in candela?
Luminous intensity measures how much light is emitted in a particular direction. It is measured in candela. A narrow spotlight can have a high candela value because the same lumens are concentrated into a small solid angle.
What is illuminance in lux?
Illuminance describes how much light falls on a surface. It is measured in lux, which is lumens per square metre. Higher lux means the surface is lit more brightly.
How does the calculator decide what to solve for?
The calculator checks which of the main fields you have filled in. If you enter only one value, it uses the advanced geometry inputs to work out the other quantities. If you enter two of lumens, candela, and lux, it fills in the remaining one whenever there is enough information from the beam angle, distance, or area fields.
Why do I need beam angle and distance?
Beam angle and distance describe how the light spreads from the source. The calculator uses the full beam angle to estimate a solid angle and the shape of the light cone. With the distance, it can work out either the luminous intensity from the illuminance or the illuminance from the intensity, using an inverse square law model.
What does the source radiation angle input mean?
The source radiation angle is the full beam angle of the light, measured in degrees or radians. It is the angle from one edge of the useful beam to the other, as seen from the light source. The calculator converts this to a solid angle and beam footprint area when needed.
What is the distance from source used for?
The distance input sets how far the illuminated surface is from the light source. When candela and distance are known, the calculator can estimate surface illuminance. When lux and distance are known, it can estimate candela. It assumes a point-like source and free space between the source and the surface.
How does the surface area input affect the result?
If you enter an illuminated area, the calculator uses it directly to relate lumens to lux. Lux is lumens divided by area. When area is missing, the calculator tries to estimate the beam footprint from the beam angle and distance. If it cannot do that, it falls back to a default area of one square metre.
Can I convert between lumens and lux without entering distance?
Yes. If you know how many lumens reach a surface and the surface area, you can convert directly between lumens and lux. In that case you do not need distance or candela. The calculator uses your lumens and area inputs to compute lux or vice versa.
What happens if I only enter lux?
If you enter only lux, the calculator needs extra information from distance, beam angle, or area to estimate lumens or candela. If you also enter a surface area, it multiplies lux by that area to give lumens. If you enter distance instead, it uses an inverse square law to estimate candela from the measured lux.
Is this calculator accurate for real lighting design?
The calculator uses standard relationships between lumens, candela, lux, solid angle, and distance. It assumes a point-like source, a clear path, and an even beam within the stated angle. Real lamps have complex distributions, reflections, and losses. For detailed lighting design in buildings or outdoor spaces, you should treat these results as quick estimates and refer to manufacturer data and specialised lighting software where needed.
Calculator Methodology and Sources
This calculator relates three core photometric quantities: luminous flux in lumens (lm), luminous intensity in candela (cd), and illuminance in lux (lx). It uses your inputs, plus optional beam angle, distance from the source, and illuminated area, to solve for missing values under standard lighting-geometry assumptions.
Luminous flux measures the total visible light output of a source and is expressed in lumens. Luminous intensity describes how much of that flux is sent in a given direction and is expressed in candela. By definition, a source with an intensity of 1 candela that emits uniformly into a solid angle of 1 steradian has a luminous flux of 1 lumen, so flux and intensity are linked by the relation Φ = I × Ω, where Φ is in lumens, I in candela, and Ω in steradians. These SI definitions follow standard photometry references and summaries from the International Light glossary, BEGA lighting notes, and the lumen entry in general reference works.
Illuminance describes how much light arrives on a surface. It is defined as luminous flux per unit area, E = Φ / A, with units of lux, where 1 lux equals 1 lumen per square metre. The calculator uses this relation to convert between lux, lumens, and illuminated area: if any two of these are known, the third is computed. This matches the equations used in practical lux–lumen converters and lighting design tools that treat lux as lumens per square metre.
When candela and distance are available, the tool also applies the point-source inverse square law, which approximates illuminance on a small target as E ≈ I / r², where r is the distance from the source. This relation is standard in lighting engineering examples and teaching material that show illuminance falling off with the square of distance.
To link beam geometry to these quantities, the calculator treats the light as a cone with a full beam angle. It converts the full angle to radians, derives the corresponding solid angle Ω of the cone using the usual cone formula, and uses that to convert between lumens and candela. It also estimates the illuminated area on a surface at distance r as a circular spot with radius r × tan(half-angle), so A = πr² for that spot. If you do not provide a beam angle, the calculator defaults to a 360 degree emission (a full sphere, Ω = 4π). If you do not enter an area but do give a distance, it uses the beam geometry to estimate the area; otherwise, it falls back to a 1 m² reference area so that flux and lux can still be related in a simple way.