LED & Light
mcd to Lumens Converter
Convert an LED's millicandela rating into approximate lumens, using the beam angle to work out the solid angle it covers.
mcd to Lumens
Convert intensity and beam angle into approximate flux.
Intensity, Flux and Solid Angle
Millicandela and lumens measure different things, which is why converting between them needs extra information. Candela measures luminous intensity: how much light travels in a particular direction. Lumens measure luminous flux: the total light emitted in all directions the source covers.
A narrow-beam LED can have a very high candela figure while emitting modest total flux, because all of its output is concentrated. A wide-angle LED with the same total flux shows a much lower candela figure. The bridge between them is the solid angle of the beam, measured in steradians.
The angle θ is the full beam angle from the datasheet, usually quoted as the angle within which intensity stays above half its on-axis value. It is halved in the cosine term because the cone is measured from the axis outward.
How to Use the mcd to Lumens Converter
Enter the millicandela figure from the LED datasheet, then the full beam angle in degrees. Typical indicator LEDs quote something between 15° and 60°; wide-angle and diffused types can reach 120° or more.
Select Calculate. The results show the candela equivalent, the solid angle in steradians, the half-angle used inside the cosine, and the approximate luminous flux in lumens. Because every intermediate step is displayed, you can verify the arithmetic rather than trusting a single number.
Beam angle has a much larger effect on the result than most people expect, so it is worth taking the figure from the datasheet rather than estimating it.
Worked Example: A Narrow-Beam Indicator LED
An LED rated 8000 mcd with a 30° beam angle.
Now widen the beam to 60° while keeping 8000 mcd. The solid angle grows to about 0.842 sr and the flux to roughly 6.74 lm, nearly four times as much. Same intensity figure, very different total light output, which is exactly why comparing two LEDs on millicandela alone is misleading.
Reading Your Luminous Flux Results
The candela figure is simply the millicandela value divided by a thousand, and it describes intensity along the beam axis. The solid angle describes how much of the sphere around the LED the beam actually fills; a full sphere is 4π, or about 12.566 steradians.
The lumen figure is the product of the two and represents the approximate total light output. Treat it as an estimate. It assumes the intensity is uniform across the cone and zero outside it, which no real LED achieves.
Where a manufacturer publishes a lumen rating directly, use that instead. This conversion exists for the many components, particularly indicator LEDs, that are only specified in millicandela.
Where Millicandela Ratings Appear
The most useful application is comparison. Two LEDs quoted at 5000 mcd and 12000 mcd may produce similar total light if the first has a 60° beam and the second a 20° beam. Converting both to approximate lumens puts them on a common footing.
It also helps with product selection for lighting rather than indication. Lamps and fixtures are specified in lumens because that is what determines how well a space is lit, so converting a component's rating gives a rough idea of how many would be needed.
Optical design uses the relationship in reverse: narrowing the beam with a lens raises the candela figure without adding any light, simply by concentrating the same flux into a smaller solid angle. This is why a torch with a focusing head appears far brighter on a distant object without drawing any more current from its battery.
The conversion also helps when estimating illumination. Once you have an approximate lumen figure, dividing by the area the beam covers at a given distance gives a rough illuminance in lux, which is the quantity lighting guidance is usually written in. That chain of reasoning is approximate at every step, but it is often enough to decide whether a component is remotely suitable before ordering samples.
Limits of the mcd to Lumen Conversion
- Real beams are not uniform cones. Intensity falls off gradually toward the edge, so measured flux is usually below the calculated figure.
- Beam angle definitions vary. Most datasheets use the half-intensity angle, but some use different conventions, which changes the result substantially.
- Light emitted outside the quoted cone is ignored. Diffused and side-emitting LEDs therefore convert particularly poorly.
- Both units are photometric, weighted for human vision. They tell you nothing about radiated power in watts or about infrared and ultraviolet output.
Use the result for comparison and rough sizing, not for photometric design or regulatory compliance.
Questions
mcd to Lumens FAQs
Why can't millicandela be converted without a beam angle?
Because candela measures light in one direction while lumens measure total output. Without knowing how wide the beam is, there is no way to know how much of the sphere that intensity covers.
Where do I find the beam angle?
It is on the LED datasheet, often written as the viewing angle or 2θ1/2. If only the half-angle is given, double it before entering it here.
Is the result exact?
No. It assumes a perfectly uniform cone, so it tends to overestimate slightly. Manufacturer-measured lumen figures are more reliable when available.
Can I convert lumens back to millicandela?
Yes, by dividing the flux by the solid angle to get candela, then multiplying by a thousand. The same beam angle assumption applies in both directions.
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