This calculator allows you to do an approximate conversion between millicandelas
(or candelas) and lumens for an LED where you know the beam angle.
The reason it’s approximate is because the specifications don’t usually include information about how the luminous intensity (in candelas) was measured.
This calculator is perhaps most useful in comparing the light output of LEDs with different beam angles.
Luminous intensity (mcd)
Luminous intensity is a measure of the light that falls on a given area. The value you provide here should come from your LED datasheet, catalog page, or manufacturers specs.
The wizard assumes the luminous intensity you enter is the average luminous intensity for the entire beam angle. This is one reason the calculation is an approximation.
Beam angle (degrees)
There’s not really a standard way to measure beam angle, which unfortunately adds to the confusion.
This is a value that should come from your LED datasheet, catalog page, or manufacturers specifications.
Values in the 15° to 30° range are common for 5mm and 3mm indicator-type LEDs.
Other types can vary widely, so consult the specifications on your datasheet.  The value to look for is usually labeled 2θ1/2.