The unit of measure commonly used to describe LED intensity is the millicandela (mcd), 1000 millicandela equals 1 candela. Candelas measures how much light is produced as measured at the light source.

The candela is the Luminous Intensity, in a given direction, of a source of monochromatic radiation of frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

The unit of measure commonly used for most other light sources is the Lumen.
Lumens are units of Luminous Flux and they measure how much light actually falls on a surface.
The Luminous Flux (lumens) from a light source is equal to the Luminous Intensity (candelas) multiplied by the solid angle over which the light is emitted, taking into account the varying intensities in different directions.
So you can see that it is not a simple matter to accurately convert millicandelas (mcd) to lumens but here is link to a handy program that will do it for you: Luminous Intensity to Lumen Converter

To use leds for 12 Volt applications you must simply connect a resistor in series with the led or leds. The resistor will drop the excess voltage and limit the current. See the Forward Voltage spec on the spec sheets (click the LED part numbers on the main LED page) for typical and maximum forward voltage, they're not all the same. Use a 1/8th watt or larger resistor soldered to the anode of the led and connect the +12 to the resistor. The chart below shows you the forward current for each color led with a single led in series with a 470 ohm resistor connected to 12 VDC. The current (and led brightness) will be slightly higher when the vehicle's engine is running because the battery charging system keeps the voltage up at around 13~14 volts. You can connect up to 4 red, orange, or yellow leds in series with each other and one resistor to power with 12 volts. The value of the current resistor must be lowered, depending on how many leds are in series. A maximum of 3 of the other colors can be in series across 12 volts, because of their higher forward voltage requirement.

Purchase LED Resistors

See our LED vehicle bulb pages for LED bulbs designed to work on 12 Volt systems

The Current State of Solid State Lighting - SSL
Circuitboards.com
Don's LED Pages
LED Center