To determine the rated current of the led is empirically impossible. This device parameter, you must ask the seller when buying it. If you know the diode type, type it in any search engine - it is likely that it will be located reference data, including rated current.
In case no data on the led there, we can assume that the device in the package type SMD rated current equal to 3 mA, round with a diameter of 3 mm to 5 mA, the rectangular cross-section 3 to 5 mm to 10 mA, in a round with diameter 5 or 10 mm to 20 mA.
The voltage drop across the led depends on its color. In the infrared it is 1.2 V, red for 1.8, green for 2.2, blue, white and purple - from 3 to 4.
The voltage drop across current-limiting resistor is define by the following formula: Ur=Up-Ud, where:
Ur - the voltage drop across the resistor;
Up - voltage power source;
Ud - the voltage drop across the led, V.
To calculate the resistance of the resistor, use Ohm's law. Before calculating put of the nominal led current in amperes, what is its value expressed in milliamps, divide by 1000. For example, 20 mA=0,02 A. Then, determine the value of the resistor according to the following formula: R=Ur/IR, where:
R is the desired resistance of the resistor in Ohms,
Ur - the voltage drop across the resistor, calculated by the previous formula;
Inom - nominal current of the led A.
The last step is the calculation of power resistor. To do this, multiply the voltage drop across the resistor to current flowing through it (also pre-translated in amps): P=Ur*Inom where:
Ur - the voltage drop across the resistor;
Inom - nominal current of the led A. to Use a resistor of higher power is possible, less not.
To improve the overall efficiency of the system if too large a voltage LEDs with the same current rating can be connected in a sequential group so that this chain had about 2/3 of the supply voltage, and the resistor is of the order of 1/3. In this case, before performing the calculation of the nominal voltage of the diodes need to be added.