Resistor calculator for LEDs

Led voltage drop	Nr of leds in series	Voltage of power supply	Desired current
1.7v   (red) 1.9v   (bright red) 2.0v   (yellow) 2.1v   (green) 3.4v   (bright white/ bright green) 4.6v   (bright blue)  Other    volt	1 2 3 4 5	5 volt 7 volt 9 volt 12 volt Other    volt	15 mA 20 mA 30 mA Other    mA

Using 1 led with 1.9 volt drop on a 12 volt power supply using a 505 ohm resistor
gives a current of 20 mA.

The nearest higher standard valued resistor is 560 ohm. This gives a current of 18 mA.

The resistor should look like this:

The first three colour rings have the following colors: [Green] [Blue] [Brown]