Another helpful LED referance chart.

LEDs can be confusing. Especially when there are so many different types of LEDs, as well as a multitude of different applications for them. LED stands for Light Emitting Diode. So an LED bulb is more than just a light bulb it is actually effecting the current you are sending through your circuit. LEDs require resistors to drop the amount of current that reaches them so they function at the appropriate brightness, they don't interfere with the workings of the circuit, and so they don't just explode. LEDs come in a variety of sizes and colors, and if you don't have the appropriate resistor in the circuit for the specific LED you are using your circuit may not function properly. The LED's function isn't based solely on the resistor, but the color also factors into the equation. If you are into building noise machines or synthesizer's like our technician Joshua, you will or have already been plagued with redesigning a circuit because it didn't function properly, when all you really needed to do was swap out an LED or the resistor in the LED portion of the circuit. Here is the chart he has saved to his desktop so he can always reference it.To calculate the resistor value needed you use this equation
R=VIN-Vf over If
So on a 9 Volt ( VIN ) circuit using a 3mm red water clear lens LED that has a Maximum Forward Voltage (Vf) rating of 2.6 and an If of 20ma the R ( Resistance value ) equals 320 ohms. So you would need a 320ohm resistor for the circuit to work. 

Don't worry, there's a link below the chart to an online calculator!