I used a 220 ohm resistor with a 9V battery and the LED works fine. However, most sources say the forward voltage in a red LED is typically only ~2 volts. Going by this logic, the LED should have burned out, as resistors only limit current not voltage. If this is a stupid question, I’m sorry. I’m relatively new at electronics.
Using a battery that is not brand new so its loaded voltage is 8.0V and using a 3.2V blue LED, then the current is (8V - 3.2V)/220 ohms= 21.8mA. No problem when most 5mm blue LEDs have a maximum continuous current of 30mA.
Here is how this works -----
I picked a 180 ohm resistor, because at 20 ohms/volt we are guaranteed a 50 milliamp intersection at the left axis. Thus we scale 20 ohms/volt by 9 volts, finding a 180 ohm resistor provides that current.
Note other LEDs will have lower forward voltage, moving solution-intersection to the left and raising the solution-current closer to 50mA.