6mm 5mW Red Laser Module (3.5~4.5V)

Series resistor in ohms is calculated by supply voltage minus 4 divided by 0.045 amps.
so r=12-4/0.045
so the resistance r in ohms =
the voltage is 12 volt
minus the voltage across the laser 4 volt
for maximum brightness you need 45 milliamps thats 0.045 amps
so you need a 177.7777 ohm resistor just go up to the next highest value. 180 ohm.
if you increase the current beyond 45 ma the laser gets dimmer and hotter and it burns out. with these things its always better it be safe and so go up to the next value of 200 ohm or 220 ohm. don't forget, if your using say a car battery on a running car it can go up to 14.5 volts which means the resistor would be
14.5-4/.045= 233 ohms and add a bit on so say a 250 ohm resistor.