How do you convert gallons to milliliters?

You convert gallons to litres and then to millilitres.
I will use Imperial gallons. (The conversion factor for US gallons is 1 US gal = 3.785 L).
EXAMPLE:
Convert 10.0 gallons to millilitres.
10.0 gal ##(4.546 L)/(1 gal)## = 45.5 L
45.5 L ##(1000 mL)/(1 L)## = 45 500 mL = 4.55 10 mL
Notice how we set up the conversion factors to make the units cancel.