Multiply the number of liters by 1,000 to convert to an equivalent measurement in milliliters. For an example, 15 liters multiplied by 1,000 converts to 15,000 mL.
Shift the decimal point three places to the right to convert liters to milliliters -- if the measurement has no decimal point, then add three zeros to the right end of the measurement. For example, 2.8755 liters becomes 2,875.5 mL when the decimal point is moved three places to the right, and 30 liters becomes 30,000 mL after adding three zeros to the measurement's right end.
Convert the volume measurements online with a conversion calculator like the one embedded into Google. Type the number of liters as a search, followed by the phrase "liters to milliliters" and then click "Search" -- the search engine will display the conversion before its first result.