One liter is equal to 1,000 milliliters. In the metric system, the prefix "milli-" is used before basic units, such as liter, gram and meter, to indicate that the measurement is one-thousandth of that basic unit.
Other common prefixes that precede basic units are deci-, centi- and kilo-. Deci- indicates one-tenth of the basic unit, so 10 deciliters make 1 liter. Centi- means one-hundredth, so 100 centiliters make up 1 liter. Kilo- indicates 1,000. A kiloliter is 1,000 liters. Because the metric system is based on powers of 10, conversions within the system often require little more than moving the decimal point to the right or left.