It is a common unit used to determine a liquid's volume.The liter is a metric volume measurement that is spelt differently in American English and German. The liter is not an SI unit, but (along with units such as hours and days) is listed as one of the "units outside the SI that are accepted for use with the SI." The SI unit of volume is the cubic meter (m³).Hence, 1 liter is equal to 1000 milliliter (ml).