It is the International System of Units (abbreviated as SI, from the French Système international.
UK measures - a cubic foot of water weighs 1000 ounces, a gallon of water weighs 10 pounds - go from there. (See - the imperial system does have some logic!!)
Pounds measures weight and kilometers measures length. There is no direct conversion between the two.
Standardized weighs and measures or Established standards and physicans
three pre-decimal British pennies
The density is 1,477 g/cm3.
The British decimal Penny weighs 3.56 grams.
A gallon of liquid argon weighs approximately 25.5 pounds.
One liter of liquid chlorine weighs approximately 1.45 kilograms.
ml is for milliliters, and measures volume of liquid (how much room the liquid takes up). mg is for milligrams, and measures how much something weighs. So you can't really compare them (if you have 1,000 ml of pancake syrup, that's going to weigh a lot more than 1,000 ml of gasoline). The only time they are the same is if you're talking about water, since the metric system uses the properties of water as a standard. So one milliliter of water takes up one cubic centimeter and weighs one gram. In that case, your 6ml of water would weigh 6,000mg (6 grams). If it's not water, you can convert between volume and weight using the 'specific gravity' of a liquid. This is a comparison of how much it weighs compared to water. So if you look up the specific gravity of some liquid, and it's 1.3, then you can take the number of milliliters of the liquid that you have and multiply by 1.3 and that's how many grams it should weigh.
That's going to depend on what the liquid is, and how much the empty can weighs.
One gallon of liquid beeswax weighs approximately 8.5 pounds.
One gallon of liquid chrome generally weighs around 10-12 pounds.