Should you want the actual formulas so you can work out your own conversions to scientific accuracy, Wikipedia has a very detailed article explaining Conversion of Units that encompasses every possible unit of measurement.
As a measurement of mass, the Pound (lb) is a unit of Mass / weight in the Imperial system. Although superseded by the Metric System, it's still commonly used in The United Kingdom, USA, Canada, Australia, New Zealand, The Republic of Ireland.
Measuring liquids: When measuring any liquid that has a mass similar to water, it can be easier and sometimes mopre accurate weigh the fluid. 1 litre of water weighs 1 kg (kilogram). 500 ml (milliliters or milliliters) of water weighs 500g (grams), 56ml of water weighs 56 grams. Etc.