If you need the actual formulas so you can make your own calculations to scientific accuracy, Wikipedia has a brilliant article covering Conversion of Units that covers a massive number of formulas for every conceivable unit of measurement.
Pounds and Ounces: Unit of Mass
The Pound (abbreviated to lb) as a measurement of weight is a unit of Mass from the British Imperial System. Although generally superseded by the Metric System, it is still in common use in the following countries; The United Kingdom, USA, Canada, Australia, New Zealand, The Republic of Ireland. The British Imperial System is also used in India, Malaysia, Sri Lanka (formerly Ceylon) and the former British Empire colony: Hong Kong. All of these countries now use a mix of the Imperial System and the Metric System.
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.