If you need the actual formulas so you can do your own conversions to scientific accuracy, Wikipedia has a brilliant article covering Conversion of Units that encompasses every a huge array of calculations for every conceivable unit of measurement.
Pounds and Ounces: Unit of Mass
As a measurement of weight, the Pound (abbreviated to lb) is a unit of Mass in the Imperial system (British Imperial System). Although it has generally been superseded by the Metric System, it is still in common use in the following countries; The United Kingdom, USA, Canada, Australia, New Zealand, The Republic of Ireland.
Measuring Fluids: When measuring any fluid that has a similar mass to water, it is often easier and more accurate to weigh the liquid, than it is to measure it by volume. 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.