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 a massive number of calculations 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 Although generally superseded by the Metric System, it is still in common use in the following countries; The United Kingdom, USA, Canada, Australia, New Zealand, The Republic of Ireland.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.
Fluid measurement: As a rough guide, if a fluid has a similar mass to water, a simple way to measure it is to weigh the liquid. 1 liter of water weighs 1 kg (kilogram). 500 ml (milliliters or milliliters) of water weighs 500g (grams), 56ml of water weighs 56 grams. Etc.