For your convenience, below you will find a table listing equivalences between US weights and metric (SI) weights.
|Pound (lb)||1 Pound = 16 oz||453.5920 g|
|Ounce (oz)||1 Ounce||28.3495 g|
|Kilogram (kg)||1 Kilogram = 1,000 gram||1,000.0000 g|
|Gram (g)||1 Gram = 1,000 mg||1.0000 g|
|Milligram (mg)||0.0010 g|
Where it concerns equivalences between US volume measurements customary in cooking, and metric volumes, conversions are precise, as indicated in the table below.
|Gallon (gal)||1 Gallon = 4 quart = 128 fl oz||3,785.4080 ml|
|Quart (qt)||1 Quart = 2 pint = 32 fl oz||946.3520 ml|
|Pint (pt)||1 Pint = 4 Gills = 16 fl oz||473.1760 ml|
|Cup (C)||1 Cup = 2 Pint = 8 fl oz||236.5880 ml|
|Gill (gi)||1 Gill = 4 fl oz||118.2940 ml|
|Fluid Ounce (fl oz)||1 Fluid Ounce = 3 Tablespoons = 8 fl dr||29.5735 ml|
|Tablespoon (T)||1 Tablespoon = 3 Teaspoons||9.8578 ml|
|Fluid Dram (fl dr)||1 Fluid Dram = 60 minims||3.6967 ml|
|Teaspoon (t)||1 Teaspoon = 8 Dashes||3.2859 ml|
|Dash (d)||1 Dash = 2 Pinches||0.4107 ml|
|Pinch (p)||1 Pinch = 2 Smidgens||0.2054 ml|
|Smidgen (s)||0.1027 ml|
|minim (min)||0.0616 ml|
British, Canadian and other equivalencies
Unfortunately, there is little standardization of volume measurements across the world. In Britain, for example, a gallon is a so-called "Imperial Gallon" and is equivalent to 4.54 liters, instead of the 3.785 liters used in the US. Consequently all related values (quarts, pints, etc.) are different as well.
There is an old cooking phrase which addresses the weight of water and other similar liquids (and in some cases, solids) used in the preparation of foods: "A pint's a pound the whole world 'round." Actually, a pint of water weighs 1.04375 pounds. The difference is large enough to matter for something as precise as baking, but you can use the 1-1 ratio as a quick estimator for a conversion. Note that this approximation is only close for US measurements. As stated, the Imperial Gallon is 20% larger, and so is the Imperial Pint. That's not even close.
Relationship of volume and weight
A substance of a certain weight can take up different volume depending on how the substance is placed in the volume, and the temperature. This relationship is called specific gravity. For pure water the equivalence of 1 cup (C) of water to 236.588 ml means that at a temperature of 4C 1 cup (C) of water weighs 236.588 gram.
Relevance of "packing"
While liquids generally occupy the same volume for a given weight, no matter what (assuming it is not under some kind of pressure), this is typically not true for solids. En example might be seen with sugar. It is not hard to understand that if you stack sugar cubes into a 1 cup volume, lots of room (air) remains between the cubes. If you would use granulated sugar, you could pack more weight into that 1 cup because not only would you use up all the space taken by the cubes, but a substantial amount of sugar would fit in the space between the cubes.
Therefore the coarseness of a material has an influence on the conversion from weight to volume, but so does the "packing" methodology. This is more easily described with a substance such as flour. If you sift flour into a cup (loosely packed) you will be able to fit much less weight compared to pushing (densily packing).
If one is working with recipes that are based on weight (or baker's percentages) and wishes to convert to volume measurements, it is important to not only describe the ingredient, but also its coarseness. E.g. "Salt, table", vs. "Salt, Kosher", or "Cheese, finely grated" vs. "Cheese, medium cubed".
Relevance of temperature
Solids generally occupy the same volume for any temperature relevant in cooking and baking. Solids do expand with increasing temperature, but much higher temperatures are needed before the effect becomes relevant for our purposes. This is less true for liquids, but still the effect is small enough that for all practical intents and purposes it can be ignored. This is why the stated equivalence of volume and weight for water will be used across the normal temperature range for measuring; between 32 F (0 C) and 120 F (50 C).