Answer:  That conversion depends upon the density of the substance that is being converted. For example, one cup of water has a different weight than one cup of salt. Each substance has a different density, which is the ratio of a substance's weight to its volume. We do not currently have any density tables available on our site. We hope to provide some in the future. Until then, you can either lookup the density of the substance that you are trying to convert elsewhere on the Internet, or you can perform a simple experiment by weighing a specific volume of the substance using a scale. From that ratio, you can then calculate the mass (weight) of any other specific volume of the substance; also, given a mass (weight), you can calculate the volume of the that substance.

Another Answer:  The answer depends upon the density of the substance you are measuring. The density is the substance's mass for a given volume. For example, the density of water is approximately one gram per milliliter (approximately 8.3454 pounds per gallon). The density of a substance also varies depending upon its temperature and pressure. For example, the density of gases vary greatly depending upon their temperature and pressure. Liquids and solids do not vary as greatly as gases. For specific chemical elements and substances, you can usually look up its density in a chemical handbook. The easiest way to calculate the density of common materials (such as foods) is the use a scale to weigh a given volume of the material. For example, weigh a cup of strawberries to determine their density, and from that, you can determine the weight of different volume of the item.

See also: Convert Ounces and Fluid Measurements