Why doesn Calc have a conversion from teaspoons etc. to ounces?
This question crops in many forms, in which a conversion is needed between a measure of volume and a measure of mass. Cups, teaspoons, fluid ounces, pints, litres and cubic feet are all measures of volume, whereas ounces, pounds and kilogrammes are measures of weight (or more strictly mass). Because they are different properties there is no direct conversion; it depends on the density of the substance. For example flour has a typical density of about 600 kg/cu.m, or 0.6 times that of water. From this you can do the conversion. But this density will vary with the type of flour and how it is stored and handled. In many cases a more practical solution is to weigh the container before and after filling to determine the equivalent weight.