Andy M. said:
250 gm is only equal to 250 ml when measuring water! One is a measure of volume and the other is a measure of weight.
A pound of anything is equal to 454 gms.
It is true that 250gm of water corresponds to 250ml of water. However, the same is true for any other substance that has a density equal to that of water. So the question boils down to comparing the density of water to that of powdered sugar. If powdered sugar has the same density as water, then 250gm of powdered sugar will measure 250ml in volume. If powdered sugar has a greater density than that of water, then 250gm of powdered sugar will measure less than 250ml in volume. Finally, if powdered sugar has a lesser density than that of water, 250gm powdered sugar will measure more than 250ml in volume.
While gm is a measure of weight and ml is a measure of volume, it is quite common and indeed practical to use either in order to describe the quantity of a substance. As far as cooking and recipes are concerned, volume units (eg. ml, cups, liquid oz) is the obvious choice for liquids. However, when it comes to powders (eg. sugar, flour etc) one can use either volume or weight units. Volume units are probably easier to measure by using standard measuring equipment such as measuring cups, a set of tablespoon, teaspoon and fraction etc. Therefore, one might wish to know how to translate volume units to weight units for a given substance. This can be done accurately only if one knows the density (bulk density for powders) of the substance in question.
In the case of powdered sugar, since Gobo wanted to know how many ml 250gm of sugar would measure, I thought of giving him a quick answer without having to resort to such lengthy explanations as I had to employ in this post. Not having any quick access to powdered sugar bulk density info, I assume that the bulk density of powdered sugar is not that much different than that of water. This is why I said that 250gm powdered sugar corresponds to 250ml. Now if anyone does know for sure the exact value of the bulk density of powdered sugar, let him/her do the calculation and come up with the exact answer. I don't think it will suffice to simply point out what is a measure of volume and what is a measure of weight without translating one into the other. After all, a certain amount of any given substance has a certain weight and occupies a certain volume. It is often useful if not necessary to know both.