Answer

The halakhic decisors disagree about this, and the Ashkenazic practice is to go by volume, while the Sephardic practice is to go by weight.
The volume measure depends on what exactly is being discussed. Each kind of item is checked differently. In principle, the volume of something rectangular can be calculated simply. If there are air spaces, then one is supposed to crumble it and see the volume of the actual material alone. Usually people estimate and leave a safety margin.
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Questioner:
Hello Rabbi
A. Rabbi Messas writes that the measure is assessed by volume, and that this was the practice of the sages of North Africa, not only the Ashkenazim
B. Could the Rabbi explain more what is meant here? Is there a mathematical calculation for this, or do you just crumble it and see whether a certain amount of material fits?
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Rabbi:
A. As stated, this is a dispute among the halakhic decisors.
B. When there are no gaps, you can calculate it or estimate it based on the dimensions (width, length, and height). This of course depends on the shape of the food (how, and whether, it can be calculated). When there are air spaces, then in principle you need to crumble it if you are going by volume. For weight, of course, that makes no difference.