There are several ways to determine the volume of a mould.
Calculation of the weight of glass needed
Calculate the amount in the metric system of measures, as that gives much easier calculations. Cubic centimetres of volume times the specific gravity of glass (2.5) will give you the number of grams of glass required.
This works best on regular geometric forms. Rectangles and parallelograms are easy to measure the length, width and depth in centimetres. Multiply together and you obtain cubic centimetres. That times the specific gravity – 2.5 – will give the number of grams to fill the mould. The frit will of course be mounded above the levelled surface, because of the air spaces between the frit pieces.
Irregular shaped moulds
The moulds which are irregular in shape or depth are more difficult to calculate.
You can determine the volume by starting with a measured amount of water. Quickly fill the mould to the surface, so that no water is absorbed into the mould. Empty the water from the mould into the drain so it does not become soaked. The difference between the starting and finishing amount of water is the volume of glass required to fill the mould.
You can use that volume in cubic centimetres times the specific gravity (2.5) to get the number of grams of glass required.
However, it is much easier to put the frit into the water until the measure shows the same amount as before the mould was filled. Then you only need pour off the water and allow glass and mould to dry. No calculation required.