Normally you would do:
Width(a) height(b) and length(c)
Volume = a x b x c = ?
That's for a regular rectangular prism.
This could be considered an irregular prism?
In which case you would just go by:
Base x height = volume.
In this case... Base would be the width like above and height would be the length of the curved edge.
It's 9:30am and technical calculations class isn't clicking this early. If could be close or completely off
There's always the bucket method....4-5g at a time till it's filled and count how much you used lol