Even better, we can show you a site that will do it for you (and teach you while it does)!
Just follow the link below and click on the shape that closely approximates your form. Then enter the dimensions, and select the type of plaster that you're using (USG #1 Pottery Plaster is selected as a default) and it'll give you the weight of plaster and water using several different formulas.
https://plaster.glazy.org