You can try this link:

http://www.pentairpool.com/includes/...l/poolcalc.htm

but I think that you'll get something that's a bit high (I got 23,000 based on the numbers you posted).

If you separate the top and bottom of the kidney and use oval as your basis, I get something closer to 19,500 gallons.

Rest assured, somebody with more knowledge than I will be along to refine this number for you.