15' diameter by 3 ft tall?
pie x 7.5^2 = 176.625 square feet surface area multiplied by a height of 3ft comes to 529.875 cubic feet. x 7.5 for gallons = 3974.0625 gallons filled to the brim.
Intex typically calculates their gallons at 70% fill, so if you do manage to get it in the ground you'd probablly be able to fill it close to the brim. giving you an extra 1000 gallons. not bad.
keep up the good work, I still need to update my pond thread tonight with some pics of my progress. I agree digging with alittle one at home is hard. (my little girl is 7 mos old)