I need some sand/gravel dumped on a specific area of my yard in order to build that spot up for placing an outdoor shed on it. The area I'm wanting to fill will be a 20 ft. x 20 ft., and a depth of 2 ft. How many 'yards' of sand/gravel does that equal? What is the formula used to work that problem (for future referrence). Thank you very much for any help with this!!!

Ron