Example 1
Standard Driveway or Pad to be covered with Stone, 6" Deep

15' x 25' = 375 Square Feet
375' x 50 (6" Depth) = 187.5 Cubic Feet
187.5 / 27 = 6.94 Cubic Yards (round up to 7 yards)

Example 2
L-Shaped Driveway or Pad to be covered with Stone, 7" Deep

This would be done in two sections.
18' x 40' = 720 Square Feet
10' x 40' = 400 Square Feet
400' + 720' = 1,120 Square Feet
1,120 x .58 (7" Depth) = 649.60 Cubic Feet
649.60 / 27 = 24.06 Cubic Yards

Example 1
Length Around a Circle, 18' Round Swimming Pool

18' x 3.1416 = 56.55' (total linear feet around pool)
To add 2' of stone around this pool 3' deep:
56.55' x 2' = 113.10'
113.10' x .25 (3" depth) = 28.28 Cubic Feet
28.28 / 27 = 1.05 Cubic Yards

Example 2
Square Footage Inside a Circle, 24' Round Swimming Pool

24' x 24' = 576'
576' x .7854 = 452.39 Square Feet (total inside pool)
To add 4" of sand evenly across pool:
452.39' x .33 (4" depth) = 149.29 Cubic Feet
149.29 / 27 = 5.53 Cubic Yards
To be safe, always round up. You would order 6 Cubic Yards of sand.