Standard Driveway or Pad to be covered with Stone, 6″ Deep:

15′ x 25′ = 375 Square Feet
375′ x .5 (6″ Depth) = 187.5 Cubic Feet
187.5 / 27 = 6.94 Cubic Yards (round up to 7 yards)

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

24' Round Swimming Pool:

24' x 24' = 576'
576' x .7864 = 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.

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