How it works
Multiply the area by the depth to get the volume, then convert to the units suppliers use:
Cubic feet = Length × Width × (Depth in inches ÷ 12) · Cubic yards = Cubic feet ÷ 27
Gravel weighs roughly 1.4 tons per cubic yard (it varies by type).
Example
A 10 ft × 10 ft area at 2 in deep needs about 0.6 cubic yards (~0.9 tons).