Use this formula to determine how much crushed stone you need for your project:
(L'xW'xH') / 27 = cubic yards of crushed stone needed
In the construction world, most materials are measured in cubic yards. Multiply the Length, in feet, by the Height, in feet, by the Width, in feet, and divide by 27. This will tell you how many cubic yards of crushed stone you need.
As an example, if your patio is 20 feet long and 10 feet wide, and you needed 6 inches of crushed stone for the base, you would use the formula like this:(20'x10'x0.5') / 27 = 3.7 cubic yards
When using this equation, make sure all of your measurements are in feet. Since we needed 6 inches of crushed stone, I used 0.5 feet for my height.
If your number comes out as a fraction, and it probably will, round up. In the example above, you would round the 3.7 to 4 cubic yards of crushed stone. It's better to have a little extra than to run short.