Basically for a Attenuation type soakaway the design needs a surface area and a attenuation volume.

Pipe has a internal volume. The formula for the

**volume** of cylinder is: cylinder

**volume** = π * radius² * height . For a pipe use its length instead of height: pipe

**volume** = π * radius² * length , where radius = inner diameter/2 . The

**volume** of a pipe is equal to the

**volume** of a liquid inside (if a pipe is fully filled with it).

View attachment 20997 Then there is the surface area for the soakaway which is just the area of the stone surround. Can use whatever size you need.