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