[[file:Hydraulic radius.png|thumb|Three footprint areas of 9 m<sup>2</sup>.<br>
From left to right x = 12 m, x = 14 m, and x = 16 m]]
The 3 dimensional equations make use of the hydraulic radius (''P''/''x''), where ''x'' is the perimeter (m) of the facility. <br>
Maximizing the perimeter of the facility directs designers towards longer, linear footprint shapes such as [[infiltration trenches]] and [[bioswales]].
To calculate the required depth:
:<math>d=a[e^{\left ( -bD \right )} -1]</math>
Where:
:<math>a=\frac{P}{x}-\frac{i I}{q}</math>
and
:<math>b=\frac{xq}{nP}</math>
The rearrangement to calculate the required footprint area of the facility for a given depth is not available at this time. Submissions are welcome.
To calculate the time (''t'') to empty the facility: