The Minimal Impact Design Standards calculator requires a user to input a value for water stored in an underground infiltration practice below the outflow (overflow) pipe and the top of the underlying soil or engineered media. This is not a straightforward calculation because there are a wide variety of underground pipe shapes and because two different calculations must be made - one for storage in the pipe and another for storage in the media surrounding the pipe. If engineered media is used in the practice, the calculator also requires a calculation of the surface area of the engineered media. This calculation is fairly simple. Both these calculations can be made using a spreadsheet developed for this purpose.
To calculate the volume of water stored in the pipes, the shape of the pipe is needed. For hexagonal or rectangular pipes, the volume (Vpipe is given by
<math> V_{pipe} = (D_O * W_{max} * L_{pipe}) - (2 *