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Before beginning the sizing calculations most of the following parameters must be known or estimated.
The exceptions are the depth (''d'') and Permeable area (''P''), as only one of these is required to find the other.
Note that some of these parameters are limited:
*The ''maximum'' total depth will be limited by construction practices i.e. not usually > 2 m.
*The ''maximum'' total depth may be limited by the [[Infiltration| conditions underground]] e.g. the groundwater or underlying geology/infrastructure.
*The minimum total depth may be limited by the need to support vegetation i.e. not < 0.6 m.
*The area P for a [[green roofs]], [[absorbent landscapes]] and [[permeable paving]] may be very similar or equal to the catchment area, so that the I/P ratio is close to 1.
*[[Infiltration trenches]], [[Infiltration chambers| chambers]] and [[bioretention cells]] have a maximum recommended I/P ratio of 20.
{|class="wikitable"
{|class="wikitable"
|+ Inputs
|+ Inputs
|-
|-
!style="background: darkcyan; color: white"|Symbol
!style="background: darkcyan; color: white"|Symbol
|''d''||m||depth of Infiltration facility or BMP
|''d''||m||depth of Infiltration facility or BMP
|-
|-
|''P''||m2<sup>2</sup>||Permeable area i.e. area of the facility or BMP
|''P''||m<sup>2</sup>||Permeable area i.e. area of the facility or BMP
|}
|}
To calculate the required depth, where the area of the facility is constrained:
<math>d=\frac{D\left (\left [ I/P \right ]i-q \right )}{n}</math>