Before beginning the sizing calculations most of the following parameters must be known or estimated.
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The exceptions are the depth (''d'') and Permeable area (''P''), as only one of these is required to find the other.
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Note that some of these parameters are limited:
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*The ''maximum'' total depth will be limited by construction practices i.e. not usually > 2 m.
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*The ''maximum'' total depth may be limited by the [[Infiltration| conditions underground]] e.g. the groundwater or underlying geology/infrastructure.
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*The minimum total depth may be limited by the need to support vegetation i.e. not < 0.6 m.
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*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.
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*[[Infiltration trenches]], [[Infiltration chambers| chambers]] and [[bioretention cells]] have a maximum recommended I/P ratio of 20.