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*''d'' = Stone bed depth (m)
*''d'' = Stone bed depth (m)
*''Q<sub>c</sub>'' = Depth of runoff from contributing drainage area, not including permeable paving surface (m)
*''Q<sub>c</sub>'' = Depth of runoff from contributing drainage area, not including permeable paving surface (m)
*''R'' = A<sub>c</sub>/A<sub>p</sub>; the ratio of contributing drainage area (''A<sub>c</sub>'') to permeable paving area (''A<sub>p</sub>''). Note that the contributing drainage area (''A<sub>c</sub>'') should not contain pervious areas.
*''R'' = ''A<sub>c</sub>''/''A<sub>p</sub>''; the ratio of contributing drainage area (''A<sub>c</sub>'') to permeable paving area (''A<sub>p</sub>''). Note that the contributing drainage area (''A<sub>c</sub>'') should not contain pervious areas.
*''P'' = Rainfall depth (m)
*''P'' = Rainfall depth (m)
*''f''' = Design infiltration rate (m/day)
*''f''' = Design infiltration rate (m/day)
*''f''' = Infiltration coefficient for native soils (m/hr)
*''f''' = Infiltration coefficient for native soils (m/hr)
*''V<sub>R</sub>'' = Void space ratio for aggregate used (typically 0.35 for 50 mm clear stone)
*''V<sub>R</sub>'' = Void space ratio for aggregate used (typically 0.35 for 50 mm clear stone)
*''t'' = Time to drain (design for 48 hour time to drain is recommended)}}
*''t'' = Time to drain (design for 96 hour time to drain is recommended)}}
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The value for native soil infiltration rate (f') used in the above equations should be the [[design infiltration rate]] that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation.
The value for native soil infiltration rate (f') used in the above equations should be the [[design infiltration rate]] that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation.