Bioretention: Sizing

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This article is specific to bioretention, vegetated systems that infiltrate water to the native soil.
If you are designing a planted system which does not infiltrate water, see advice on Planters: Sizing.

The vertical storage zones in a bioretention cell include: ponding, mulch, filter media, choker course, pipe diameter reservoir and the storage reservoir.

Many of the dimensions in a bioretention system can be predetermined according to the function of the component. There is greatest flexibility in the ponding depth and the depth of the storage reservoir beneath the optional underdrain pipe. The order of operations in calculating these dimensions depends on whether an underdrain is desired.

Component Recommended depth (with underdrain pipe) Recommended depth (no underdrain pipe) Typical void ratio (VR)
Ponding (dp) 300 mm See below 1
Mulch 75 ± 25 mm
  • 0.7 for wood based
  • 0.4 for stone
filter media (dm)
  • 300 mm to support turf grass (and accept only rainwater/roof runoff)
  • 600 mm to support flowering perennials and decorative grasses
  • 1000 mm to support trees
  • 0.4 for sandy mix
  • 0.35 for a more loamy mix.
Choker course 100 mm 0.4 typical
Perforated pipe Is equal to underdrain pipe diameter Not applicable 1.0
Storage reservoir (ds) See below See below 0.4

Calculate the maximum overall depth[edit]

  • Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above.
  • Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth. *Note that this component does not apply if a downstream riser is being used to control an extended saturation zone.
  • Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (ds, max, mm):


  • f' = Design infiltration rate (mm/hr), and
  • t = Drainage time (hrs). Check local regulations for drainage time requirements.

Additional step for system without underdrain[edit]

  • Step 4: Determine maximum permissible ponding depth (dp, max):


  • f' = Design infiltration rate (mm/hr), and
  • 48 = Maximum allowable drainage time for ponded water (hrs)
  • Note that in designs without underdrains, conceptually the drainage of ponded water is limited by exfiltration at the base of the practice.
  • Step 5: Sum total depth of bioretention, and compare to available depth above the seasonally high water table and/or bedrock elevation. Adjust if necessary.

Calculate the remaining dimensions[edit]

  • Step 6: Multiply the depth of each separate component by the void ratio and then sum the total to find the 1 dimensional storage (in mm).
  • Step 7: Calculate the required total storage (ST, m3):


  • RVCT is the Runoff volume control target (mm),
  • Ac is the catchment area (Ha), and
  • 10 is the units correction between m3 and mm.Ha.

Additional calculations[edit]

Calculating infiltration practice drainage in 1 dimension[edit]

This spreadsheet compares drainage in a single dimension under zero head conditions, mean head conditions and falling head conditions. It provides a more conservative measurement of the drainage time for the purposes of groundwater mounding (where a shorter drainage time causes a greater impact).

Download drainage time calculator(.xlsx)

Drainage time (3D)[1][edit]

Two practice areas of 9 m2.
P = 12 m (left), P = 20 m (right)

In some situations, it may be desirable to reduce the size of the bioretention required, by accounting for rapid drainage. Typically, this is only worth exploring over sandy soils with rapid infiltration.

Note that narrow, linear bioretention features (or bioswales) drain faster than round or blocky footprint geometries.

  • Begin the drainage time calculation by dividing the area of the practice (Ap) by the perimeter (P).
  • To estimate the time (t) to fully drain the facility:


  • VR is the void ratio of the media,
  • Ap is the area of the practice (m2),
  • f' is the design infiltration rate (mm/hr),
  • P is the perimeter of the practice (m), and
  • dT is the total depth of the practice, including the ponding zone (m).

Groundwater mounding[edit]

When you wish to model the extent of groundwater mounding beneath an infiltration facility. This tool uses Hantush's derivation (1967).
Download groundwater mounding calculator(.xlsm)
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and hosted by the USGS.

  1. Woods Ballard, B., S. Wilson, H. Udale-Clarke, S. Illman, T. Scott, R. Ahsley, and R. Kellagher. 2016. The SuDS Manual. 5th ed. CIRIA, London.