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Soak Pit Sizing Calculator

Work out soak pit sizing instantly with clear inputs, formula shown and shareable results.

About Soak Pit Sizing Calculator

A soak pit disposes of effluent through its base and side walls, so the required infiltration area is daily volume times a safety factor divided by the soil percolation rate. Percolation ranges from over 50 L/m2/day in sand to under 10 in clay, where a soak pit may not be viable at all.

Formulas

Infiltration area

Area = daily effluent x safety factor / percolation rate

Pit depth

Depth = (area - base area) / circumference

Worked example

With the default values below, Soak Pit Sizing Calculator returns Infiltration Area Required of 37.500. Change any input above and every figure here is recalculated in your browser.

Inputs used

Daily Effluent Volumeminimum 0 L/day
750 L/day
Soil Percolation Rateminimum 0 L/m2/day
30 L/m2/day
Pit Diameterminimum 0 m
1.5 m
Safety Factorminimum 1
1.5

Results

Infiltration Area Required
37.500
Effective Pit Depth
7.583
Pit Volume
13.400
Base Infiltration Area
1.7671

Step by step

  1. 1Values used
  2. 2Infiltration area
  3. 3Pit depth
  4. 4Infiltration Area Required= 37.500
  5. 5Effective Pit Depth= 7.583
  6. 6Pit Volume= 13.400
  7. 7Base Infiltration Area= 1.7671

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Soakaway design must comply with local rules on groundwater separation, setback distances from wells and percolation testing. Poorly sited pits contaminate drinking water sources.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Soakaway design must comply with local rules on groundwater separation, setback distances from wells and percolation testing. Poorly sited pits contaminate drinking water sources.

Frequently Asked Questions

How is percolation rate determined?

By a percolation test: dig a test hole, presoak it, then time the fall of water level over a measured drop. Standards specify the procedure and the conversion to a design loading rate.

What minimum separation to groundwater is needed?

Typically 1-2 metres of unsaturated soil below the pit base, and prescribed minimum distances from wells, boundaries and buildings. These are regulatory requirements, not guidelines.

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