How To Do A Percolation Test (Soakaway Size Calculator)

21 Aug 2021


The purpose of a percolation test is to see how long a length of field drain (soakaway) you need for your Sewage Treatment Plant. One might also call it a soakaway size calculator. See this document for more information. You'll need to record your results here.

 Dig at least two holes for the percolation test

Step 1: Dig

Excavate at least three holes 300mm square to a depth at least 300mm below the proposed invert level (300mm) of the infiltration pipe, spacing them along the proposed line of drainage.

If the water table is reached, please note at what depth.


Fill the holes with water to do a percolation test

Step 2: Fill

Fill with water to 300mm and allow to seep away completely. If the water drains rapidly (within 10 mins), then the hole should be filled again up to a maximum of 10 times.


Is It Suitable?

If the water continues to drain away rapidly, then the ground is not suitable.

Also if the water has not soaked away within 6 hours, the area is not suitable in this case either.

Diagram for percolation test


  • Determine the percolation rate by refilling each hole with water to a depth of at least 300mm and observing the time in seconds for the water to seep away from 75% full to 25% full (225mm to 75mm).
  • Divide this time in seconds by 150. This gives the average time in seconds required for the water to drop 1mm.
  • Repeat the test at least 3 times in each hole.
  • Take the average figure from the tests to produce the percolation value Vp (in seconds).
  • Obtain the average figure for the percolation value (Vp) by summing up all the values and dividing by the number of values used.



The time difference is the vp rate or percolation rate. Email the results to This email address is being protected from spambots. You need JavaScript enabled to view it..

We will calculate the length of drains needed. We will need the number of dwellings involved and the number of bedrooms.

Record the results on this chart:

Percolation Test Results Form


Here is an example of a completed form:

Percolation Test Example