The percolation rate is calculated for each test hole by dividing the time interval used between measurements by the magnitude of the last water level drop. This calculation results in a percolation rate in terms of minutes per inch.

The percolation rate is determined by conducting a percolation (perc) test. The percolation test measures the amount of time it takes for water in a test hole to drop 1 inch.

For septic tank systems and wells, the minimum requirement varies from 15-60m depending on whether the well is up, across or downgradient, the percolation value, soil type and depth.

Soils with a percolation rate faster than five minutes per inch are acceptable if a 12-inch-thick loamy sand soil liner with a percolation rate of 15 to 20 minutes per inch is installed in the trench or bed. The trench or bed is then sized based on this soil liner percolation rate.

After placing the sand or fine gravel in the percolation test hole, the hole must be slowly filled with clear water to a minimum depth of 12 inches over the sand or gravel. This water level should be maintained for at least two hours.

Percolation test method to calculate area of drainage field for Septic tanks or sewage treatment systems. Step 1: Excavate a test hole 300mm square x 300mm deep below proposed. Step 2: Fill the test hole with water and allow drain away over night. Step 3: Refill to a depth of 300mm and note time taken in seconds to drain.