Percolation test (perc test)

A percolation test measures how quickly water drains through the soil at a property — specifically, the rate at which water level drops in a test hole. The test is required by code before installing a new septic drain field; results determine whether a conventional septic system is feasible and how to size it.

Procedure: dig test holes (typically 2-4 holes, 24-36 inches deep) at the proposed drain field location; pre-soak the holes with water (often 24+ hours of saturation); refill the holes with water and measure the rate at which the water level drops over a defined period.

Results are typically expressed in minutes per inch (mpi). Faster percolation (under 30 mpi) supports conventional septic systems. Slower percolation (30-60 mpi) requires larger drain fields or alternative system types. Very slow percolation (over 60 mpi) often disqualifies the property for conventional septic, requiring engineered alternative systems (mound systems, sand filters, aerobic treatment units) at much higher cost.

For septic operators, perc testing is licensed work in most jurisdictions, often performed by registered professional engineers or licensed septic system designers. Some septic operators add this certification to their service offerings; others refer the work and focus on installation/maintenance after the design is completed by others.