Perc Test (Percolation Test)

A percolation test — usually called a "perc test" — measures how quickly water drains through soil at a specific site. It's required in most counties before you can install a septic system.

The test involves digging holes to the depth of the proposed drain field (usually 2-3 feet), saturating the soil, and measuring how fast the water level drops. Results are expressed in minutes per inch (MPI). Most counties require results between 1 and 60 MPI — too fast means effluent reaches groundwater unfiltered, too slow means the soil can't absorb it.

USDA soil survey data can help you predict perc test results before buying land. Well-drained soils with Group A or B hydrologic groups typically pass. Poorly drained soils with Group D almost always fail for conventional systems.

Look up your address to see the specific soil conditions at your property, or browse the full glossary for more terms.