What Is a Perc Test in MS? Complete Guide for Hattiesburg-Area Properties

A percolation test — universally called a "perc test" — is the foundational soil evaluation that determines what type of septic system can be installed on a given property. In Hattiesburg and throughout Forrest and Lamar County, where clay-heavy soils create significant variation in drainage characteristics, the perc test result is often the single most consequential factor in septic system design and cost.

What a Perc Test Measures

A perc test measures the rate at which water moves through soil — specifically, how many minutes it takes for one inch of water to drop in a test hole of defined dimensions. This "minutes per inch" (MPI) result directly determines the minimum required drainfield area for a given daily flow rate, and whether a conventional system is even permissible. MS MDEQ uses perc test results along with soil profile evaluation to classify sites and specify appropriate system types. Sites with perc rates between 1 and 60 minutes per inch are generally suitable for conventional systems, with slower rates requiring larger drainfield areas. Sites slower than 60 MPI typically require alternative systems — mound, ATU, or sand filter.

Perc Test Results in Hattiesburg's Clay

In Hattiesburg's clay-dominant soils, perc test results commonly run 30 to 90+ minutes per inch. Results above 60 MPI — common in the heavier clay areas of Forrest County — disqualify properties from conventional drainfield installation under MS code. This is why mound systems are so common in the Pine Belt — they are frequently the only compliant option for sites where native soil simply cannot perc fast enough for a conventional system.

When Perc Tests Are Required

In MS, perc testing is required for all new septic installations and for major repair or replacement projects. The test must be conducted by or under the supervision of a licensed professional and the results submitted to the county health department as part of the permit application. Perc test results remain on file with the county and are available to future owners as part of the permit documentation.