Regulatory & Homeowner
Percolation Test (Perc Test)
A percolation test (perc test) is a soil evaluation that measures how quickly water absorbs into the ground at a specific location. It is used to determine whether a site can support a conventional drainfield and, if so, how large the drainfield needs to be. The test involves digging test holes, saturating the soil, and then measuring the rate at which a standardized amount of water drains over a measured time period.
Perc test results are expressed in minutes per inch (MPI) — how many minutes it takes for the water level to drop one inch. Fast-draining sandy soils may show 1–5 MPI; slow-draining clay soils may be 30–60 MPI or more. Most Minnesota regulations require percolation rates between 3 and 60 MPI for a conventional drainfield; soils outside this range require alternative system designs. Perc tests must be conducted by licensed soil evaluators in Minnesota.
What This Means for You
If you are purchasing undeveloped land in Minnesota to build on, or if you are considering a septic system on a property that doesn't have one, a perc test is one of the first steps. It determines what type of system is feasible and whether the lot can support the size of home you're planning. Do not buy rural land for a primary residence without confirming the soil will support a system.
Related Terms
Drain Field (Leach Field) · SSTS (Subsurface Sewage Treatment System) · Septic System Permit