A percolation test, commonly referred to as a perc test, assesses the soil's water absorption rate, which is essential for planning a septic drain field or infiltration basin. The outcomes of this test are crucial for the proper design of a septic system. Essentially, the test measures how quickly a specific volume of water seeps into the subsoil of a drilled hole with a known surface area. While regulations may vary by jurisdiction regarding the calculations for line length and pit depth, the testing methodology remains consistent. Typically, sandy soils exhibit a higher absorption capacity compared to clay-rich soils or areas with a shallow water table.

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The procedure involves excavating one or more holes to a designated depth, soaking them by maintaining a high-water level, and then conducting the test by filling the holes to a predetermined level and timing the water level drop as it percolates into the surrounding soil. Various empirical formulas exist to guide the design based on the facility size, percolation test results, and other factors; however, design services are not offered at this time.