Perk Test

A percolation test, or perk test, is a way to evaluate the absorption rate of a particular area of soil. Perk tests are important because septic systemscannot be designed appropriately without understanding the absorption rate of the soil. Regulator agencies require that all land undergoes a perk test before a septic tank is installed. Even if a septic system is not being installed, a perk test can be beneficial in that it determines the quality of the soil in a given tract of land.


  • The first step of the perk test consists of drilling holes into the ground. A minimum of three test holes will be drilled, and the holes can vary from 6 inches to 4 feet in diameter and 3 to 20 feet in depth. The holes will then be filled with water, and the testers will time the water to see how long it takes for the holes to drain completely. This measurement allows the testers to calculate the rate of absorption of the entire area mathematically.

Soil Testing

  • During the test, the testers will also categorize the different layers of soil according to type. For example, they will label each layer as “silt,” “loam,” “clay” or “sand.” Some layers will be a blend of these soil types. It could be that each hole will reveal a different type of soil, in which case further testing may be required.

Differences in Tests

  • The tests and their results may differ depending on the purpose of the tests. If the facilities that will be built include water-using appliances–such as dishwashers, washing machines or commercial kitchens–the absorption rate must be higher to accommodate them. Vertical septic pits also require higher absorption rates than most lines that must be laid closer to the surface.

Skewed Results

  • Sometimes the results of a perk test can be skewed. For example, if you use a post hole digger or auger to dig the holes, the tool may smear the soil along the edges of the hole as it drills. This can result in a significant change in the results. Results are more likely to be skewed if the test is done by the landowner rather than by an expert. Professionals are licensed and experienced to deal with the technical details that are part of a perk test.


  • Although a perk test can be an expensive investment, it is just that–an investment. A perk test determines the value of the soil. For investors, the quality of the soil is of the utmost importance. For landowners who intend to build, knowledge of the soil is definitely necessary. In fact, legally a tract of land must undergo a perk test in order for certain devices to be installed, such as septic tanks.

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Dexter Michigan Real Estate Agent. Providing professional full time residential home sales expertise throughout Washtenaw County.

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