Why and How to do Perc Test for a Septic System

digging a hole for perc test
What You'll Need
Tape Measure

A perc test, or percolation test, is a soil test that is performed before installing a septic system tank. The perc test is extremely important because it measures the level of liquid absorption of the soil where the proposed septic tank will be located. It determines how quickly the material from the septic system will be absorbed into the soil.

A septic system works by allowing material from the septic tank to flow into leach lines that are placed adjacent to the tank. As the organic material slowly seeps into the surrounding soil, it is naturally absorbed into the ground and eventually processed through the soil. If the soil surrounding the location of a prospective septic system is not capable of absorbing large amounts of liquid, then a new location will be necessary or the septic system will fail and result in expensive repairs.

Additionally, the placement of septic tanks and entire septic systems are regulated by local building codes, even in rural areas. In many areas, a soil test and perc test are required before a new building permit will be issued. In some cases, these tests must be performed by an approved county engineer in order for approval by the building department. A professional inspector will typically dig several holes around the entire proposed septic system area when performing a perc test for the purposes of building permit approval.

Generally, a perc rate of less than 15 minutes per inch or greater than 105 minutes per inch is unacceptable. However, all areas have specific guidelines and requirements for acceptable perc rates.

Step 1

Dig a hole 2 feet deep where you plan to locate the septic tank. Use the measuring tape to make sure of the depth of the hole. The width of the hole is not important.

Step 2

Place water in the hole and let the water completely saturate the surrounding soil and dirt. Refill the hole again with water and use the tape measure to determine the depth of water in the hole.

Step 3

Use the timer to wait exactly 30 minutes. At the end of 30 minutes, measure the depth of the remaining water in the hole.

Step 4

Calculate the percolation rate using the following formula: 30 minutes divided by (initial water depth minus final water depth).


Initial water depth at the start of the perc test was 24 inches and 20 inches at the end of 30 minutes. The calculation for this example would be 30 divided (24 minus 20) or 30/(24-20) = 30/4 = 7.5.

In our example, the perc rate would be 7.5 minutes per inch.

Step 5

Compare the result of the perc test with local building codes to determine if the soil meets the perc test requirements for your area. Every county and municipality has different requirements and variables that are included in the building codes, such as size of the tank, size of the house (number of bedrooms and bathrooms), and the number of leach lines.