A percolation test — or perc test — is one of the first steps in designing any new septic system. Here's what it measures and what the results determine.
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A percolation test measures how quickly water drains through your soil. Specifically, it measures how many minutes it takes for the water level in a test hole to drop one inch. This rate — expressed in minutes per inch (MPI) — determines whether your soil can support a conventional drain field.
In most jurisdictions, perc tests must be conducted by or witnessed by a licensed professional — licensed soil evaluator, sanitarian, or professional engineer. The county health department typically requires a permit to conduct the test.
A failed perc test doesn't necessarily mean you can't have a septic system — it means you can't have a conventional one. Alternative systems (mound, drip, aerobic treatment unit) are designed for poor-perc soils.
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Use the Free Calculator →Perc tests typically cost $150–$500 depending on your location, the number of holes tested, and whether a licensed engineer or soil evaluator is required to conduct or witness the test.
Any time you're installing a new septic system or replacing a drain field on a property without existing test results. Some jurisdictions also require updated tests when adding bedrooms.
It means a conventional drain field isn't feasible, but you still have options. Alternative systems — mound, drip, aerobic treatment units — are designed for sites with slow-percolating soils.
In most jurisdictions, no. Perc tests for septic permitting must be conducted or witnessed by a licensed professional and results submitted to the county health department.