Updated 18 April 2026
A percolation (perc) test measures how fast water drains through the soil at your proposed drain field location. The result - measured in minutes per inch - determines which septic systems are legally permitted on your lot. It is required by virtually every county in the US before a septic permit will be issued. Typical cost: $500-$1,500, not usually included in the septic installer's quote.
The percolation test measures the rate at which water soaks into the soil at the proposed drain field location, expressed in minutes per inch (min/inch). A result of 20 min/inch means the water level in the test hole drops 1 inch every 20 minutes. A result of 120 min/inch means it drops 1 inch every 2 hours.
This rate directly determines how large a drain field you need and which system types are permissible. Fast-draining sandy soils (10-30 min/inch) can handle a conventional system on a relatively small footprint. Slow-draining clay soils (100+ min/inch) cannot support a conventional drain field at all, and the effluent would surface or pond before treating.
| Rate (min/inch) | Rating | Systems Permitted |
|---|---|---|
| Under 30 min/inch | Excellent | Any conventional system |
| 30-60 min/inch | Good - Passes | Conventional gravity or chamber |
| 60-90 min/inch | Marginal | Alternative system usually required |
| 90-120 min/inch | Poor - Fails | Alternative system mandatory |
| Over 120 min/inch | Denied | No conventional permit possible |
A licensed soil scientist or engineer digs 3-5 test holes at the proposed drain field location to a depth of 12-18 inches. The holes are pre-saturated - filled with water and allowed to drain repeatedly over 24 hours - to saturate the surrounding soil as it would be at seasonal peak. This pre-saturation step is critical; testing dry-season soil without saturation produces artificially fast results that do not reflect worst-case conditions.
After pre-saturation, the test begins. The holes are filled to a set level and the drop rate is measured at 30-minute intervals over 4-6 hours. The slowest result from all test holes is typically used as the design rate, providing a conservative basis for the permit. Many counties require a county health department inspector to witness the test; in those cases, you schedule the test for a specific date and the inspector attends.
Some lots have variable soil conditions - a rocky clay area near the driveway may test poorly while a sandy area near the tree line tests well. A second test at a different location costs another $500-$1,500 but can yield a passing result if soil varies across the property.
A failed perc means you need an engineered alternative: mound ($10,000-$25,000), ATU ($10,000-$20,000), drip irrigation ($8,000-$18,000), or sand filter ($6,000-$15,000). An engineer certifies the design for your specific site conditions. This is the most common path forward.
A deep hole test to 5-8 feet assesses the soil profile, water table, and restrictive layers. In some cases, a deep pit evaluation reveals that a lower soil horizon actually drains acceptably and can support an engineered system at greater depth than the standard perc test measured.
The worst-case perc test results occur in late winter and early spring when the soil is fully saturated from snowmelt and winter precipitation. Summer tests in the same location can yield dramatically faster results - sometimes 2-3x faster - because the soil is drier. Many states require testing during the wet season specifically to prevent underestimating the water table and soil saturation. Always check your county's seasonal testing requirements.
If you are buying a rural lot in summer and the seller provides a perc test result from July, request a copy and ask the county when the test was performed and whether re-testing in the wet season is required before a permit will be issued. A summer perc test that passes may fail in March.
| Professional Type | Typical Cost |
|---|---|
| Licensed soil scientist | $600-$1,200 |
| Professional engineer (PE) | $800-$1,500 |
| County-certified evaluator | $500-$900 |
| Licensed septic installer | $500-$1,000 |