Safety rule: Call 811 before anyone digs, and do not enter deep test pits or trenches. Excavation, soil testing and system design for a septic permit are work for licensed or certified professionals and the local health authority.
If you plan to build on land without public sewer, the first septic question is not which tank to buy. It is whether the soil can take the wastewater. A percolation test, usually called a perc test, is one of the ways that question is answered. It measures how quickly water soaks into the ground at the spot where a drain field might go.
This guide explains what the test measures, how Nebraska Extension describes doing it, how a county health department in Maryland handles the process, and what Virginia’s code says about how much weight the result should carry. These are examples from three places, not a national standard. Procedures, who may do the test, when it may be done and what it costs all vary by state and county, so ask your local health department before you spend any money. We do not give prices because we could not find a reliable government figure.
Septic perc test at a glance
| Question | Short answer |
|---|---|
| What does it measure? | How long it takes water in a test hole to drop 1 inch, expressed as minutes per inch (mpi) |
| Why does it matter? | Slower soils need larger drain fields, and very slow or very fast soils may be unsuitable for a standard one (Nebraska Extension) |
| Is it the whole site evaluation? | No. Virginia’s code says percolation tests are only one of many criteria in judging soil suitability |
| Who does it? | Depends on the jurisdiction. In Nebraska, a certified inspector, installer, soil evaluator, registered environmental health specialist or licensed engineer. In Harford County, Maryland, the test is arranged through the Health Department |
| Does passing mean approval? | No. Harford County says a satisfactory time test is not an approval to install a system |
What a perc test actually measures
Nebraska Extension describes the percolation rate as how quickly water moves through soil. That matters because a septic tank only does part of the job. The liquid that leaves it is treated as it moves through the soil, which is why soil drives system design. See what a septic drain field is and how septic systems work for that last stage.
The test itself is simple in concept. Water is put into a hole in the soil, and the time it takes for the water level to drop 1 inch is recorded. The result is given in minutes per inch. A low number means water drains quickly. A high number means it drains slowly. Nebraska Extension says that soils with slower rates need larger drain fields to handle the same amount of wastewater, and that soils with very slow rates may be unsuitable for drain fields at all.
Why the result is only one part of the evaluation
A perc number does not settle the question by itself. Virginia’s sewage regulations say a soil evaluation for an absorption system should follow a systematic approach that considers the physiographic province, topography, available area, slope and the soil profile, including the thickness, color, permeability and texture of each soil layer. The code adds that topography, available area, seasonal water table, drinking water supplies, bodies of water, shellfish growing areas, soil depth and rate of absorption must be considered. It says a percolation test may be required before a permit is issued and that percolation tests are analyzed as only one of many criteria.
Harford County, Maryland, makes the same point in practical terms. Its Health Department says a satisfactory time test should not be taken as approval to install a system. The approving authority also looks at topography, geology, soil classification, hydrology, surface and subsurface drainage, soil descriptions, any need for seasonal testing, the history of septic systems in the area, and where the required disposal area can go.
How the test is done: the Nebraska Extension method
Nebraska Extension’s NebGuide G1472 describes a procedure based on Nebraska Department of Environmental Quality Title 124. It says local requirements may be more stringent and specifics may vary, so treat the following as an example of how a careful test is run, not as instructions to follow yourself.
| Step | What the guide describes |
|---|---|
| 1. Dig holes | At least three, preferably four, holes spaced evenly where the drain field is planned. If the soil type varies a lot, at least four, preferably six. Holes 4 to 12 inches in diameter and as deep as the proposed trench; 24 to 30 inches is called a good average. No tests on disturbed or frozen soil |
| 2. Roughen the hole | Tools can smooth or seal the sides and bottom, so scratch them and add about 2 inches of clean gravel to prevent scouring |
| 3. Presoak | Keep 12 inches of clear water in the hole for at least four hours, preferably overnight. In clay soils, at least 12 hours so the soil can swell. Soils with more than 30 percent clay may need several days of saturation when dry. Sandy soils may need no soaking |
| 4. Measure | Usually the day after saturation. Keep 6 inches of water over the gravel and measure the drop at 30-minute intervals for four hours, using the last 30-minute drop. In sandy soils where 6 inches drains in under 30 minutes, use 10-minute intervals |
| 5. Calculate | Minutes elapsed divided by inches dropped. Example in the guide: a 2-inch drop in 30 minutes is 15 minutes per inch |
| 6. Average the field | Average the final rates of all holes. If individual holes vary by more than 20 mpi, do not average; design for the slowest rate or consider another location with less variation |
The guide also says to contact the local underground-utility locating service before excavation (811 in the United States; Nebraska requires at least two business days’ notice) and to record the readings and store them with the onsite system records.
What fast and slow results mean in Nebraska
The guide’s example shows how the numbers are read. In Nebraska, if the percolation rate is faster than 5 mpi, the soil is considered unsuitable for a drain field system because wastewater would travel too quickly to be treated properly, which could contaminate groundwater, especially where the water table is shallow. If the rate is slower than 60 mpi, the soil is unsuitable for a traditional soil absorption system. Clay swells when wet, which reduces permeability further, and wastewater could pond near the surface or back up into the house.
The guide adds that in some cases a soil-based treatment system can still be installed outside this range, but it must be designed by a professional engineer and meet state approval, and alternative systems need a state construction permit. Nebraska also suggests that when the rate is slower than 60 mpi on a lot of at least three acres, a lagoon system may be considered. Those limits are Nebraska’s. Other states set different ranges, so never use these numbers to judge a site elsewhere.
If a site fails the standard range, that does not always mean no system. See types of septic systems for alternatives and septic mound systems for one design used where natural soil is limited.
How a county runs the process: Harford County, Maryland
Harford County’s Health Department explains that under its county code and Maryland regulations, soil percolation testing is required as part of the site review for a new, repaired or replaced onsite disposal system when public sewer is not available. The property owner must first obtain a sanitary construction permit from the Health Department. To start the percolation process, the owner submits:
- An application for a soil percolation test.
- A site plan drawn to scale showing property lines and easements, existing or proposed buildings and driveways, the proposed septic or reserve area, and wells and other septic systems within 200 feet.
- The required fee.
- Copies of any earlier soil testing or environmental assessments.
Staff then check the application for completeness and decide whether a test is permissible at this time. Certain soil classifications can only be tested in the wet season, which the department says is typically February 1 through April 30, depending on soil moisture. If testing cannot be done then, the application and fee are returned. Otherwise staff contact the designated person to schedule it.
The owner is responsible for the digging equipment. Harford County lists a backhoe and qualified operator able to dig to 14 feet with a 2-foot bucket, a shovel, and enough water, typically 15 to 20 gallons per lot. The owner must also contact the utility-marking service before digging. Results come by mail after the test and review, and the department says no decision is final until it is received in writing. Bad weather can mean rescheduling.
When to do it, and what to ask first
- Ask before you pay. Ask the local health department who may perform the test, whether staff or a private professional does it, and what the application needs.
- Plan for the season. Harford County restricts some soils to a wet-season window, and Nebraska says not to test frozen ground. A calendar can matter more than you expect.
- Do it before you commit. A buyer of vacant land can ask whether any earlier soil testing exists. Harford County asks applicants to submit prior results.
- Keep the records. Nebraska Extension advises storing the data with the onsite system records. See how to find septic system records.
- Do not treat a pass as a permit. It is one input to the decision, as both Virginia’s code and Harford County say.
What does a perc test cost?
We did not find a government or university source that gives a current cost, and the documents we read describe fees only in general terms. Harford County requires a fee with the application but its procedure sheet does not list the amount. Cost depends on who does the work, the number of holes, equipment such as a backhoe, access to the site and local fees. Get the fee schedule from your health department and written quotes from any private professional. For general context, see septic system cost and septic tank size.
Frequently asked questions
What is a septic perc test?
A test of how fast water soaks into soil at the proposed drain field site. The result is the number of minutes it takes water to drop 1 inch.
Can I do my own perc test?
Often not for a permit. Nebraska limits the test to certified or registered professionals, and in Harford County it is arranged through the Health Department. Ask your local authority.
What is a good perc rate?
It depends on the state. In Nebraska’s guidance, rates faster than 5 mpi or slower than 60 mpi are treated as unsuitable for a standard system without modification or a different design. Other states use other limits.
Does a passing perc test mean I can build a septic system?
No. Harford County says a satisfactory test is not an approval. The approving authority also reviews topography, soil, hydrology, drainage and the disposal area.
Why is perc testing sometimes restricted to certain months?
Some soils can only be judged under wet conditions. Harford County says certain soil classifications may only be tested in the wet season, typically February 1 through April 30.
What if the land fails the test?
Ask the health department about alternatives. In Nebraska, an engineer-designed system or another site may be options. Never assume a failed test means the lot is unbuildable without asking.
Sources and further reading
- University of Nebraska–Lincoln Extension: Residential Onsite Wastewater Treatment: Conducting a Soil Percolation Test (G1472)
- Harford County (MD) Health Department: Percolation Test Application Procedure
- Virginia Administrative Code, 12VAC5-610-450: Evaluation criteria for subsurface soil absorption systems
This article provides general U.S. homeowner information based on a few state and county sources. Soil testing procedures, limits, timing and fees vary by state, county and site. Always follow your local health department’s requirements and the advice of a licensed septic professional or soil evaluator.