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What is a Perc Test & When Do You Need One?

Written by JD Esajian

Whenever land is purchased, transferred, or transformed, a perc test may be necessary to check for the soil’s drainage rate. Perc tests are hugely important aspects of legal and safe property transfers or improvements. After all, every property owner wants to know whether their property can take a new or improved septic system!

Not sure how to get a perc test or why you even need one? In this guide, you’ll learn what a perc test is, how it works, when you would need to get one, average costs, and other important information you should know.

What is a Perc Test?

In brief, a perc test measures soil’s moisture absorption rate. More specifically, a perc test determines how long it takes for the soil to drain moisture added to the ground. Perc tests are often important for any property that utilizes a septic system, as perc tests can help determine the best place to install a drain, leach field, or other septic system elements.

Most perc tests are completed by trained professionals that specialize in either septic system design or land surveying.

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percolation tests

When Do You Need a Perc Test?

Imagine an instance where a property is improved, but a new septic tank is placed inside soil that can’t drain moisture properly. In such a scenario, the consequences would likely be costly (not to mention disgusting).

Perc tests can help land surveyors and construction workers determine the right or wrong locations to install a septic system. On top of that, many local jurisdictions require perc tests before new septic systems can be built or old septic systems can be replaced.

Septic tanks hold the wastewater long enough that it naturally separates into solids and liquids. The clarified liquids are then placed in a drain field or funnel through trenches several feet below ground. At this point, the liquids drain into the surrounding soil.

As you can imagine, the soil for a septic tank must be drainable enough that this system works perfectly. To perform a perc test, experts will assess the slope of the soil, check the mix of sand and gravel content, and other elements. Typically, soil with a lot of clay or rock is not suitable for installing a septic system because its drain rate is usually not very high.

Furthermore, perc tests may be crucial for rural sites that don’t benefit from municipal sewage systems. A failed test may mean that houses can’t be built in certain areas. For developers or investors, that means perc tests are doubly important as it determines what land you should make an offer on.

Who Can Perform a Perc Test & How Do They Work?

A perc test is always completed by a septic tank specialist or a land surveyor who has the equipment and insight necessary to complete the test. Although official tests must be completed by licensed professionals, they aren’t always that complex.

In fact, most perc tests are fairly simple. The specialist digs one or more holes and sees how long it takes for water to drain through the holes to measure the drainage rate of the surrounding soil. They specifically measure the “percolation rate”, which they get by examining how much water was put into the hole(s) and dividing that by how much time it took to drain.

For example, if it takes two hours for the water to drain about 2 inches, the soil has a perc rate of 1 in./h.

Some complex perc tests might involve a more advanced process:

  • The specialist may dig one or more holes into the soil of a proposed drain field

  • After digging the holes, loosened soil is scraped away and a specialist fills the holes with gravel up to about 2 inches

  • The soil is pre-soaked to mimic the saturation conditions of a typical septic system

  • 12 inches of water are placed in the hole for at least four hours during pre-soaking

  • The specialist returns the next day, then tests the holes by filling them with water (usually covering the gravel by at least 6 inches)

  • The specialist then takes measurements every 15 to 30 minutes or so to determine the percolation rate

Though perc tests can be simple, home test kits aren’t the best choice. Where holes are dug, who digs the holes, and what to look for when interpreting results are all crucial elements. That means investors, prospective homeowners, and developers alike should contact a specialist to complete perc tests where possible.

County health and environmental agencies often have specialists available who can come out to a given plot of land and perform a perc test. Alternatively, developers or investors may hire independent engineers. Note that there are specific rules about how often perc tests must be repeated on certain land parcels.

Usually, perc tests must be repeated every 2 to 5 years when they are necessary in the first place.

Perc Test Laws & Regulations

When ordering a perc test, you should also note that there are different laws or regulations depending on where the test takes place. Therefore, you should always contact your county’s health department to see if any local laws or restrictions may affect septic installation procedures or perc tests.

For example, lots of counties require a health department employee to witness the test. This requires that both the tester and the health department professional be at the property simultaneously, which could minimize the appropriate dates for the test.

Other counties may require the perc test to be performed by a “licensed soil professional” or require you to choose a tester from a list of pre-licensed individuals. On top of all that, many counties have regulations regarding how many tests can be performed in short order and how often repeated perc tests must be performed to ensure soil drainage over the long term.

As noted above, most perc tests need to be repeated every 2 to 5 years. But the exact length of time can vary heavily from county to county.

How Much Does It Cost to Get a Perc Test?

The cost of a perc test can vary heavily, usually by municipality or County. The average labor costs for a standard perc test range between $100 and $200 per hour, although some counties may have specialists that charge a flat fee of $300 for the whole test.

As you can see, perc test costs can be heavily influenced by how long it takes to complete the test. Undeveloped sites may only take between one and three hours, but sites that need septic system replacements may take longer. At the high-end of things, perc tests could run you as much as $750 or more.

Average Cost for a Perc Test by Land Size

When it comes to a percolation test, the more holes dug will allow the specialist to make more accurate predictions about the percolation rate for the surrounding soil. Therefore, most perc tests require at least two holes to be dug with about 40 feet of space between both.

The average cost of a perc test by acre or land size is broken down below:

  • 0.25 acres – $150-$250

  • 0.5 acres – $300-$500

  • 0.75 acres – $450-$750

  • 1 acre – $600-$900

  • 2 acres – $900-$1400

  • 5 acres – $1100-$2000

Generally speaking, the more land you need to survey, the more holes need to be dug, which increases the labor costs for the perc test.

Perc Test Cost by Hole Size

Perc tests costs may also be affected by the sizes of holes that must be dug. Most test holes will be between 2 and 3 feet deep, but some may be as far as 6 feet below the ground. Perc test hole width often begins at 6 inches but may reach up to 3 feet in diameter.

You can typically expect the cost of your test to increase by between $100 and $150 for each foot of depth or width you add to the hole. So, for example, a 2-foot deep hole may cost $200, but a 3-foot hole may cost $300, and so on.

Additional Costs and Considerations

Depending on the specifics of your land parcel, you may face additional costs and considerations when performing a perc test, including:

  • Permit fees, though permits are not usually required for a percolation test

  • Technician fees if a certified technician is required by your county

  • Septic planning requirements and fees

  • Location costs. Some perc tests must be performed on slopes or slippery soil, which can increase labor costs

  • Lab testing, which may be necessary to test soil for the content of specific minerals or drainage factors

  • Extra testing or packages required to complete the perc test

What Time of Year is Best to do a Perc Test?

Perc tests are best completed in the driest part of the year, usually toward the end of summer or the beginning of winter. That’s because the water table beneath the soil will be at the lowest point, and the dry soil will absorb liquid more readily. In contrast, if the soil is frozen or has recently absorbed a lot of rain/meltwater, percolation results will naturally be lower.

If you absolutely must complete a perc test in cold weather, remember to follow testing procedures for cold-weather excavations, such as digging below the frost line.

what is a perc test

What is a Percolation Test Report?

A percolation test report is the official results of the soil and its percolation rate. Perc test reports are usually required to renovate or plan a new septic system. The report is offered to the local health department for your county and is officially a part of the public record.

Perc test reports include common-sense data points like the test’s date, the technician’s name, the results, and the conditions of the perc test itself. Most perc test reports are 1 to 2 pages long and are relatively straightforward.

When you get your results, look for a perc rate of between 1 and 30 minutes per inch of soil. This is the ideal percolation rate for a septic system.

Failed Perc Test

However, some perc tests may reveal that your soil fails the test since it doesn’t drain water fast enough or even at all. A perc test may also fail if the soil drains water too quickly, especially if the land is on a steep slope.

Many properties with low water tables have a higher chance of passing a test than properties with high water tables.

Failed Perc Test Alternatives

If the soil on a parcel you own fails a perc test, you aren’t totally out of options, nor does it mean the soil is completely unusable. For starters, you can always order a second opinion and get another test or speak to a local health department representative.

They may be able to provide records of previous tests so you can get an idea of how the water table may have changed over time. If your property failed an initial perc test, it might simply be an issue of not performing the test in the right area.

You can also dig holes in another area where soil content may be different. Those areas may be more suitable for a new or renovated septic system. Or you can excavate deeper and consider placing your septic system deeper beneath the soil.

Other alternative options include:

  • Appealing the results with your local county department of health

  • Installing a treatment system with your septic tank

  • Installing a larger septic tank than you planned

  • Using alternative septic systems, like grazing fields or mount systems

  • Linking your property up to municipal water systems


In the end, perc tests are necessary for land development, especially if the property in question is unable to connect to a municipal water system. A perc test is important to ensure that any septic system for the property functions correctly. Make sure that you understand the requirements and appropriate benchmarks for a perc test before ordering one from your county’s health representatives.

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