A perc test (percolation test) measures how fast water drains through soil, expressed as a rate in minutes per inch (MPI). Reading the results comes down to one number: how many minutes it takes for the water level to drop one inch in a test hole. Most jurisdictions require that number to fall between 1 and 60 MPI for a standard septic system to be approved. A lower number means faster drainage; a higher number means the soil absorbs water slowly.
What the Perc Rate Number Means
The perc rate is calculated by dividing the total test time by the total inches of water drop. If you ran a 30-minute test and the water level dropped 3 inches, your perc rate is 10 minutes per inch (30 รท 3 = 10 MPI). That single number tells you, and your local health department, whether the soil can handle effluent from a septic drain field without backing up or contaminating groundwater.
A rate between roughly 1 and 60 MPI is the general passing window, though your county may use tighter limits. Los Angeles, for example, caps the acceptable range at 1 to 60 MPI for standard leach trench designs. Some jurisdictions set the slow end at 45 MPI instead of 60. Always check your local code, because the EPA does not regulate single-family septic systems. Permitting is handled entirely by state, tribal, and local health departments.
Fast, Slow, and Failing Results
A perc rate under 1 MPI means water disappears almost instantly. That sounds good, but it’s actually a problem. Soil that drains too fast, typically sandy or gravelly ground, doesn’t filter wastewater before it reaches the water table. If you fill the test hole with 12 inches of water and it vanishes in under 10 minutes, the soil is considered excessively permeable. Your county may require an engineered system or additional treatment steps rather than a conventional drain field.
On the slow end, soil that percolates at rates slower than 1 inch per 30 minutes (30 MPI or higher) is more prone to clogging over time if a standard system is installed. Rates above 60 MPI generally mean the soil can’t accept enough effluent to support a conventional septic system at all. Clay-heavy soils are the usual culprit. A failing result doesn’t necessarily mean you can’t build on the property, but it typically means you’ll need an alternative system design, which costs more.
Between those extremes, a rate in the 5 to 25 MPI range is considered good to moderate drainage and usually qualifies for a standard gravity-fed drain field with the least design complications.
How the Test Is Conducted
Understanding the procedure helps you spot problems in the results. A standard perc test involves three holes, each 4 to 12 inches in diameter and about 36 inches deep, distributed evenly across the proposed drain field area. The holes are dug with a post-hole digger or auger (not a backhoe, which smears the soil walls and distorts results). The sides and bottom are scratched to expose a natural soil surface, and loose dirt is removed. A 2-inch layer of gravel goes in the bottom to prevent the water stream from eroding the soil.
Before any measurements happen, the soil must be pre-soaked to simulate real-world conditions where wastewater is constantly flowing into the ground. For most soils, this means maintaining 12 inches of water in each hole for at least 4 hours, then letting the soil rest and swell for at least 12 hours before testing. Clay soils may need 12 or more hours of continuous soaking, and soils with more than 30 percent clay can require several days of saturation when dry to produce an accurate reading. Sandy soils are the exception: if two consecutive fills of 12 inches drain in under 10 minutes, you skip the soak and test immediately.
The actual measurement typically starts the day after pre-soaking. Water is added to about 6 inches above the gravel layer, then measured at regular intervals, usually every 5 minutes, to track how fast the level drops. The results from all three holes are recorded, and the slowest hole often determines the official rate for the site.
Reading a Perc Test Report
A perc test report from a soil evaluator or engineer will typically include several pieces of information beyond the raw MPI number. Here’s what to look for:
- Number and location of test holes: Three holes spread across the drain field area is standard. If only one hole was tested, some jurisdictions won’t accept the results.
- Pre-soak duration: The report should state how long the holes were soaked before measurement. A test conducted without adequate pre-soaking, or one done more than 30 hours after soaking began, may not reflect real conditions. The valid window is typically 15 to 30 hours from the start of the pre-soak.
- Individual hole rates: Each hole gets its own MPI reading. If one hole reads 8 MPI and another reads 45 MPI, that variation tells you the soil isn’t uniform. The system design will need to account for the slowest-draining area.
- Soil description: Good reports note the soil type encountered at different depths (sandy loam, clay, gravel). This context explains why the rate came out the way it did.
- Water table observations: If water seeped into the hole from below during digging or soaking, that indicates a high water table, which can limit drain field depth regardless of the perc rate.
Factors That Affect Accuracy
A perc test is a snapshot of soil conditions on a specific day, and several variables can skew results. Testing should not happen during a rainstorm, within 24 hours of more than half an inch of rain, or when the ground is frozen. Wet soil absorbs water more slowly than dry soil, so a test after heavy rain may produce a slower (worse) rate than the soil’s true average. Conversely, testing during a prolonged dry spell on soil that wasn’t adequately pre-soaked can produce an artificially fast result.
Seasonal groundwater levels also matter. A site that tests well in August may have a water table just a few feet below the surface in March. If you’re not testing during the wet season, a qualified professional may need to assess the seasonal high water table separately. Some counties require perc tests to be done during specific months to capture worst-case conditions.
What Happens After a Passing Result
A passing perc rate doesn’t automatically mean your septic permit is approved. The rate determines the required size of the drain field: slower drainage means a larger field to give effluent enough soil contact area. A site that percs at 5 MPI might need a much smaller drain field than one that percs at 40 MPI, which directly affects cost and how much usable land remains on the property.
Your local health department will use the perc rate alongside other factors, including lot size, setback distances from wells and property lines, soil depth to bedrock, and the number of bedrooms in the proposed home, to determine whether and how a septic system can be installed. If the rate falls outside the acceptable range, you may be offered alternatives like mound systems, aerobic treatment units, or sand filter systems, all of which add complexity and expense but can make otherwise unbuildable lots viable.