What Is a Perc Test?
A perc test — short for percolation test — is a soil evaluation that measures how quickly water drains through the ground on your property. It is one of the most critical steps in the septic permitting process. In Arizona, counties require a perc test (also called a soil absorption test) before any septic system can be designed or permitted.
The test determines how fast water moves through the soil, expressed as a percolation rate (minutes per inch). This rate directly influences what type of septic system can be installed on your lot — whether a simple conventional gravity system or a more advanced alternative system.
Why Is a Perc Test Required in Arizona?
Arizona's Department of Environmental Quality (ADEQ) and county environmental services departments require percolation testing to protect groundwater and public health. The results confirm that your soil can safely absorb and filter wastewater from a septic system.
Without a passing perc test and site evaluation, you cannot obtain a septic permit — and without a permit, you cannot build. It's the first domino in the construction process for any property on well and septic in Maricopa County, Yavapai County, Pinal County, and throughout Arizona.
How Does a Percolation Test Work?
The perc test process involves several steps:
- Test holes are dug — Typically two or more holes are excavated to the depth of the proposed drainfield, usually 18 to 36 inches depending on county requirements.
- Holes are pre-soaked — Water is added to the holes and allowed to saturate the surrounding soil. Pre-soaking simulates the worst-case conditions your septic system will face.
- Water drop is measured — Fresh water is added to a set level, and the rate at which it drops is measured at timed intervals. This produces the percolation rate.
- Soil profile is documented — The evaluator records soil texture, color, layers, restrictive horizons (like caliche or clay), and any signs of groundwater.
What Do the Results Mean?
Perc test results are reported as a rate — for example, "5 minutes per inch" means the water level drops one inch every five minutes. Here's how the results typically break down:
- Fast percolation (under 5 min/in) — Sandy or gravelly soil. Good for conventional systems, though very fast rates can sometimes require engineered solutions to prevent inadequate treatment.
- Moderate percolation (5–60 min/in) — Ideal range for most conventional septic systems. This is the sweet spot where gravity drainfields work well.
- Slow percolation (over 60 min/in) — Clay-heavy or compacted soil. Typically requires an alternative septic system with advanced treatment technology.
How Long Does a Perc Test Take?
The on-site fieldwork for a percolation test typically takes a few hours, depending on site conditions and the number of test holes required. Report delivery is usually within 48 hours after fieldwork is completed. At Perc Test AZ, we offer rush scheduling when your build timeline is tight.
How Much Does a Perc Test Cost?
Perc test costs in Arizona vary based on the size of your lot, accessibility, number of test holes, and county jurisdiction. Contact us for a free project quote — we'll provide a clear price based on your specific property and requirements.
Perc Test vs. Septic Soil Testing: What's the Difference?
In the septic world, people often use "perc test" and "soil testing" interchangeably — but they're not the same thing.
A standard perc test (USIR percolation test) measures how quickly water drains through soil at the depth of the proposed drainfield, typically 18 to 36 inches deep. It produces the percolation rate that determines what type of system your property can support.
Septic soil testing (also called a site and soil evaluation or Phase 1 evaluation) goes much deeper. Up to 3 test holes are dug down to a depth of 12 feet or until refusal. This deeper evaluation documents the full soil profile — texture, color, layers, restrictive horizons (like caliche or clay), and any signs of groundwater. The shallower we are able to dig due to soil conditions, the more limited we are in the system design.
Arizona counties require both the percolation data and the deeper soil evaluation before a septic system can be designed. In practice, both are typically performed together as part of the same site visit. Learn more about our perc testing and soil evaluation services.
What Happens After the Perc Test?
Once your soil evaluation is complete, the results feed directly into the septic design process. Your designer uses the percolation rate, soil profile, and lot layout to determine the right system type, size the drainfield, and prepare permit-ready plans for county submission.
At Perc Test AZ, we handle both testing and design — so your data flows seamlessly from fieldwork to finished plans without delays or miscommunication between separate firms.