Perc Test vs Soil Evaluation: What's the Difference
Short answer: a perc test (percolation test) times how fast water drops in a pre-soaked hole, in minutes per inch. A soil evaluation (soil morphology evaluation) has a trained evaluator dig a pit or boring and describe each soil layer: its texture, structure, consistence and color, and the depth of any seasonal water table, rock or dense layer. Both are used to choose a loading rate and the system type. Many states now rely mainly on the soil evaluation and use the perc test, if at all, to confirm it. Your state decides which one is required.
This guide explains how each test works, what it can and can't tell you, and what to expect on your lot.
What a perc test measures
A perc test answers one question: how quickly does water move into this soil?
A typical procedure, in general terms:
- Dig or bore holes, often several, at the depth of the proposed drain field.
- Roughen the sides, add a little gravel to the bottom, and fill with water to presoak, often for hours or overnight, so the soil is near saturated as it will be in use.
- Refill to a set level and measure the drop in water level over timed intervals.
- Repeat until the readings stabilize. The rate is expressed in minutes per inch (MPI).
The rules fix every detail: hole diameter, depth, presoak time, interval, how many holes and how the results are averaged. They differ a lot between states and counties, so the procedure your health department publishes is the one to follow.
The result is looked up in a table that gives a loading rate. A soil that drops an inch in a few minutes takes water quickly; one that takes an hour or more takes it slowly. Very fast results can also be a problem, because effluent that moves too fast isn't treated.
Limits of the perc test
A perc test measures water moving into a disturbed hole on one day. It doesn't directly show:
- a seasonal high water table that is absent on the day of the test,
- the layers below and above the test depth, or a dense layer a foot down,
- variation across the site, beyond the holes tested.
Results also depend on weather, how the hole was dug (smeared sides slow it down) and how well it was presoaked. The EPA's Mound Systems fact sheet puts it this way: "Percolation tests are used in some jurisdictions to estimate the soil permeability because they are empirically related to the loading rate," but loading rates should be based on the soil texture, structure and consistence, "using the percolation test only to confirm morphological interpretations."
What a soil evaluation looks at
A soil evaluation reads the soil itself, layer by layer, usually in a backhoe pit or with a hand auger. The evaluator records each horizon (layer):
| Property | What it tells the designer |
|---|---|
| Depth of each horizon | Where the usable soil starts and stops |
| Texture (sand, silt, clay) | How fast water can move; often judged by feel in the field |
| Structure (granular, blocky, platy, massive) | Whether there are paths for water; platy and massive soils pass water poorly |
| Consistence (loose, friable, firm) | How dense the soil is |
| Color (Munsell notation) | Gray colors and mottles (redoximorphic features) show where the soil is saturated part of the year |
| Coarse fragments and roots | Gravel and rock content; how deep roots and water go |
From those, the evaluator finds the limiting condition: the shallowest depth that stops treatment, such as:
- a seasonal high water table (perched or apparent), shown by gray depletions and mottles even in a dry season,
- bedrock, hard or weathered,
- a restrictive layer, such as a fragipan or dense till,
- excessively permeable material, such as coarse sand and gravel.
The system must keep your state's required separation above the most limiting of these. The EPA's onsite wastewater systems manual covers site and soil evaluation in detail for designers who want the background.
Perc test vs soil evaluation, side by side
| Perc test | Soil evaluation | |
|---|---|---|
| What it measures | Rate water drops in a hole | Soil layers, texture, structure, color |
| Result | Minutes per inch | Horizon descriptions and the depth of the limiting condition |
| Shows a seasonal water table | Only if water is present that day | Yes, from soil color (redoximorphic features) |
| Time on site | Several hours, often over two days with presoak | Usually one visit per pit |
| Weather sensitive | Yes | Less, though wet pits are hard to read |
| Who does it | Depends on the state | Usually a soil scientist or licensed evaluator |
| Used for | Loading rate (in states that use it) | Loading rate, separation, system type |
Some states require only one, some require both, and some accept either. Requirements can also differ by county or by system type. Start with your state on septic rules by state, then confirm with your health department.
Who does the test
Depending on the state, site and soil evaluations are done by a health department sanitarian, a licensed soil scientist or soil evaluator, a licensed onsite designer or installer, or a professional engineer. In some places a county staff member must witness the test or the pits. Book early: evaluators are busy in the spring, and wet seasons can delay perc tests.
What the NRCS soil survey can and can't do
The USDA NRCS soil survey maps soil types across the country, and it's a good first look at what to expect: map units with slow permeability or a high water table are a warning before you buy a lot. But the survey is mapped at a scale that can miss what is under one drain field. It is a guide, not a site test: the field evaluation decides.
How to prepare for the test
- Ask what's required. Which test, how many holes, where and how deep, and who must be present.
- Mark the area. The test should be where the drain field (and any reserve area) will go, clear of setbacks.
- Keep traffic off it. Driving over the area compacts the soil and changes the result.
- Clear access for a backhoe if pits are required.
- Think about the house. Testing before the house is placed gives the designer the freedom to put each where it works best.
How landplan.io records soil tests
In landplan.io a soil test is a point on the map with the full test data:
- Soil test forms for your state: horizons with Munsell color, texture, structure and consistence; limiting conditions with their depths, or N/A when none was observed; and the loading rates from your state's table.
- Percolation data: each hole's depth, diameter and presoak, and its timed readings; landplan.io shows each reading's rate, marks a hole steady when its last two readings agree within 10%, and averages the holes.
- Soil Field Log (beta): log a test hole from the surface down at the pit, on a phone.
- The NRCS soil survey and water layers on the map, as the guide before you dig.
The design then uses the soil test nearest the system, picks the most limiting condition, and sizes the drain field from the loading rate your state's rules give. See:
For how the loading rate turns into a drain field size, see Drain Field Size.
Frequently asked questions
What is a good perc test result?
There is no single "good" number. Your state's table gives a loading rate for a range of results, and very slow or very fast results may need a different system or none at all. Ask your health department what range their table covers.
Can you fail a perc test?
A site can fail to qualify for a conventional system, but that rarely means no system is possible. A mound, an at-grade bed or pretreatment can often work where trenches can't. See Septic System Types.
How long is a perc test or soil evaluation valid?
Many jurisdictions accept a test for a set number of years, or for as long as the site isn't disturbed. Ask your health department.
Can I do my own perc test?
In most places, no: the test must be done or witnessed by a licensed or authorized person. Check your state's rules.
Why did my evaluator find a water table when the hole was dry?
Because soil color records saturation over many years. Gray colors and mottles at a depth show the soil is saturated there part of the year, even if it was dry the day of the visit.
Design it from your address
Create a free account, type the address, enter your soil test or perc readings, and landplan.io picks the loading rate and designs a system that fits your county's rules.