A perc test (percolation test) measures how long the soil takes to absorb one inch of water, in minutes per inch. That number is what turns a bedroom count into a drainfield size, and it is what tells the county whether a conventional field is legal on the lot at all. The test is a hole, a bucket of water and a watch, done to a written procedure, by someone the county will accept. Guessing from how the garden drains after rain is how people buy a lot that needs a $25,000 mound.
Who performs a perc test
Whoever your county will put a stamp on. That is almost never the homeowner. Typical licensed roles, depending on the state:
- A licensed soil evaluator or registered environmental health specialist, the most common answer in the East and Midwest.
- A licensed on-site installer or designer in states that let the trade run the test under county witness.
- A professional engineer, required on some engineered systems and in some jurisdictions for any new field.
- A licensed site evaluator, the Texas OSSF title, and the equivalent in a few other states.
- The county sanitarian, in counties that still run the test themselves rather than reviewing someone else's.
Call the environmental health office and ask two questions: who is allowed to perform a perc test on this parcel, and do they also require a soil profile pit. Many counties have moved to a full soil evaluation (horizons, redox features, depth to water or rock) with the perc holes as one input, not as the whole answer. A cheap hole-only test that the county will not accept is not a cheap test.
Perc test cost
Plan on $250 to $800 for the perc holes alone, and $400 to $2,000 for a soil evaluation that includes the profile pit, the write-up and the county fee. That range is the same "permit and perc test" line the install calculator uses ($400 to $2,000). Difficult access, winter conditions, a witnessed test that has to be rescheduled for rain, or a site that needs holes in more than one candidate area all push it up. It is still the cheapest money on the project. A failed guess costs a redesign.
How the test is run
- Holes go to the proposed trench depth, commonly 24 to 36 inches, in the actual area of the future field, not in the front lawn because it was convenient.
- Sides are scraped clean so smeared clay does not fake a slow result. A little gravel goes in the bottom.
- The soil is presoaked, often overnight, so swelling clays finish swelling before anyone times anything.
- The hole is refilled to a measured depth and the drop is timed. The result is minutes per inch, the time for the water to fall one inch.
- Readings repeat until they stabilize. One fast reading after a dry August is not a perc rate.
- Several holes, then a rule. Counties average, or they take the slowest hole, or they throw out the best and worst. Ask which before you celebrate a single fast hole.
A perc test in August on a lot that ponds in April is a test of the wrong day. Several counties require wet-season testing, or they read redox features in the profile pit as a stand-in for the seasonal high water table. If you are buying the lot, do not close before this is done. The test is also the moment you find out the reserve area is too small.
How to read the result
Minutes per inch maps to an application rate, gallons per day the soil will accept per square foot of trench bottom. Slower soil (higher minutes) means a bigger field. The common baseline this site uses, from EPA onsite guidance, is the table below. Your county's table controls, and some states refuse a conventional field well before 120 minutes per inch.
| Perc rate | Soil | gal / day / sq ft | Conventional field |
|---|---|---|---|
| Under 1 min/in | Gravel or coarse sand | 0 | No. Too fast to treat. |
| 1 to 5 min/in | Sand | 1.2 | Yes, sometimes with a rate cap |
| 6 to 15 min/in | Sandy loam | 0.8 | Yes. Smallest usual field. |
| 16 to 30 min/in | Loam | 0.6 | Yes. The common case. |
| 31 to 45 min/in | Silt loam | 0.5 | Yes. Noticeably larger. |
| 46 to 60 min/in | Clay loam | 0.45 | Yes, near the limit in many states |
| 61 to 120 min/in | Silty clay | 0.2 | Yes, very large, or alternative |
| Over 120 min/in | Clay | 0 | No. Mound or aerobic. |
A three bedroom house at 120 gallons per bedroom is 360 gallons a day. On loam at 0.6 gallons per square foot that is a 600 square foot field. On silty clay at 0.2 it is 1,800 square feet, three times the trench, or a conversation about an aerobic unit or a mound. The drain field size calculator does that arithmetic for your bedrooms and perc rate and draws the plan.
What if the lot fails?
Failing a perc test means a conventional gravity field is off the table in that spot, not that the lot is worthless. The next questions are whether another area on the parcel percs, whether a chamber or pressure-dosed field gets you over the line, and whether the site is a mound, sand filter or aerobic job. Those alternatives are priced on the system without a leach field page and typically run $10,000 to $35,000. The perc test is what tells you which page you are on.
Buying a house or a lot
On a vacant lot, make the offer contingent on a current soil evaluation. A 20-year-old perc test is a historical document. Codes and water tables move. On an existing house the original permit file is the first document to pull, and a new perc is what you pay for when the field has to be replaced. Either way, the person who performs it should be the person the county named when you called, not the seller's cousin with a post-hole digger.