Updated October 20, 2026
Short answer
A percolation (“perc”) test measures how fast water moves through your soil. In Texas it sits inside a larger requirement: §285.30 calls for a complete site evaluation — two soil borings, read by a licensed site evaluator or professional engineer — on every tract where a septic system will be installed. The soil class that comes out of it sets your system type, your drainfield size, and most of your cost.
What a perc test measures
In its classic form, a percolation test is exactly what it sounds like: holes are dug in the proposed disposal area, pre-soaked, filled with water, and the drop in water level is timed. The result is a rate — how many minutes the water takes to fall an inch — which translates into a loading rate, the number of gallons per square foot per day that the soil can reasonably accept. That loading rate is the divisor for your whole dispersal field. Faster soil, smaller field. Slower soil, bigger field, or a different system entirely.
Texas requires a site evaluation, not just a timed test (§285.30)
This is the part most articles get wrong, and it changes what you should be asking for. Chapter 285 requires that a complete site evaluation be performed on every tract of land where an on-site system will be installed, and that it include two soil borings in the area where the system will go. A timed percolation test is one accepted way to characterise the soil; a soil texture analysis read from those borings is the other, and around here it is the method that usually settles the design. So when a homeowner asks for a perc test, what they actually need — and what the county will accept — is the site evaluation.
Who is licensed to perform it
The evaluation has to be done by a licensed site evaluator or a professional engineer. Chapter 285 also licenses the rest of the chain: Installer I for standard systems, Installer II for aerobic, spray and drip systems, maintenance providers for aerobic units under contract, and the county's designated representative who reviews and inspects. A contractor cannot sign off on their own soil, a seller's word is not evidence, and a report without a licensed signature will not clear the permit desk. It is worth confirming who will sign before you pay anyone.
What actually happens on your lot
The evaluator walks the property, identifies where the dispersal field can realistically go given the structures, the well, the property lines and the drainage, and then drills the borings there rather than in a convenient corner. Out of each boring come the things the design needs: soil texture and class at depth, the depth at which a restrictive layer appears, the depth to groundwater, and any evidence of seasonal saturation. Those findings, with the site sketch, become part of the permit application your county reviews. It is usually a single visit; the report and the permit review are what take time.
Soil classes and what each one allows (§285.31)
Chapter 285 sorts soils into classes, and each class carries a loading rate. The table below shows the classes that matter most in this region alongside the drainfield area a three-bedroom home's 240 gallon-per-day design flow would require in each. The spread is dramatic: the same house needs roughly 630 square feet of field on sandy loam and about 2,400 square feet on clay. When a neighbour tells you their system cost half of your quote, this table is usually the reason — not a difference in contractors.
The rule that fails more lots than perc rate does (§285.33(b)(1))
A conventional absorptive drainfield may only be used in suitable soil, and there must be two feet of suitable soil between the bottom of the excavation and either a restrictive horizon or groundwater. On the coastal prairie, plenty of lots have workable surface soil but clay or a seasonal water table within a couple of feet, and that separation requirement — not the percolation rate — is what disqualifies the conventional system. It is also why so many properties around here end up on aerobic treatment with spray or drip dispersal, and why a soil boring tells you more than a surface look ever will.
What happens if the soil “fails”
Very few lots have no permittable option at all. A poor result almost always means the design moves up rather than stops: heavy clay or a high water table points to an aerobic treatment unit with spray or drip dispersal, and a tight or irregular lot points to drip specifically. What genuinely disqualifies land is different — most often the lot-size rule for platted subdivisions, or no room to fit the field and keep the required setbacks. If we find that, we will tell you plainly rather than sell you a system that cannot be permitted.
Lot size can rule a property out before the soil does (§285.4(c))
For subdivisions platted after 1988, Chapter 285 sets minimum lot sizes for on-site systems: at least half an acre where the home is on public water, and at least one acre where it is on a private well. This catches out buyers of cheap quarter-acre lots in old-platted rural subdivisions more often than any soil condition does. Check the lot size and the water source before you fall in love with the price — no soil report rescues a lot that is too small to qualify.
Our free land pre-check, and what it cannot replace
Before you pay for anything, run the address through our free land pre-check. It reads the USDA soil survey, FEMA flood mapping, and the national wetlands inventory for the parcel and returns the likely soil limitation, flood zone, wetland presence and most probable system type in under a minute. Be clear about its limits: soil survey data is mapped at landscape scale, not lot scale, so it cannot tell you the texture at your excavation depth, spot fill soil on an old homesite, or catch a perched water table this season. It is a screening tool. Chapter 285 still requires the on-site evaluation.
When it happens in a real project, and what it costs
The evaluation comes first — before design, before the permit application, and ideally before you close on land. We can run it for you, whether you're about to install or just want to know whether a lot is buildable; either way it's quoted separately from the installed system price. Our perc test cost guide covers what an evaluation runs in Texas and what drives the range.
Free tool
Will your lot need an aerobic system? Check the address.
USDA soil rating, FEMA flood zone, and mapped wetlands for any Texas address — plus the system type and flat price you're likely looking at. About ten seconds, no account.
Soil class, loading rate, and the field a 3-bedroom home would need
| Soil class | Typical texture | Loading rate (gal/ft²/day) | Drainfield, 3-bed home |
|---|---|---|---|
| Ib | Sandy loam, loamy sand | 0.38 | ~630 sq ft |
| II | Loam, sandy clay loam | 0.25 | ~960 sq ft |
| III | Clay loam, silty clay loam | 0.20 | ~1,200 sq ft |
| IV | Clay, silty clay | 0.10 | ~2,400 sq ft |
Perc test, site evaluation, and our free pre-check — what each one is
| What it produces | Who performs it | Accepted for a permit? | |
|---|---|---|---|
| Percolation test | A timed drainage rate for the test holes | Licensed site evaluator or professional engineer | Yes — as one method inside the site evaluation |
| Site evaluation (§285.30) | Soil class and depth from two borings, water table, restrictive layers, site sketch | Licensed site evaluator or professional engineer | Yes — this is the requirement |
| Free land pre-check | Likely soil limitation, flood zone, wetlands and probable system type from public map data | Automated, from USDA / FEMA / USFWS data | No — screening before you buy or book |
Frequently asked questions
What is a perc test?
A percolation test times how quickly water drains out of test holes dug in the proposed disposal area. The rate converts into a loading rate — the gallons per square foot per day the soil can accept — which is what sizes your drainfield.
Does Texas require a perc test?
Texas requires a complete site evaluation on every tract where a system will be installed, including two soil borings (§285.30). A timed percolation test is one accepted method within it; a soil texture analysis from the borings is the other and is more common here.
Who can perform a perc test or site evaluation in Texas?
A licensed site evaluator or a professional engineer. It is not something a contractor, a seller, or a homeowner can self-certify, and the county checks the signature on the report.
Can I rely on the perc test from the lot next door?
No. The evaluation is required on every tract where a system will be installed, and soils change over short distances. A neighbour's borings tell you nothing dependable about where your field would go.
What is a good perc rate?
Faster is better, but the useful number is the soil class and its loading rate. Sandy loam classes absorb roughly four times what heavy clay will take for the same house, which is why the same design needs a far smaller field on lighter soil.
What happens if my land fails the perc test?
Usually the design changes rather than the project ending — an aerobic treatment unit with spray or drip dispersal handles soils a conventional drainfield cannot. Outright disqualifiers are more often lot size or setback space than soil.
How long does a perc test take?
The field work is typically one visit. The written report and the county's permit review are the longer part of the calendar, so start the evaluation before you need the permit rather than after.
How much does a perc test cost in Texas?
The evaluation is quoted on its own, separate from the installed system price, whether or not we install your system. Our perc test cost guide covers the market range and what moves it.
Should I get a perc test before buying land?
Yes, or at minimum make the offer contingent on a passing evaluation. Start with our free land pre-check to screen the parcel in a minute, then pay for the on-site evaluation on the lots that survive that screen.
Is a perc test the same as a septic permit?
No. The site evaluation (what most people call the perc test) is one piece of the permit application. The county's authorized agent issues the permit, called an authorization to construct (§285.3), after it reviews the evaluation together with the system design. Passing the evaluation doesn't give you a permit by itself.
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