Reading your result
What your perc rate means
The gauge converts your drop time (T, minutes per inch) into a design loading rate with 1.2 ÷ √T gal/ft²/day, then classifies the soil. Faster soil — a lower T — carries more wastewater per square foot, so it needs a smaller field; slower soil needs a bigger one, and past about 60 min/in a conventional gravity field usually won't pass. Feed the number straight into the drain field calculator to size the field.
Typical perc rate by soil type
If you haven't had a test done yet, the soil texture on your lot gives you a rough expectation to plan against. These are the defaults the calculator uses when you pick a soil type instead of entering a measured number — treat them as a starting point, not a substitute for the real test:
| Soil texture | Typical perc rate | What it means |
|---|---|---|
| Sand | ~5 min/in | Pretreatment likely |
| Sandy Loam | ~15 min/in | Standard field OK |
| Loam | ~30 min/in | Standard field OK |
| Clay Loam | ~45 min/in | Oversize the field |
| Clay | ~90 min/in | Engineered system |
Real lots vary widely inside a texture class — a clay loam with good structure can outperform a compacted sandy loam.
How much the rate changes the field size
Slow soil doesn't just fail or pass — it scales the drain field. Because area is the design flow divided by the loading rate, and the loading rate falls as 1.2 ÷ √T, the required area grows with the square root of your perc rate. Relative to a fast 10 min/in site:
| Perc rate | Loading rate | Field area vs 10 min/in |
|---|---|---|
| 5 min/in | 0.537 gal/ft²/day | 0.71× |
| 10 min/in | 0.379 gal/ft²/day | 1.00× |
| 20 min/in | 0.268 gal/ft²/day | 1.41× |
| 30 min/in | 0.219 gal/ft²/day | 1.73× |
| 45 min/in | 0.179 gal/ft²/day | 2.12× |
| 60 min/in | 0.155 gal/ft²/day | 2.45× |
So a 60 min/in lot needs roughly two and a half times the absorption area of a 10 min/in lot for the same house — which is why a slow result raises the excavation and materials bill long before it forces an alternative system. For the actual square footage and trench length, run your number through the drain field size calculator.
What the perc rate doesn't tell you
The drop time is only one of the findings that decide your system. A site can perc beautifully and still fail on depth to bedrock, seasonal high water table, slope, or setback distance from a well or property line — all of which are logged in the same soil evaluation. Several states have gone further and dropped the timed test as the deciding measurement: North Carolina assigns a Long-Term Acceptance Rate from soil morphology, and Wisconsin requires a full soil and site evaluation by a Certified Soil Tester rather than a perc test at all. Check what your own state actually requires on the septic cost and rules by state pages before you book anything.
Full guide: Reading your perc test results → — how to run a falling-head test, what each soil class means, and what to do with slow soil.
Frequently asked questions
What is a good perc rate for a septic system?
The ideal range is roughly 5–30 minutes per inch. Faster than 5 min/in drains so quickly it may not filter effluent; slower than 60 min/in is generally unsuitable for a conventional drain field and needs an alternative system.
How do you do a percolation test?
Dig test holes to the proposed trench depth, pre-soak them, then fill with water and time how long the level takes to drop one inch. Average several holes. Most jurisdictions require a licensed evaluator to perform or witness the test.
What does minutes per inch mean?
It is the time for water in a saturated test hole to fall one inch. A lower number means faster-draining soil. The figure converts to a design loading rate (gallons per square foot per day) that sizes the drain field.
What if my perc rate is too slow?
Above about 60 min/in, a conventional gravity field will not treat wastewater fast enough. You will likely need a mound, sand filter, or aerobic treatment unit — an engineered design.
How much does a perc test cost?
A percolation test typically runs about $150–$1,500, depending on your area, how many test holes the health department requires, and whether an engineer or soil scientist has to witness it. Rural sites with easy access are cheapest; multiple deep holes on a difficult lot cost the most. The test is separate from the septic design and permit fees.
How is the loading rate calculated?
We use the standard relation loadingRate = 1.2 ÷ √T, where T is the perc rate in minutes per inch. The drain field area is then the daily design flow divided by that loading rate.
How long does a perc test take, and when do you get results?
The field work usually takes a couple of hours plus the pre-soak — many jurisdictions require soaking the holes for several hours or overnight before timing. The evaluator often knows the rate on site the same day, but the written report the health department accepts typically follows in a few days to two weeks.
Is it a perc test or a perk test?
Both spellings are used, but it is short for percolation test, so "perc test" is the standard form — that is what health departments and septic codes write. A "perk test" is the same thing.
Sources & methodology
- Soil loading rate from percolation rate (1.2 ÷ √T) — EPA / Wisconsin onsite design model
- US EPA — Onsite Wastewater Treatment Systems Manual (EPA/625/R-00/008): 60 gal/person/day design flow
- Your state or county health department — the authoritative source for local code, minimums, setbacks, and permits
Estimates compiled from the sources above and standard engineering formulas — not engineering, professional, or legal advice, and may not reflect your local code. See our full methodology and disclaimer.