Percolation test
Perc rate & soil test calculator
Enter the time for water to drop one inch in your test hole. We classify the soil, give the design loading rate, and tell you whether a conventional drain field will work.
Estimate only. A permitted perc test must be performed or witnessed by a licensed evaluator; local rules govern.
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.
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 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.