Percolation test calculator

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.

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.

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.

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.

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.

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.

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.

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.