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.

Divide the elapsed minutes by the inches the water fell: perc rate (min/in) = minutes ÷ inches of drop. If the level fell 1½ inches during a 30-minute reading, that is 30 ÷ 1.5 = 20 min/in. Use the final, stabilised reading of each hole, not the first one — the early readings run fast while the soil is still wetting. Codes differ on whether you then average the holes or design to the slowest one, so follow the rule your health department publishes.

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.

No. The tank is sized from bedrooms (the design flow) under your state code, and the perc rate does not change it. What the perc rate sizes is the drain field, through the soil loading rate. So a "perc test for a septic tank" is really a perc test for the tank's drain field — size the tank separately with the septic tank size calculator.

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.

Same idea, different units. The UK method (BS 6297 and Building Regulations Approved Document H) reports Vp in seconds per millimetre of drop. To convert, multiply minutes per inch by 60 and divide by 25.4: 20 min/in ≈ 47 s/mm, and the UK acceptable band of roughly 12–100 s/mm corresponds to about 5–42 min/in. This calculator is built on US codes, so use it for the conversion, not for a UK drainage-field design.

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.

It means a percolation test has been run on the lot and the soil passed for a septic system — usually a conventional one — so the land can be built on without a sewer connection. Ask for the actual report: the approved rate, the test date, and where the holes were dug, because many counties treat a perc result as valid only for a limited period and for the specific area tested. A lot that "won't perc" may still be buildable with an engineered system, at a higher cost.

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.