A percolation test measures how fast your soil absorbs water — the single most important factor in whether, and how, a septic system can be built on your land.
What minutes-per-inch means
The result, T, is the number of minutes for water to drop one inch in a saturated test hole. Lower is faster. You pre-soak the holes, then time the drop and average several holes. Most jurisdictions require a licensed evaluator to perform or witness the test.
Soil classes
The loading rate
The perc rate converts to a soil loading rate with the standard relation:
Divide your home's daily design flow by that loading rate and you have the required absorption area. Faster soil carries more water per square foot, so it needs less field.
Too slow or too fast
Above about 60 min/in, a conventional gravity field can't treat wastewater fast enough — you'll need a mound, sand filter, or aerobic unit. Below about 1 min/in, water moves so fast it may reach groundwater before it's filtered, and the design has to compensate.
Don't perc test right after heavy rain — saturated soil reads slower than reality. Most codes specify dry-season conditions.