A percolation, or “perc,” test measures how quickly water drains from a set of test holes dug into a lot’s soil, expressed as minutes per inch of drop. That rate is used, often alongside a fuller soil evaluation, to help determine whether a conventional drainfield will work on the site and how large it needs to be.
The classic method digs several holes to the depth the drainfield trenches will sit at, presoaks them so the test measures the soil’s working absorption rate rather than its dry, initial rate, then fills them again and times how long the water takes to drop a measured distance. Faster drainage (a lower minutes-per-inch number) generally means the soil can accept more effluent per square foot, allowing a smaller field; slower drainage means a larger field is needed, and very slow or very fast rates can rule a conventional system out entirely.
A perc test measures drainage rate specifically; it does not by itself describe soil texture, structure, or depth to water table and restrictive layers the way a full soil evaluation does. Because of that, some states require a full soil evaluation instead of or in addition to a perc test, since drainage rate alone can be misleading on sites with layered or unusual soils.
Results are site-specific and can vary meaningfully across even a single lot, which is why multiple test holes are dug rather than one, and why perc results from a neighboring property are not a substitute for testing the actual site.
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How fast water drains from presoaked test holes in the soil, expressed as minutes per inch, used to help size a drainfield.
No. A perc test measures drainage rate only; a soil evaluation also examines soil texture, structure, and depth to water table and restrictive layers, and some states require it instead of or alongside a perc test.
Yes, which is why multiple test holes are dug across a site rather than relying on a single result or a neighboring property's test.
Guide last reviewed 2026-08-31.