How to Read Your Perc Test Results

The one number that matters most: MPI

The headline result is the percolation rate, written in minutes per inch (MPI). It’s exactly what it sounds like: how many minutes the water in the test hole took to drop one inch. A low number means fast-draining soil, a high number means slow soil. Everything else on the report is there to support or explain that figure.

If your report lists several holes, expect a range. Testers usually go by the slowest (worst-case) reading, or an average, to size the system conservatively. Don’t panic over one slow hole. What matters is how the county reads the whole set.

What counts as fast, slow, and passing

There’s no single national cutoff; each county sets its own. But this general scale tells you where you stand:

• Under ~5 MPI is very fast (sandy). It can drain too quickly and may need extra treatment.
• ~5–30 MPI is the sweet spot. Supports a conventional system with a modest drain field.
• ~30–60 MPI still passes, but expect a larger field.
• ~60–120 MPI is slow (clay). Often marginal, and an engineered or mound system may be required.
• Over ~120 MPI is very slow. Usually fails a conventional system.

We break down the full MPI-to-soil-to-LTAR relationship, with the numbers, in the perc test cost guide.

From your rate to your drain field size

Your MPI converts to a Long-Term Acceptance Rate (LTAR), measured in gallons per square foot per day, and that sizes the field. The math: your home’s design flow (commonly 150 gallons per day per bedroom) divided by the LTAR gives the required field area. Faster soil, smaller field. Slower soil, bigger and pricier field. You don’t have to run it by hand. Plug your state, soil, and bedrooms into the Perc Test Cost Calculator for the size and a full-system estimate.

What else is on the report

The MPI is the star, but these supporting details shape your design just as much:

• Soil texture / profile. Sand, loam, or clay, and how the layers stack, is what explains the rate.
• Depth to seasonal water table. A high water table shrinks the usable soil and can force a mound.
• Depth to bedrock or a restrictive layer. Shallow rock limits how deep the system can go.
• Number and depth of holes. Confirms the test met county requirements.
• Test location. It should match where the drain field will actually be built.

Pass, fail, or conditional

Reports don’t always come back as a clean yes or no. A pass means a conventional system is viable. A fail means it isn’t as-is — but a retest or alternative system may still work (see Failed a perc test? What to do). A conditional or “suitable for an alternative system” result means the site works, but only with a specific, usually costlier design. Read which one you’ve got before you budget.

Red flags in a report

• No clear MPI figure, or no county-accepted signature on the report.
• The tested location doesn’t match your planned building or field area.
• A borderline rate with no note on what system it actually supports.
• An expired result. Many are only good for two to five years.
• “Pass” with a slow rate and no mention of a larger field or alternative design.

How long is your result good for?

A perc result isn’t permanent. Depending on the county, a passing test is typically valid for two to five years, after which you may need to retest before a permit will be issued. A few things can also void an otherwise-current result: regrading or adding fill to the test area, a code change in the intervening years, or increasing the home’s size (more bedrooms means more design flow than the original test was sized for). If your report is more than a couple of years old, confirm it’s still accepted before you build a plan around it.

Sharing your results with the county and buyers

Your results rarely stay just for you. For a permit, the written report, signed by whoever your state authorizes, goes to the health department as the basis for the system design. In a land sale, many states require perc results to be disclosed to buyers, and a clean, current report is a real selling point. It removes the single biggest unknown about undeveloped land.

Keep the original signed document, not just a verbal “it passed.” If a contractor or seller can’t produce the paper the county will accept, treat the result as unconfirmed until they can.

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