Perc Test Results Explained: What Your MPI Number Actually Means

Your perc test results came back as a single number, something like 47 MPI, and now you’re staring at it with no idea whether that’s good news or the end of your plans.

That’s what this is for. Put your perc rate in and get a plain reading of where it puts you, what it means for your drain field, roughly what you’re looking at cost-wise, and what to actually do next.

Worth knowing what rides on this number. Your perc result decides what septic system you’re allowed to install, and the septic system is what gets you a certificate of occupancy. No approved waste system, no legal occupancy, which is the part that catches people out when they’re planning an off grid cabin and treating the building as the hard bit.

Why perc test results mean nothing on their own

MPI stands for minutes per inch. It’s how long the water in the test hole takes to drop one inch. Lower is faster drainage.

The trouble is that a raw number means nothing without the thresholds, and the thresholds aren’t printed on the report. Most counties pass conventional septic systems somewhere in the 5 to 60 MPI range, but that upper limit genuinely varies, and a few counties set a lower limit too.

So 47 MPI is a comfortable pass in one county and a conversation with an engineer in the next one over.

The part that surprises people

Everybody expects slow drainage to be the problem. It usually is. Dense clay, compacted ground, anything past 60 MPI and you’re into engineered system territory.

But fast drainage can fail too.

If water rips through sand and gravel at under 1 MPI, the soil isn’t filtering the wastewater before it reaches groundwater. That’s the whole reason the test exists, so some counties set a minimum rate and require a sand filter or imported soil to slow things down.

Nobody expects to fail a perc test for draining too well, and yet.

Why bedrooms matter and square footage doesn’t

The tool asks how many bedrooms you’re planning, which catches people off guard.

Septic systems are sized by bedroom count rather than house size, because bedrooms are the proxy for how many people live there and therefore how much wastewater gets produced. A 3,000 square foot two-bedroom needs a smaller system than a 1,200 square foot four-bedroom.

Your perc rate then multiplies that. Slow soil absorbs the same daily volume over more area, so the trench footprint grows. Which matters twice over, because you need room for the field and room for a replacement field alongside it.

What this tool is and isn’t

It’s a read on where your number sits and what it usually implies. It is not a permit decision.

Perc thresholds, testing procedures, how long results stay valid, whether alternative systems are approved at all, all of that is set county by county and some of it is set by the individual inspector. Two parcels a mile apart can get different answers.

So use this to know whether you should be relaxed, cautious or worried, and then call the health department with your actual number. They’ll tell you in one phone call what no tool can.

A poor result on an existing system usually means the field is failing rather than that you need a new design. Different problem, different cost, and I’ve covered it in the drain field replacement cost guide.

If you want the full picture on what a perc test costs, how the test itself works, and what your options are if the result is bad, that’s all in the perc test guide. And if your number puts you into engineered system territory, the septic system cost guide breaks down what each type actually costs to install, plus the continuous power draw an aerobic unit puts on an off-grid system.