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Septic & DrainageCalculators

Septic

Perc Rate Calculator

Enter how far the water dropped and how long it took, and this calculator converts that observation into minutes per inch and inches per hour. It reports the arithmetic on your reading — it does not judge the result.

Your measurements

Results update as you type. Every unit dropdown converts for you.

How far the water fell during the interval you timed.

min

Percolation rate

30.00minutes per inch

Drop rate
2.000 inches per hour
Drop measured
1.000 in
Time measured
30.0 min

Planning estimate: This calculator provides general mathematical estimates and is not a septic-system design, permit determination, soil evaluation, engineering recommendation, or substitute for local requirements. Septic regulations, design flows, loading rates, setbacks, reserve areas, and installation requirements vary by jurisdiction and site conditions.

Show the calculationOpen
  1. Minutes per inch30.0 min ÷ 1.000 in=30.00 min/in
  2. Inches per hour60 ÷ 30.00=2.000 in/hr

Reading your result

Minutes per inch is the conventional way to express the rate: a larger number means water leaves the hole more slowly. Inches per hour is the same observation inverted, which some references prefer.

This is arithmetic on one timed interval. It is not a percolation test, and the calculator deliberately produces no pass or fail — whether a rate is usable depends on the system type, the design method and rules that vary by jurisdiction.

Rates typically slow across successive intervals as soil saturates, which is why formal procedures presoak and take repeated readings. A single fast early reading is usually optimistic.

How this calculator works

A percolation rate describes how fast water leaves a test hole. Divide the time you timed by the distance the water fell and you get minutes per inch: a bigger number means slower drainage.

Inches per hour is the same observation expressed the other way round. Sixty minutes divided by the minutes-per-inch figure gives the distance the water would fall in an hour at that rate.

One interval is one data point. Real test procedures involve presoaking, repeated readings across multiple holes, and specific timing rules, and the numbers usually settle down as the soil saturates.

This calculator produces no pass or fail verdict. Whether a rate is acceptable, how the test must be run, and who is qualified to run it are all determined by your local authority and site conditions.

The formula

Minutes per inch
elapsed minutes ÷ inches dropped
Inches per hour
60 ÷ minutes per inch

Worked example

The water level in a test hole falls 1 inch over a timed 30-minute interval.

  1. 01Minutes per inch30 min ÷ 1 in=30 min/in
  2. 02Inches per hour60 ÷ 30=2 in/hr

The observed rate is 30 minutes per inch, equivalent to 2 inches per hour. Whether that reading is usable for your project is a question for your soil professional and local authority.

How to take the measurements

  • Fix a reference point across the top of the hole — a board or a stake — and measure down from the same point for every reading.
  • Record the exact clock time at the start and end of each interval rather than relying on an approximate duration.
  • Follow the presoak and timing procedure your local authority specifies. Readings taken on dry soil before presoaking usually run optimistically fast.
  • Take readings in more than one hole across the proposed area. Soil changes over short distances.
  • Keep the raw readings, not just the calculated rate. Anyone reviewing the test will want the times and depths.

Practical considerations

  • Season affects results. The same hole tested on saturated spring ground and in a dry summer can give very different readings.
  • A slow result is not necessarily the end of a project. Alternative system types exist for difficult soils, though they cost more and need a designer.
  • If soil suitability affects a land purchase, test before closing. Finding out afterwards is an expensive way to learn.
  • Ask your local authority what procedure they require before digging test holes. Hole dimensions, presoak time and the number of tests are usually specified.

Assumptions and limitations

  • One interval is one data point. Real procedures specify hole dimensions, presoak duration, reading intervals and the number of holes, and the calculator assumes none of that.
  • Percolation rate is one input among several. Soil texture, structure, depth to a limiting layer and seasonal water table all matter and none are captured here.
  • In most jurisdictions a test that will be relied upon for a permit must be performed or witnessed by a licensed or certified professional under a defined procedure.

Frequently asked questions

Does a faster rate mean better soil?

Not necessarily. Very slow drainage and very fast drainage are both treated as complications in septic planning, for different reasons. Interpretation belongs to your soil professional and local authority.

Can I run my own perc test?

For your own curiosity, yes. For anything that has to be permitted, most jurisdictions require the test to be performed or witnessed by a licensed or certified professional under a defined procedure. Check locally before you dig a test hole.

Why do my readings change between intervals?

Soil takes time to saturate. Early intervals often drain faster than later ones, which is why test procedures generally call for presoaking and multiple readings.

Does this calculator tell me if my soil passes?

No. It converts your reading into two standard units and stops there. Acceptance criteria vary by jurisdiction and depend on far more than a single number.

Does a faster rate mean better soil?

Not necessarily. Very slow and very fast drainage are each treated as complications in septic planning, for different reasons. Interpretation belongs with your soil professional and local authority.

Can I run my own perc test?

For your own understanding, yes. For anything that has to be permitted, most jurisdictions require a licensed or certified professional to perform or witness the test under a specified procedure.

Why do my readings change between intervals?

Soil takes time to saturate. Early intervals commonly drain faster than later ones, which is why formal procedures presoak and take repeated readings.

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