Percolation Rate Calculator
A percolation rate tells you how fast water moves down through soil, the number health departments ask for before approving a septic system and the number farmers and builders use to plan drainage. This calculator takes your test-hole readings, the water level before and after plus the elapsed time, and reports the rate with a hydraulic conductivity figure, a drainage class and a septic reading. Two corrections land in this version: the conductivity conversion is fixed, and five form fields that never changed any result are gone.
Quick Answer
Water falling from 12 inches to 10 inches in one hour percolates at 2.000 inches per hour, which is 50.80 mm per hour, with an approximate hydraulic conductivity of 1.41 times 10 to the power minus 3 cm per second. That conductivity figure is the corrected one: the previous version printed 1.42 times 10 to the power minus 4 for the same run, a factor of ten too small, and every conductivity figure on the old page carried the same error.
How to use the percolation rate calculator
Enter the two water levels and the elapsed time, then click Calculate. The rate is the water drop divided by the time, converted to inches per hour whatever units you entered. The tool also converts the rate to millimetres per hour, derives the approximate hydraulic conductivity, inverts the rate into minutes per inch of drop, and classifies the drainage and the septic suitability from the rate. Reset returns the worked example above.
The fields that used to sit below the time box are gone, deliberately. Hole diameter, hole depth, soil type, test method and temperature were form fields that the calculation never used: the old code did not read the first four at all, and it read temperature only to convert it and then discard it. They now live where they belong, in the how-to guidance below, as notes for running a clean test, not as inputs pretending to change the answer.
The formula this calculator actually uses
Percolation rate equals the water drop divided by the elapsed time, with both converted to inches and hours. Hydraulic conductivity converts that rate from inches per hour to centimetres per second by pure dimensional arithmetic: one inch is 2.54 cm and one hour is 3,600 seconds, so one inch per hour equals 2.54 divided by 3,600, which is 0.0007056 cm per second. The previous tool multiplied by 0.000071 instead, which is 9.94 times too small, so its conductivity output was wrong by an order of magnitude on every run. A unit conversion is arithmetic, not a method choice, and no authority supports the old factor, so it is corrected here rather than debated. The septic bands and drainage classes are the page’s own reading guides, kept unchanged from the previous version: they interpret the rate, they do not change it.
Worked example: 12 inches to 10 inches in one hour
Drop is 12 minus 10, which is 2 inches. Time is 1 hour. Rate is 2 divided by 1, which is 2.000 inches per hour. In millimetres that is 2 times 25.4, which is 50.80 mm per hour. Conductivity is 2 times 0.0007056, which is 0.0014112 cm per second, displayed as 1.41 times 10 to the power minus 3. Time per inch is 60 divided by 2, which is 30.0 minutes. At 2 inches per hour the drainage class reads moderately well drained (the well drained band starts strictly above 2 inches per hour) and the septic reading reads excellent for septic systems. Every figure is a tool output for the default inputs.
Running a clean test: the notes-only fields, explained
Hole diameter and depth still matter for running the test, just not for this division: a standard test hole is dug to the proposed drain depth, presoaked, and refilled to a marked level before timing begins, and your health department’s procedure governs the exact hole size and presoak. Soil type explains a surprising rate after the fact but does not enter the arithmetic. Test method matters for comparability between tests, not for the division itself. Temperature affects water viscosity in full laboratory conductivity work, but this field tool reports the direct rate and a dimensional conversion of it, so a temperature box that silently did nothing was worse than no box at all. For related ground math, the soil infiltration rate calculator covers infiltration from a different test setup, the irrigation system cost calculator prices watering the same ground, and the plant area calculator turns area and spacing into plant counts for what grows in it.
Reading the bands
The tool classifies every rate twice, once for septic suitability and once for drainage, from the thresholds in its own code. The septic reading is: above 4 inches per hour, too fast and may not filter properly; 1 to 4, excellent for septic systems; 0.2 to 1, good; 0.05 to 0.2, marginal and may require special design; below 0.05, poor for a conventional system. The drainage class is: above 8, excessively drained; above 2, well drained; above 0.5, moderately well drained; above 0.1, somewhat poorly drained; above 0.01, poorly drained; at or below 0.01, very poorly drained. The two readings answer different questions, so a rate can be well drained for planting yet marginal for a septic field, and the health department’s own limits always govern approval.
Frequently asked questions
What is a good percolation rate for a septic system?
In this tool’s reading guide, 1 to 4 inches per hour reads excellent for septic systems, 0.2 to 1 reads good, and below 0.05 reads poor for a conventional system. Your health department’s own limits govern approval, and they vary by jurisdiction.
Why did the hydraulic conductivity change by a factor of ten?
Because the old conversion factor was wrong. One inch per hour equals 2.54 cm divided by 3,600 seconds, which is 0.0007056 cm per second. The old tool used 0.000071, about ten times too small, so every conductivity figure it printed was an order of magnitude low.
Where did the hole diameter, soil type and temperature boxes go?
They were removed from the form because the calculation never used them. Hole size, soil type, test method and temperature still matter for running a proper test, so they are explained in the guidance above, but a box that cannot change the answer should not sit in the form.
Is the conductivity figure exact?
No, it is labelled approximate. It is a dimensional conversion of the field rate, not a laboratory measurement, and real conductivity also depends on soil structure, saturation and temperature. Use it for comparison, not as a lab result.
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References
- Unit conversion used for hydraulic conductivity: 1 inch equals 2.54 cm and 1 hour equals 3,600 seconds, giving 1 in/hr equals 0.0007056 cm/s by dimensional arithmetic. No external conductivity dataset is cited because the correction is arithmetic, not a sourced constant.