Calculate percolation rate, water volume, or time from any two values, and convert infiltration and percolation rates in in/hr, cm/hr, min/in, and min/cm.
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Percolation Rate Formula
The calculator uses different formulas depending on the selected mode.
Flow rate mode
- PR = percolation rate, such as mL/min, L/min, fl oz/min, cups/min, or gallons/min
- V = amount of water
- t = percolation time
This mode treats percolation as a volume passing through soil or media over time. Units are converted internally before the missing value is calculated.
Perc test mode
- PR_p = perc test rate, usually in min/in or min/cm
- T = elapsed time
- d = water level drop distance
A lower min/in value means faster drainage. A higher min/in value means slower drainage.
Infiltration conversion mode
- PR_p = percolation rate in min/in
- I = infiltration rate in in/hr
- 60 = minutes per hour
For centimeter units, the calculator uses 1 inch = 2.54 cm.
Percolation Rate Reference Tables
Use these tables as general guides. Local soil rules for septic systems, drainage fields, or stormwater design may use different cutoffs.
| Perc rate | Drainage description | Meaning |
|---|---|---|
| Less than 5 min/in | Very fast | Water drains quickly through the soil. |
| 5 to 15 min/in | Ideal to fast | Often good drainage for many soil applications. |
| 15 to 30 min/in | Moderate | Water drains at a middle rate. |
| 30 to 60 min/in | Slow | Water moves slowly through the soil. |
| More than 60 min/in | Very slow | Soil may hold water for a long time. |
| Hydrologic soil group | Preset infiltration rate | Equivalent perc rate |
|---|---|---|
| A | 1.63 in/hr | 36.81 min/in |
| B | 0.45 in/hr | 133.33 min/in |
| C | 0.20 in/hr | 300.00 min/in |
| D | 0.06 in/hr | 1000.00 min/in |
How to Check a Percolation Rate Result
Make sure the water level drop and time are both greater than zero. Use the same test method for each hole if you compare multiple locations.
Example: if the water drops 2 inches in 30 minutes, the perc rate is:
That result is a moderate drainage rate by the table above.
