Percolation Rate Calculator
Test Results:
Water Level Drop: 0 inches
Percolation Rate: 0 MPI (Minutes Per Inch)
Soil Absorption Category: N/A
What is a Percolation Rate?
A percolation rate (often called a "perk rate") measures how quickly water moves through the soil. This measurement is critical for civil engineering, landscaping, and most importantly, the design of septic system leach fields. The rate is typically expressed in Minutes Per Inch (MPI), which indicates how many minutes it takes for the water level in a test hole to drop exactly one inch.
How to Conduct a Manual Perk Test
- Dig the Hole: Dig a hole approximately 6 to 12 inches in diameter to the depth of your proposed drainage system.
- Pre-Soak: Fill the hole with water and keep it saturated for several hours (or overnight) to ensure the soil reflects real-world rainy conditions.
- Measure: Fill the hole to a specific level (the initial level). After a set amount of time (e.g., 30 or 60 minutes), measure the new water level (the final level).
- Calculate: Divide the time elapsed by the number of inches the water level dropped.
Typical Percolation Rates by Soil Type
| Soil Type | Typical Rate (MPI) | Suitability |
|---|---|---|
| Coarse Sand / Gravel | 1 – 5 MPI | Very Fast (May require liners) |
| Fine Sand | 5 – 20 MPI | Excellent |
| Sandy Loam | 20 – 45 MPI | Good |
| Silt Loam | 45 – 60 MPI | Moderate (Design careful) |
| Clay / Heavy Silts | Over 60 MPI | Poor (Often unsuitable) |
Why the Percolation Rate Matters
If the percolation rate is too fast (less than 5 MPI), the water may reach the groundwater table without being properly filtered by the soil bacteria. If the rate is too slow (greater than 60 MPI), the water will pool on the surface, causing septic failure or flooding. For most residential septic systems, a rate between 15 and 45 MPI is considered ideal.
Calculation Example
Suppose you fill a hole to the 12-inch mark. After 60 minutes, the water level has dropped to the 10-inch mark.
- Total Drop: 12″ – 10″ = 2 inches.
- Time: 60 minutes.
- Calculation: 60 minutes / 2 inches = 30 MPI.
In this example, the soil has a percolation rate of 30 minutes per inch, which is generally suitable for a standard absorption field.