How to Calculate Percolation Rate of Soil

Understanding how quickly water drains through your soil is critical for installing septic systems, planning drainage for agriculture, or designing a rain garden. Use the calculator below to determine your soil's percolation rate based on your "perc test" measurements.

function calculatePercRate() { // Get inputs var dropInput = document.getElementById('waterDrop'); var timeInput = document.getElementById('timeElapsed'); var drop = parseFloat(dropInput.value); var time = parseFloat(timeInput.value); // Validation if (isNaN(drop) || isNaN(time) || drop <= 0 || time <= 0) { alert("Please enter valid positive numbers for both water drop and time."); return; } // Calculation Logic // MPI = Minutes Per Inch (Standard for Septic) var mpi = time / drop; // IPH = Inches Per Hour (Standard for Irrigation/Drainage) var iph = (drop / time) * 60; // Update UI document.getElementById('resMPI').innerText = mpi.toFixed(2); document.getElementById('resIPH').innerText = iph.toFixed(2); document.getElementById('resultBox').style.display = 'block'; // Soil Classification Logic var classDiv = document.getElementById('soilClassification'); var classification = ""; var cssClass = ""; if (mpi = 10 && mpi <= 60) { classification = "Result: Moderate Drainage (Likely Loam). Ideal range for standard septic drain fields."; cssClass = "soil-loam"; } else { classification = "Result: Slow Drainage (Likely Clay/Compact Soil). May require a mound system or alternative drainage solution."; cssClass = "soil-clay"; } classDiv.innerText = classification; classDiv.className = "soil-type " + cssClass; }

How to Calculate Percolation Rate of Soil

The percolation rate (often called the "perc rate") is a measurement of how fast water moves through the soil. It is defined mathematically as the time it takes for the water level to drop one inch.

The Formula

To calculate the rate manually, you use the following formula:

Percolation Rate (MPI) = Time Elapsed (minutes) / Water Drop (inches)

Example: If you measure a water drop of 1.5 inches over a period of 30 minutes:

• Calculation: 30 / 1.5 = 20
• Result: 20 Minutes Per Inch (MPI).

Why is Percolation Rate Important?

Soil permeability dictates what you can build or grow on a piece of land:

• Septic Systems: Most health departments require a perc test before issuing a permit. If the soil drains too fast (under 5 MPI), it may not filter wastewater effectively before it hits groundwater. If it drains too slowly (over 60 MPI), the system may back up.
• Agriculture & Gardening: Knowing if your soil retains water (Clay) or drains instantly (Sand) helps determine irrigation schedules and fertilizer needs.
• Drainage Planning: Essential for designing French drains, rain gardens, or dry wells to prevent flooding.

How to Perform a DIY Perc Test

1. Dig the Hole: Dig a hole 6 to 12 inches in diameter and about 2 to 3 feet deep (or to the depth of your proposed drain field).
2. Prepare the Bottom: Scratch the sides and bottom to remove smeared soil surfaces (which act as a seal) and add 2 inches of gravel to the bottom.
3. Presoak: Fill the hole with water and keep it relatively full for at least 4 hours (ideally overnight). This saturates the soil to simulate wet season conditions.
4. Measure:
   • Adjust water level to exactly 6 inches above the gravel.
   • Wait for a set time interval (e.g., 30 minutes).
   • Measure how much the water level dropped.
5. Calculate: Input your drop measurement and time interval into the calculator above.

Interpreting Your Results

While local regulations vary, generally accepted guidelines for septic suitability are:

• 0 – 10 MPI (Fast): Often implies coarse sand or gravel. Good for drainage, but may require advanced treatment units for septic systems.
• 11 – 60 MPI (Moderate): The "Goldilocks" zone. Usually silty loam or sandy clay loam. Standard gravity-fed drain fields often work here.
• 60+ MPI (Slow): Heavy clay or compacted soil. Water is likely to pond. May require mound systems or aerobic treatment units.