Percolation Rate Calculation

Soil Percolation Rate Calculator

Your Percolation Rate:

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Understanding Soil Percolation Rates (MPI)

A soil percolation test, commonly known as a "perc test," is a critical procedure used to determine the absorption rate of soil for a septic drain field or leach field. The result is 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.

The Importance of the Perc Test

Before installing a septic system, local health departments require a perc test to ensure the ground can effectively filter and treat wastewater. If the water drains too quickly, the effluent won't be treated properly, potentially contaminating groundwater. If it drains too slowly, the system could back up into your home or create surface puddles in your yard.

How to Interpret Your Results

Standard guidelines generally categorize percolation rates as follows:

• 1 – 5 MPI: Very fast. May require special fill soil or engineering to prevent groundwater contamination.
• 6 – 45 MPI: Ideal range for standard gravity-fed septic systems.
• 46 – 60 MPI: Moderate drainage. May require a larger leach field area.
• Over 60 MPI: Slow drainage. Often requires specialized systems like aerobic treatment units or mound systems.

Calculation Example

If you dug a hole, pre-soaked it, and measured that the water level dropped 1.5 inches over a period of 45 minutes, your calculation would be:

45 minutes / 1.5 inches = 30 MPI

In this case, 30 MPI is considered a good rate for most standard septic designs.

Step-by-Step Perc Test Method

1. Dig the Hole: Usually 6 to 12 inches in diameter and as deep as the planned drainage trench.
2. Scarify the Surface: Scratch the sides and bottom of the hole to remove smeared clay and expose natural soil pores.
3. Pre-Soak: Fill the hole with water and keep it filled for several hours (or overnight) to saturate the surrounding soil.
4. Measure: Adjust the water level to a fixed point (usually 6 inches deep) and record the time it takes for the water to drop a specific distance.

function calculatePercRate() { var drop = parseFloat(document.getElementById('waterDrop').value); var time = parseFloat(document.getElementById('timeElapsed').value); var resultContainer = document.getElementById('percResultContainer'); var rateOutput = document.getElementById('percRateOutput'); var classOutput = document.getElementById('percClassification'); if (isNaN(drop) || isNaN(time) || drop <= 0 || time <= 0) { alert("Please enter valid positive numbers for both the water drop and time elapsed."); return; } var mpi = time / drop; var roundedMpi = mpi.toFixed(2); rateOutput.innerText = roundedMpi + " MPI"; resultContainer.style.display = "block"; var classification = ""; var bgColor = ""; var textColor = "#000"; if (mpi = 5 && mpi 45 && mpi 60 && mpi <= 90) { classification = "Slow Drainage (Specialized design required)"; bgColor = "#fdf2e9"; textColor = "#a04000"; } else { classification = "Very Slow / Unsuitable for standard systems"; bgColor = "#fdedec"; textColor = "#943126"; } resultContainer.style.backgroundColor = bgColor; classOutput.style.color = textColor; classOutput.innerText = classification; }