How to Do a Percolation Test Yourself: Step-by-Step Protocol

Whether you are buying a plot of land to build a new home, or your existing drain field has failed and you need to assess the soil for a replacement, a soil percolation test (perc test) is the mandatory first step. It determines one critical fact: how quickly water drains through your soil.

While official tests for permitting must be conducted by a licensed engineer or soil scientist, many homeowners want to run a preliminary perc test themselves to understand their soil's capacity before spending thousands on professional services.

This guide breaks down the exact procedural protocol to perform a DIY percolation test, allowing you to gauge your soil's health and absorption rate accurately. For a deeper understanding of the science behind these numbers, review our guide on what a percolation test measures scientifically.

What You Will Need

Before you start digging, gather the right equipment. Precision matters.

• Post-hole digger or auger: For digging precise cylindrical holes.
• Tape measure or yardstick: To measure water levels accurately.
• Stopwatch or timer: For consistent interval recording.
• Bucket and water source: You will need a significant amount of water (10-20 gallons per hole).
• Clean gravel (pea gravel): To prevent scouring the bottom of the hole when pouring water.
• Notepad and pen: To record your measurements.

Step 1: Dig the Test Holes

A single hole does not provide an accurate average for an entire drain field area.

1. Dig 3 to 6 holes evenly spaced across the proposed drain field area.
2. Each hole should be 6 to 12 inches in diameter.
3. Dig to the depth of the proposed drain field trenches (typically 24 to 36 inches deep).
4. Scarify (scratch) the sides and bottom of the holes with a sharp tool to remove any smeared clay or compacted soil caused by the digging process. Smeared soil acts like a ceramic pot and blocks water absorption, ruining the test.
5. Place 2 inches of clean pea gravel at the bottom of each hole. This protects the soil bottom from scouring and sealing when you pour water in.

Step 2: Presoak the Soil

You cannot measure percolation in dry soil. The soil must be saturated to simulate the worst-case scenario: a drain field operating during the wet season.

1. Carefully fill each hole with clear water up to at least 12 inches above the gravel.
2. Keep the hole filled for at least 4 hours, ideally overnight.
3. If the water drains away completely in less than 10 minutes during the presoak, you have extremely sandy/gravelly soil, and you can proceed immediately to the test without waiting overnight.
4. For all other soils (loam, clay), allow the soil to swell and saturate overnight (at least 12 hours) before proceeding.

Step 3: Conduct the Measurement

Once the soil is pre-soaked and swollen, the actual test begins.

1. Fill the hole with exactly 6 inches of water above the gravel layer.
2. Using your tape measure, record the exact water level from a fixed reference point at the top of the hole.
3. Start your timer.
4. For sandy/fast-draining soil: Measure the water drop every 10 minutes for 1 hour. Refill to 6 inches after each measurement if it drains too quickly.
5. For normal/slower soil: Measure the water drop every 30 minutes for 4 hours.
6. The percolation rate is calculated from the final reading (the last 10-minute or 30-minute interval), as this represents the stabilized absorption rate.

Step 4: Calculate Your Perc Rate (MPI)

The result of a percolation test is expressed in Minutes Per Inch (MPI). It tells you exactly how many minutes it takes for the water level to drop one inch.

If the water dropped 0.5 inches in the final 30-minute interval: 30 minutes ÷ 0.5 inches = 60 MPI

Interpret Your Results Immediately

You don't need to do the math manually or guess what it means for your property. Take your raw measurements and input them directly into our interactive tool:

Understanding Pass/Fail Results

Once you have your MPI, you can compare it against standard health department regulations:

• 1–30 MPI (Excellent): Ideal for a conventional gravity-fed septic system.
• 30–60 MPI (Acceptable): Suitable for a conventional system, but will require a larger drain field footprint to compensate for the slower absorption.
• 60–120 MPI (Marginal): Too slow for a standard drain field. You will likely need an engineered alternative system, such as a mound system or aerobic treatment unit (ATU).
• > 120 MPI (Failing): The soil is primarily clay and essentially impermeable. A traditional septic system cannot be installed here.

If your existing system is failing and your soil perc test shows very slow absorption due to biomat buildup rather than natural clay, biological restoration is often possible before resorting to expensive mechanical replacements.