What is a passing percolation rate for septic systems?

Most jurisdictions require percolation rates between 1 and 60 minutes per inch (MPI) for conventional septic systems. The ideal range is 10-30 MPI. Rates under 1 MPI (very fast draining sand) may not filter properly, while rates over 60 MPI (heavy clay) may not absorb effluent adequately.

How many percolation test holes are required?

Most codes require a minimum of 3 test holes spaced evenly across the proposed drainfield area. Some jurisdictions require more holes for larger systems or variable terrain. Holes should be at the same depth as the planned drainfield trenches, typically 24-36 inches.

Who can perform an official percolation test?

Requirements vary by state and county. Some jurisdictions require a licensed soil scientist or engineer, while others allow licensed septic installers or the county health department to conduct the test. In all cases, the test must follow local procedures and be documented on approved forms.

Can I do a perc test myself before hiring a professional?

You can do an informal percolation test to get a rough idea of soil conditions before investing in an official test. However, official tests for permit applications must be conducted by authorized personnel following specific protocols. Your informal results can help you decide if the site is worth further evaluation.

All Septic Tools

Sizing & Design

Soil Percolation Test Calculator and Step-by-Step Guide

A soil percolation test measures how fast water drains through soil at a proposed drainfield site. The test involves digging holes, pre-soaking the soil, and timing how quickly water levels drop. Ideal percolation rates fall between 10 and 60 minutes per inch. Results directly determine drainfield sizing and whether a conventional system is feasible for the property.

Step-by-Step Percolation Test Guide

Dig Test Holes

Dig 3 holes, each 8 inches in diameter and 30 inches deep, evenly spaced across the proposed drainfield area.

Prepare Hole Bottom

Scarify the bottom and sides of each hole. Add 2 inches of coarse gravel to the bottom to prevent smearing.

Pre-Soak

Fill each hole with water and maintain the water level for 12 hours (overnight recommended). This saturates the soil to simulate wet conditions.

Measure Water Drop

Refill to 6 inches above the gravel. Measure the water drop every 30 minutes for at least 4 readings. Record start and end levels each interval.

Calculate Rate

Divide the time interval (minutes) by the water drop (inches) for each reading. Average the last few readings for your perc rate in minutes per inch (MPI).

Test Parameters

Hole Depth (in)

Diameter (in)

# of Holes

Pre-Soak Duration (hours)

Water Level Readings

#Start (in)End (in)Interval (min)

Average Percolation Rate

24.5 MPI

minutes per inch

Soil Classification

Soil typeLoam / Silt Loam

SuitabilityGood for septic systems

Loading rate0.5 gal/sqft/day

Individual Readings

Reading 130 MPI

Reading 220 MPI

Reading 325 MPI

Reading 423.1 MPI

Average24.5 MPI

Percolation Rate Reference

< 1 MPIGravelToo fast

1-5 MPICoarse SandCaution

5-15 MPISandy LoamIdeal

15-30 MPILoamGood

30-45 MPIClay LoamAcceptable

45-60 MPISilty ClayMarginal

> 60 MPIHeavy ClayUnsuitable

Have a question about your results?

Ask SepticMind AI about soil percolation test guide details

Automate this in SepticMind

Get AI-powered predictions, automated reminders, route optimization, and compliance tools built for septic professionals. Start your free 14-day trial.

Frequently Asked Questions

Dig test holes 6-12 inches in diameter and 24-30 inches deep at the proposed drainfield location. Pre-soak by filling holes with water for at least 4 hours (or overnight). Refill to 6 inches above the gravel at the bottom, then measure the water drop every 30 minutes until it stabilizes. Calculate the average rate in minutes per inch.

Disclaimer: This tool provides estimates for informational purposes only and should not be used as a substitute for professional engineering or regulatory guidance. Septic system requirements vary by jurisdiction. Local health departments, soil scientists, and licensed septic professionals should be consulted for site-specific designs and permit requirements. Actual regulations in your area may differ from the general guidelines used in these calculations. SepticMind assumes no liability for decisions made based on these estimates.

Related Tools