What's a good percolation rate?

It depends on the application. For septic systems, 15-60 minutes per inch is ideal. For stormwater infiltration, rates faster than 0.5 inches per hour are generally workable. Rates below that typically require alternative drainage methods.

Can you do a perc test in winter?

Yes, but it's harder. Frozen ground or saturated soil from recent rain can give misleading results. Ideally, test during dry conditions to get the most conservative (slowest) infiltration rate.

What if my soil fails the perc test?

Then infiltration-based systems won't work. For septic systems, you'd need an alternative like an engineered leach field or mound system. For stormwater, you'd use biofiltration (treats but doesn't infiltrate) or underground detention with controlled discharge.

How many perc tests do I need?

Typically at least two test pits per infiltration area, placed where the BMPs or leach field will be installed. Large sites may require more tests to account for soil variability.

Can I use an old perc test from a neighbor's property?

No. Soil conditions can vary dramatically over short distances. Every site requires its own testing.

Home›Resources›What Is a Percolation Test?

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What Is a Percolation Test?

The field test that measures how fast water drains through your soil — critical for seepage pits and stormwater BMPs.

Can Your Soil Drink Water Fast Enough?

Some engineering questions can't be answered in a lab — you have to test them in the field. Percolation testing ("perc testing") is one of those cases. It measures how quickly water infiltrates into your native soil, and that infiltration rate determines whether you can use seepage pits, infiltration basins, or other systems that rely on water soaking into the ground.

A percolation test involves digging test pits, pre-soaking the soil, filling the pit with water, and measuring how fast the water level drops. The infiltration rate — typically reported in inches per hour — tells you whether the soil can handle the design flow or if you need alternative drainage methods.

This test is required for septic system design in rural areas, but it's increasingly required for urban stormwater projects too. Low Impact Development (LID) and Water Quality Management Plans (WQMPs) often rely on infiltration-based BMPs, and those BMPs only work if the soil can actually absorb the water. That's where perc testing comes in.

What Happens During a Perc Test

Percolation testing is a controlled field procedure with specific steps:

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Test Pit Excavation

We dig test pits to the depth of the proposed infiltration surface — typically 3-5 feet deep for seepage pits or bio-retention basins.

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Pre-Soaking Period

The pit is filled with water and allowed to soak overnight. This saturates the soil and simulates realistic infiltration conditions.

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Percolation Measurement

Water is added to a consistent depth, and we measure how fast the water level drops over time. Multiple measurements are taken to verify consistency.

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Rate Calculation

The infiltration rate is calculated in inches per hour. Faster rates mean the soil drains well; slower rates mean it doesn't.

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Soil Classification

We classify the soil type (sand, loam, clay) based on visual observation and correlate it with the measured infiltration rate.

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Report & Recommendations

We document the results and recommend whether infiltration-based systems are feasible, or if alternative drainage methods are needed.

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Why Infiltration Rates Aren't Guesswork

You can't assume sandy soil will drain well, just like you can't assume clay won't. We've tested sandy soils with terrible infiltration because of buried clay layers, and we've tested clayey soils that percolate surprisingly well due to high silt content. The only way to know is to test.

Building departments and regional water boards require perc testing because they've seen projects where the designer assumed the soil would infiltrate, designed expensive infiltration BMPs, and then discovered during construction that the soil won't drain at all. Now you're stuck with a non-functional system and no easy fix.

For septic systems, this is even more critical. Septic leach fields rely entirely on the soil's ability to absorb and treat effluent. If the perc rate is too slow, the leach field won't work and sewage will back up. Health departments won't approve septic systems without documented perc testing.

When You Need a Percolation Test?

Common project types and triggers:

Septic Systems

Always required. Health departments mandate perc testing to size septic leach fields and verify the soil can handle effluent disposal.

Seepage Pits (Stormwater)

Required when using seepage pits for roof drainage or stormwater disposal in areas without public storm drains.

LID/WQMP Infiltration BMPs

Required for infiltration basins, bioretention areas, or permeable pavement systems that rely on soil infiltration.

Dry Wells

If you're installing dry wells to dispose of drainage, perc testing verifies they'll work in your soil conditions.

Agricultural Drainage

Used to evaluate whether fields can support subsurface drainage systems or irrigation infiltration.

Common Questions

What clients typically ask about a percolation test?:

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We handle a percolation test? for projects across Southern California.

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