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Technical Guide · Geotechnical

Percolation Test vs. Septic T-Time vs. Infiltration Test

They sound like three different tests. In fact it's one field test and two design numbers — here's how they fit together, and which one your Ontario project needs.

Reviewed by Tony Wang, P.Eng. · King EPCM · Last updated July 2026

The short version

One test, two design numbers

These terms get used interchangeably because they're related — but they aren't three separate things. Only one of them is an actual test; the other two are design parameters you calculate from its result.

A percolation test and an infiltration test are the same physical field method: they measure a soil's hydraulic conductivity (k) — how readily water moves through it. From that single measurement you derive a T-time for septic design, or an infiltration rate for stormwater design.

  • Percolation / infiltration test
    The field method — two names, one test. Produces hydraulic conductivity (k).
  • Design T-time
    The septic design number (min/cm), converted from the field data.
  • Infiltration rate
    The stormwater design number (mm/hr), converted from the field data.
Side by side

How the three relate

The key: one column is the physical field test; the other two are the design numbers you convert its data into — one for septic, one for stormwater.

 Percolation / infiltration testField methodDesign T-timeSepticInfiltration rateStormwater
What it isThe physical field test — two names for the same methodThe septic design parameter, derived from the field dataThe stormwater design parameter, derived from the field data
What it producesHydraulic conductivity, k (m/s) — the raw field data, independent of purposeDesign percolation time, T (min/cm)Infiltration rate (mm/hr)
How it's obtainedField methods: falling-head, Constant Head Permeameter, Guelph Permeameter, or Pask Permeameter testConverted from the field data (k), or from soil classification / grain-size (MMAH SB-6) for coarse-grained soilsAnalyzed and converted from the field data (k)
Used to designNothing on its own — it's raw data without contextSeptic leaching-bed type & sizeSoakaways, infiltration trenches, bioretention (LID)
1 · The test

The field test (percolation / infiltration)

"Percolation test" and "infiltration test" are two names for the same thing — a field measurement of how readily water moves through soil. The result is the soil's hydraulic conductivity, k (m/s): a purpose-neutral description of the soil, before any design assumption is applied.

It can be measured several ways, chosen to suit the soil and site — a falling-head test, a Constant Head Permeameter, a Guelph Permeameter, or a Pask Permeameter.

The key idea: the test produces raw data. What you're designing — a septic bed or a stormwater soakaway — decides which design number you convert that data into.
Field test pit for measuring soil hydraulic conductivity
Rural land development site requiring a septic system
2 · The septic design number

Design T-time (septic)

For septic design, the field data is converted into a T-time — the design percolation time, "T", in minutes per centimetre. Lower T means faster-draining soil.

Under OBC 8.2.1.2, T can be established from the field test, by classifying the soil (MMAH SB-6 / grain-size for coarse soils), or by converting a measured hydraulic conductivity. When a percolation test is used, at least three holes are run and the highest (slowest) reading governs.

Soil type (typical)Approx. T-time (min/cm)Typical hydraulic conductivity, k (m/s)
Clean gravel / medium–coarse sand~1–510⁻² – 10⁻⁴
Sand, silty / gravelly sand~5–1010⁻⁴ – 10⁻⁵
Sandy silt, silty sand~10–2010⁻⁵ – 10⁻⁶
Sandy silty clay~20–3510⁻⁶ – 10⁻⁸
Silty clay~35–5010⁻⁸ – 10⁻⁹
Clay~50+< 10⁻⁹
Design range: a conventional in-ground leaching bed generally suits a percolation time of roughly 1–50 min/cm; shallow buried trenches are permitted up to T < 125. Faster soil (lower T, higher k) drains quickly; slower soil (higher T, lower k) may need an alternative system.

T-time and hydraulic-conductivity figures above are typical ranges for orientation only. The design T-time must be taken from the applicable MMAH Supplementary Standard SB-6 soil description and confirmed against site conditions.

3 · The stormwater design number

Infiltration rate (stormwater)

For stormwater and low-impact development, the same field data is expressed as an infiltration rate (mm/hr), derived from the soil's hydraulic conductivity (k).

Ontario guidance recommends measuring it in the field with a permeameter or infiltrometer (for example, a Guelph permeameter) rather than estimating from grain size. The result sizes soakaways, infiltration trenches, chambers, and bioretention.

It falls under MECP and conservation-authority stormwater rules — a separate framework from the Building Code's septic provisions.

  • Measures
    Hydraulic conductivity (k) → infiltration rate (mm/hr)
  • Equipment
    Guelph permeameter · ring infiltrometer
  • Designs
    Soakaways, infiltration trenches, bioretention
  • Framework
    MECP / conservation-authority SWM & LID guidance
Decision guide

Which number does your project need?

Putting in a septic system

Building or replacing a private sewage system on an unserviced lot, or adding bedrooms that raise the flow.

→ Design T-time (from the field test or soil classification)

Managing stormwater on site

Designing soakaways, infiltration trenches, or LID features to meet a stormwater or conservation-authority requirement.

→ Infiltration rate (from the field test)

Buying or assessing rural land

Checking whether a lot can support a home before you commit — septic feasibility is usually the deciding factor.

→ Septic feasibility / T-time
King EPCM

How we help

We measure the soil and deliver whichever design number your project needs. Soil hydraulic conductivity and septic T-time evaluation is part of our geotechnical work, and stormwater infiltration testing is part of our hydrogeology work — so you get the right result for the actual requirement, not a re-test later.

Where a lot is tricky, our hydrogeology and civil design teams look at groundwater, drainage, and grading together. We work across the GTA and Ontario.

Not sure which you need?

Tell us the property and what you're building — we'll confirm whether it's a septic T-time, a stormwater infiltration rate, or both, and quote it clearly.

Request a Quote
Common questions

Quick answers

Is a percolation test the same as an infiltration test?

Essentially yes — two names for the same field method that measures a soil's hydraulic conductivity (k). The difference is what you convert the result into: a T-time for septic, or an infiltration rate for stormwater.

What is T-time and how is it determined in Ontario?

T-time is the septic design percolation time (min/cm). Under OBC 8.2.1.2 it can come from a field percolation test (≥3 holes, highest reading), from soil classification (MMAH SB-6 / grain-size for coarse soils), or by measuring the soil's hydraulic conductivity and converting it to a T-time.

What's the difference between a T-time and an infiltration rate?

Both come from the same field data (hydraulic conductivity). T-time (min/cm) is the septic design number; an infiltration rate (mm/hr) is the stormwater design number. Same measurement, different purpose, units, and governing framework.

What is a good T-time for a septic system?

Lower is faster soil. Roughly: sands ~1–15 min/cm, silty/loamy soils ~15–35, clays ~35–50+. A conventional bed generally suits T ≈ 1–50, and shallow buried trenches up to T < 125. Confirm against SB-6 and your municipality or health unit.

Does King EPCM perform these tests?

Yes — soil hydraulic conductivity and septic T-time evaluation (geotechnical) and stormwater infiltration testing (hydrogeology), across the GTA and Ontario.

General educational information only — not engineering advice for a specific property, and no substitute for the current Ontario Building Code, MMAH SB-6, or your local authority's requirements. Confirm thresholds and values for your site with a qualified professional.

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Not sure which test your project needs?

Tell us about the property and we'll point you to the right scope — and a fast, clear quote.

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