Sanitation System (C-42) · Lesson 2 of 6

Sizing a Leach Field: Soil, Square Feet, and Trench Length

"Sizing a Leach Field: Soil, Square Feet, and Trench Length" is lesson 2 of 6 in the Sanitation System (C-42) Learning Hub. It opens with "Start With the Soil Type" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Start With the Soil Type
  2. Which Gallons Do You Use
  3. Minimums, Credits, and Multipliers
  4. Turn Square Feet Into Trench Length
  5. Protect the Field and Its Reserve

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Required leaching area" through "What if the design changes?".

What you'll be able to do

  • Explain the main idea of "Sizing a Leach Field: Soil, Square Feet, and Trench Length" in plain language you can use on the exam.
  • Apply this step with confidence: "Minimums, Credits, and Multipliers".
  • Apply this step with confidence: "Turn Square Feet Into Trench Length".
  • Apply this step with confidence: "Protect the Field and Its Reserve".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Leach Field Sizing: 4-Bedroom Home on Sandy LoamTRENCH CROSS-SECTION (not to scale)groundcover ≥ 12 inpipe24 infilterunderpipe3 ft bottom width (18-36 in allowed)Orange sides = sidewall credit (12-36 in below line)Per ft of trench: 3 sq ft bottom + 2 sq ft sides = 5SIZE THE FIELDTable H 201.1(2) and Section H 301.11,200 gal / 100 = 1212 x 40 sq ft (sandy loam) = 480 sq ftEffective area per foot of trench = 5 sq ft480 / 5 = 96 ft of trenche.g. 2 lines x 48 ft (max 100 ft each)IF THE DESIGN CHANGES (same soil)Chambers: 480 x 0.70 = 336 sq ftLeaching bed: 480 x 1.5 = 720 sq ftMinimum trench bottom: 150 sq ftRequired: 480 sq ft = 96 ft of 3-ft trench (2 lines of 48 ft)1,200 gal tank on sandy loam (40 sq ft per 100 gal); tank under 5,000 gal capOver the 150 sq ft minimum; each line under the 100 ft maximum2025 CPC Appendix H: Table H 201.1(2), Sections H 301.1(1), (2), (5), and Table H 601.9CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Start With the Soil Type

Table H 201.1(2) of 2025 CPC Appendix H lists five typical soils and the leaching area each one needs per 100 gallons: coarse sand or gravel = 20 square feet, fine sand = 25, sandy loam or sandy clay = 40, clay with considerable sand or gravel = 90, and clay with a small amount of sand or gravel = 120. The same table gives the absorption capacity per square foot per 24 hours (5.0, 4.0, 2.5, 1.1, and 0.8 gallons). A site whose soil is not on the list, or that is questionable, must be percolation tested to the AHJ's satisfaction (Section H 401.2).

Step 2: Which Gallons Do You Use

Section H 301.1 says the effective absorption area is predicated on the required septic tank capacity or the estimated flow rate, whichever is greater. So the formula is: area (sq ft) = gallons / 100 x the square feet per 100 gallons for the soil. Where a percolation test is required, Section H 401.3 bars a system if the soil absorbs less than 0.83 or more than 5.12 gallons per square foot per 24 hours, with a loamy-sand replacement option for the fast soil.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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