Determining the required size of a leach field is a bit more complicated. The first thing to consider is the nature of the soil in which the leach field is to be constructed. Because water has to be absorbed in the soil, we need to know how fast it can be absorbed. This is called the percolation rate and is expressed as the time it takes for water in a test hole to decrease in level by one inch (minutes/inch). We must also know the type of soil and whether seasonal changes in the natural level of groundwater will interfere with the satisfactory operation of the system. Seasonal groundwater must be more than four feet from the bottom of the leach field trenches. Judgments regarding the soil conditions and percolation rates are best left to a professional. If the soil percolates very fast (less than one minute per inch) or very slow (greater than 60 minutes per inch) it will not be possible to install a standard leach field in the existing soil.

We must now determine the amount of water that has to be absorbed each day. As with the septic tank sizing, there are also “rules of thumb” that can be used to find out how much water must be absorbed each day for each bedroom in the house (expressed as gallons per day per bedroom). For older houses (built before 1979) we must allow 150 gallons per day (gpd) per bedroom. For houses where the toilets are limited to no more than 3.5 gallons per flush and the faucets and showerheads are limited to 3 gallons per minute or less, we must allow 130 gpd per bedroom. For houses with water-saving toilets that use only one gallon per flush we allow 90 gpd per bedroom. The required flow rate is found by multiplying the appropriate flow by the number of bedrooms (in this case, we do not have to count a garbage disposal as a bedroom).

Knowing the rate at which water can be absorbed by the soil (the percolation rate) and the flow rate (in gallons per day), we can use the following table to calculate how many square feet of absorption field is needed.

Absorption Percolation(minutes per inch) |
Application Rates(Gallons per Day per Square Foot) |

1 – 5 |
1.2 |

6 – 7 |
1.0 |

8 – 10 |
0.9 |

11 – 15 |
0.8 |

16 – 20 |
0.7 |

21 – 30 |
0.6 |

31 – 45 |
0.5 |

46 – 60 |
0.45 |

Soil with a percolation rate less than 1 minute per inch or more than 60 minutes per inch is unsuitable for a conventional system.

Required Area (square feet) = Flow Rate (gallons per day) / Application Rate (gallons per day per square foot)

Now that we know the number of square feet of absorption field that is needed, we can divide by the width of each trench to see how many feet of trench is required. The normal trench width

Assume you are buying a 3-bedroom house that was built in 1971. The leach field has failed and a new one must be installed. You have had a percolation test performed and the design professional has determined that the soil is suitable, the groundwater conditions are acceptable, and the percolation rate is 32 minutes per inch. How big an absorption field will be needed?

Since the house was built before 1979, the flow rate is 3 bedrooms times 150 gallons per day per bedroom, or 450 gallons per day. From the table above, the application rate is 0.5 gallons per day per square foot for a percolation rate of 32 minutes per inch. The required trench area is then 450 gallons per day divided by 0.5 gallons per day per square foot. You will need 900 square feet of absorption area. If the absorption trenches are 2 feet wide, you will need a total of 450 feet of absorption trench. Most health codes limit the length of any one trench (called a lateral) to no more than 60 feet, the minimum number of laterals is 450 feet divided by 60 feet per lateral, or 7.5 laterals. Where property conditions permit, it is best to keep the laterals the same length, so your design professional may specify 8 laterals, each 60 feet long. But what if there is only room on the property for laterals that are 45 feet long. In this case, you would need 10 laterals, or trenches. In addition to the area needed for the leach field, you should also allow room for possible expansion (50% expansion area is required in New York State).