How to Determine the Size of a Septic Leach Field

A leach field is a portion of a septic system. It is the last stage where the sewage exits the septic tank and enters into a series of trenches to complete the waste treatment and disposal. In some areas a septic system with leach field comprise a required form of waste disposal. Determining the correct size for your leach bed is determined based on a number of factors including local laws, the soil absorption and gallons of water used each day.

Step 1

Contact your local government agencies to verify any local, county or state requirements for septic tanks in your area. There may be zoning laws or restrictions on the distance a septic system, including leach field, can be placed from structures, roads and wetlands.

Step 2

Complete a soil perc test. Soil percolation measures how quickly water is absorbed into the ground, and it plays a major role in the size of the leach field that will be required. The slower the perc, the larger the leach field required. Contact a professional septic engineer to complete this test for you.

Step 3

Determine the number of gallons your household uses each day. For homes built before 1979 an estimated use is 150 gallons per bedroom, per day, according to the Inspectapedia website. New homes with more efficient water systems will use less water per day. Check your water meter and billing cycles to determine your average usage of water per day. Local codes may dictate the minimum gallons used to determine the size of your leach field no matter what your personal usage is.

Step 4

Divide the number of gallons per day by the perc rate of the soil to determine the amount of square footage required in your leach field. For example, a home that uses 1,000 gallons of water a day with soil that percolates at 2.5 gallons per day will require a leach field of 400 square feet.

Step 5

Plan for the unexpected. Overestimate your water needs. It is advisable to have a leach field that is bigger than what is required based on the calculation above. You never know when you might want to increase the size of your home or family, have rain for an extended period of time or any other unforeseen issue that may create a larger need for and use of water. You do not want to have the leach field back up with sewage into your home.