Soil Loading Rates – Septic System Sizing

How are septic systems sized?

Soil loading rates are numbers that describe the rate that effluent water is expected to penetrate a soil.  Loading rates are expressed in gallons per day / square foot.   Soil texture and structure are both considered when determining the soil loading rates.  Click the below graphic to see Indiana’s Loading Rate Chart for Subsurface septic systems.

The soil loading rate  is used to calculate the size of the septic system using the following equation.

Design Daily Flow / Soil Loading Rate = Square Footage of Trench Required

System Sizing Example:

Three Bedroom House (3 bedrooms x 150 gpd/bedroom) = 450 gpd flow

Fine Sand Soil Texture = Loading rate of .75 gpd/sq. ft. moderate structure

450 divided by .75 = 600 sq. ft. absorption field

Indiana Septic Code 410 IAC 6-8.2 defines Loading Rates as:

Sec. 43. “Soil loading rate” means the allowable rate of application of septic tank effluent to the soil. It is
expressed in gallons per day per square foot. (Indiana State Department of Health; 410 IAC 6-8.2-43)