Drain Field Size Calculator
Your soil determines everything about drain field sizing. A sandy lot absorbs effluent quickly and needs far less area than a clay-heavy property where water moves through the ground at a fraction of the speed. This calculator estimates drain field dimensions based on your household size, soil conditions, and system type -- giving you a realistic starting point before a professional site evaluation. If you have percolation test results from a soil engineer, enter those for a more precise estimate.
360 gallons/day design flow
Moderate drainage, balanced soil
Minutes per inch from percolation test. Overrides soil type estimate.
Standard gravel-and-pipe trenches, 3 ft wide
Household size. Every bedroom adds 120 gallons per day to the design flow, regardless of actual occupancy. A 4-bedroom home is designed for 480 gallons daily.
Soil permeability. Sandy soils absorb effluent at 1.2 gallons per square foot per day. Heavy clay absorbs just 0.2 -- meaning a clay lot needs six times the drain field area of a sandy one for the same household.
Percolation test. A perc test measures how fast water drains through your specific soil. If you have these results (in minutes per inch), enter them above for a more accurate calculation than soil type alone.
System type. Conventional trenches and chamber systems use 3-foot-wide trenches. Drip distribution uses narrower 2-foot trenches but requires a pump and more maintenance.