Septic Sizing & Design
The septic sizing & design math: minimum tank size by bedrooms / occupants, design daily wastewater flow (occupants × gpd), drain-field absorption area (flow ÷ soil loading rate), a percolation-rate → soil-loading lookup, trench length & number of laterals, and tank capacity from dimensions (L × W × liquid depth × 7.48). Sizing values are labeled planning bands — the required design is set by a licensed designer and your local health department.
Designer, driller, health department & permit: Septic design, drain-field sizing and well siting must be verified by a licensed septic designer or professional well driller and approved by your local health department. Sizing rules, setbacks and perc / soil-loading requirements vary by jurisdiction. Confirm the design, pull the required permit and get the required inspection before you dig or drill.
Septic Tank Size by BedroomsSeptic tank size calculator: minimum gallons by number of bedrooms plus design daily flow from occupants. Planning bands; confirm with your health department.Daily Wastewater FlowDaily wastewater flow calculator: occupants times gallons per person per day gives the design flow (gpd) that sizes a septic drain field. Adjustable convention.Drain-Field Absorption AreaDrain-field absorption area calculator: design daily flow divided by soil loading rate gives the required square feet. Loading rate comes from your perc test.Perc Rate to Soil LoadingPerc rate to soil loading calculator: enter minutes per inch from your percolation test and get the gpd/sq ft loading band used to size a septic drain field.Trench Length & LateralsTrench length and laterals calculator: absorption area divided by effective trench width gives linear feet, then the number of laterals by max run.Tank Capacity from DimensionsSeptic tank capacity calculator: length times width times liquid depth times 7.48 gives gallons. Use liquid depth, not full tank height, for capacity.
Every Septic Sizing & Design tool is calculated server-side — visible immediately — with its formula, a worked example and a reference table, and works on the prices you enter. Background reading is in the guides.