Septic sizing follows a fixed logical sequence: design flow from bedroom count sets the daily wastewater volume, a soil evaluation or percolation test sets the application rate the soil can handle, and together those two determine tank capacity and drainfield area. Dispersal method and setbacks are chosen last, fitted to the site’s constraints.
1. Design flow. Set from bedroom count under state code, this is the daily gallons the system has to be able to treat.
2. Soil and site evaluation. Test pits or a percolation test establish the application rate, depth to seasonal high water table, depth to any restrictive layer, and slope.
3. Tank capacity. Sized from design flow, with a code-set minimum, typically in a two-compartment configuration.
4. Drainfield area. Design flow divided by application rate. Slower soil or higher flow both increase the required footprint.
5. Dispersal method. Conventional trench where soil and site allow it; pressure distribution, chambers, a mound, a sand filter, drip dispersal, or an aerobic treatment unit where soil, water table, slope, or lot size rule a conventional system out.
6. Setbacks and reserve area. Distances to wells, property lines, surface water and foundations, plus land set aside for a future replacement field.
Each step constrains the ones after it. A soil evaluation that finds a shallow water table can eliminate an entire category of system before dispersal method is even considered; a small or oddly shaped lot can force a more space-efficient method even where the soil itself would support a conventional field. Sizing is not a single calculation but a chain of decisions, each one narrowing the options for the next.
We hold recorded septic as-built drawings and parcel-level septic-versus-sewer classification for thousands of US counties. Look up where records are kept for a specific county, or open the interactive parcel map to check a property directly.
Find your county’s septic records Open the parcel map
Design flow, calculated from the property's bedroom count under state code, since it determines the daily wastewater volume every later step is sized around.
The soil and site evaluation: application rate, depth to seasonal high water table, depth to any restrictive layer, and slope. Any of these can rule a conventional system out in favor of an alternative.
Tank capacity is set from design flow directly, while drainfield area also needs the soil's application rate, so in practice both draw on the design flow and soil evaluation together rather than one strictly before the other.
Guide last reviewed 2026-08-31.