Tank volume is only one part of the system. Soil, groundwater, setbacks and the approved design determine what can be installed.
1. Know what the tank does
A conventional septic system separates solids in a tank, then sends effluent to a drainfield for further treatment through suitable soil. The tank is not a complete drinking-water treatment system and its outlet is not clean water.
A site needs space and suitable conditions for the approved treatment arrangement. A larger tank does not correct unsuitable soil or high groundwater. Protect the planned treatment area while laying out the house, driveway and water supply.
EPA · How septic systems work ↗2. Start with an approved site assessment
Ask the local authority what soil evaluation and design submission it requires. Bring a plan showing the house, well, boundaries, watercourses, access and proposed finished levels. Keep a replacement area if the local design calls for one.
System selection can change with soil permeability, slope, groundwater, occupancy and regulation. Alternatives may involve pumps or advanced treatment and a continuing service contract. Compare installation and operating requirements together.
EPA · Types of septic systems ↗3. Use a water budget as a planning input
Estimated household wastewater is residents multiplied by daily water use, adjusted for uses that do not enter the sewer. Irrigation usually belongs outside the household wastewater total; treatment backwash may need a separate assessment.
The load calculator multiplies flow by a user-selected holding period to illustrate hydraulic volume. The default three days is a scenario, not a design standard. It excludes sludge storage, local minimums, peak loading and the design of the soil treatment area.
4. Keep water and solids under control
EPA describes typical household inspection at least every three years and pumping commonly every three to five years; actual needs depend on the system. Mechanised or alternative systems may need more frequent service. Keep the inspection report and follow the approved maintenance plan.
Fix leaks, spread large water uses over time and keep wipes, grease and chemicals out. Keep vehicles off the drainfield and divert roof runoff away. Sewage backup, persistent odour or wet ground over the field warrants professional investigation. Never enter a septic tank.
EPA · Septic care and maintenance ↗Bring this to the design appointment
| Information | Why it matters | Where to obtain it |
|---|---|---|
| Occupancy / design bedrooms | Regulated loading basis | Building plan and local authority |
| Soil and groundwater assessment | Treatment feasibility | Approved site evaluator |
| Well and boundary locations | Required separations | Survey and local rules |
| Service access | Inspection and pump-out | Site layout |
| Power availability | Pumps, alarms and treatment | Electrical plan |
Worked example
An illustrative hydraulic volume
Four residents at an assumed 150 L/day produce 600 L/day of household flow.
600 L/day × 3 days = 1,800 L = 1.8 m³ of illustrative liquid volume
Do not order a 1.8 m³ tank from this result. Approved tank capacity may be governed by different loading rules and must include the required allowances.
What to have ready
- Approved site assessment
- Permitted design loading
- Tank, field and replacement-area plan
- Service access and operating schedule
Common mistakes
- Treating retention time as the only sizing criterion.
- Building or parking on the treatment area.
- Adding treatment chemicals in place of maintenance.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.