What size septic tank do I need? Sizing calculator
Estimate your septic tank size from the people in the house, following AS/NZS 1547.
Getting the size wrong is the costly mistake. A tank that's too small doesn't settle the solids in time — they wash out and clog the absorption trench within months. Too big is just wasted money.
This estimate follows the design-flow logic of AS/NZS 1547. Add the water uses that push up the load and you get a realistic figure — but your council and soil report always have the final word.
This is an estimate: the design is finalised by a report to your council, which sizes the tank on the daily design flow and the soil's capacity to absorb the effluent. Minimum tank sizes vary by state.
If you're between two sizes, go bigger: the price difference is small, and the cost of a clogged trench is not. If your soil drains poorly, you may need an AWTS instead — check the soil first.
FAQ
What size tank for 4 people?
With a typical design flow allowance, around 3,000 litres — but the minimum and the exact figure depend on your state and council, and on how much water the household uses.
What is design flow?
The daily wastewater volume the system is designed for, based on people and fixtures, under AS/NZS 1547 — not just headcount.