Tank and soakaway volumes from your occupancy, flow and soil figures.
Electrical and plumbing. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Septic and Soakaway Worksheet tool.
A septic tank is sized by the volume of sewage it receives and the time the sewage needs to settle, and both of those come from occupancy and flow rather than from the size of the house. That is why the tool asks for the flow rate and refuses to infer it, and why the soakaway is sized from the same flow against the ground's capacity to absorb it. The output is a volume, a soakaway area and the materials for both. What it is not is an environmental approval. Discharge, setbacks, percolation testing and the treatment standard are regulated locally, and the numbers here are the inputs to that process rather than a substitute for it.
A household of six at a hundred and fifty litres a day each produces nine hundred litres a day. At a retention of two days the tank needs eighteen hundred litres of working volume, plus the sludge and scum space the standard requires, so a tank of around two thousand seven hundred litres for a household of six. A soakaway at an infiltration rate of thirty litres per square metre per day for nine hundred litres a day needs thirty square metres of base area, which at a metre wide trench is thirty metres of trench. Both numbers came from the same flow, and both change together when the occupancy changes.
The daily flow and the retention period, and the sludge and scum storage the standard requires on top of that. Flow follows the occupancy and the fixtures, and it is the number that matters, because a tank sized by bedroom count is sized by a proxy for people who may not be there. Enter the flow, choose the retention, and the working volume follows, with the materials for it.
Because a flow rate derived from a bedroom count is an assumption about occupancy, and the consequences of getting it wrong are unpleasant and expensive to fix. A house with six occupants and a house of the same size with two produce very different loads, and a guest house is different again. Since you know the intended use and the tool does not, the flow is your input, and the calculation refuses to proceed without it.
From the daily flow and the infiltration rate of the ground, which comes from a percolation test. The soakaway has to absorb what the tank discharges, so its area follows the flow divided by the rate at which the ground accepts it, and a clay soil needs many times the area of a sandy one for the same house. The materials then follow the area: pipe, granular fill and geotextile for the trench or the pit you describe.
No. It sizes a system and counts its materials. Permits, discharge consent, setback distances, groundwater protection and the treatment standard are administered locally, and the numbers here are inputs to that process rather than a substitute for it. Where a site is sensitive, the design may need to be different from the standard solution the calculator assumes.
Yes. Enter volumes in litres or gallons and dimensions in metres or feet, and the calculation stays in the unit you chose rather than converting mid way. That matters for a tank specified in gallons by a supplier and a soakaway area measured in square metres on site, which is a common combination and a common source of mistakes when the conversion happens twice.