Septic Tank Capacity Calculator
Find the tank capacity needed for a household from the number of users, the daily water use per person and the retention days.
Recent
Formula
How the calculation works
The calculator multiplies the number of users by the litres each person uses per day to get the daily flow into the tank.
It multiplies that daily flow by the retention days, which is the time the waste should stay in the tank for solids to settle.
The result in litres is also divided by one thousand to show the capacity in cubic metres for construction planning.
Common mistakes
- Building a tank smaller than the calculated capacity, which makes it fill quickly and need frequent emptying.
- Choosing too few retention days, since a short retention time lets solids leave with the liquid and block the soak pit.
When to use it
- Homeowners use it to size a septic tank correctly before building, instead of copying a neighbour's tank blindly.
- Builders use it to check whether an existing tank is large enough for the number of people who will use it.
Worked example
For 5 users at 150 litres each per day over 3 retention days, the daily flow is 750 litres, the capacity is 2250 litres and that is 2.25 cubic metres.
Common questions
What is a reasonable retention time?
Two to three days is the common figure, because waste needs that long in the tank for solids to settle and the liquid to flow out more cleanly.
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