Septic Tank Size Calculator
Work out the septic tank capacity a household needs from users, water use and retention time.
Recent
Formula
How the calculation works
The daily flow is the number of users times the litres each uses, and multiplying by the retention days gives the liquid capacity.
Sludge builds up every year and needs its own space, so it is added to the liquid volume for the total.
The result is given in litres and cubic metres, which is the unit tanks are usually sold in.
Common mistakes
- Sizing for the current household and forgetting that a growing family or guests will need more capacity.
- Ignoring the sludge space, which is what fills a tank and forces an early pump-out.
When to use it
- Use it when planning a septic tank for a house or small building.
- It is not a building permit; local rules set the minimum size and setbacks.
Worked example
Five users at 120 litres a day with 2 days retention and 30 litres sludge each need 1,200 plus 150, a total of 1,350 litres (1.35 cubic metres).
Common questions
How long should waste stay in the tank?
Two days retention is a common minimum for solids to settle properly.
Is a bigger tank better?
A larger tank means less frequent emptying, but it costs more to build and must still fit the plot.
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