Septic Tank Calculator
Size a septic tank and drain field from occupancy and soil percolation rate.
About Septic Tank Calculator
Tanks are sized on design occupancy rather than current occupancy, because bedrooms outlast households. Drain field area depends on how fast the soil accepts water, so a slow percolation rate can more than double the land needed. Percolation testing governs whether a conventional system is permitted at all, which makes it the first thing to establish on an unserviced plot.
Formula
Septic Tank
Tank volume = daily flow × retention days + sludge allowance; field area = flow ÷ loading rate
- percolation rate
- Minutes for water to fall one millimetre in a test hole
- design occupancy
- The greater of actual occupants and two per bedroom
Worked example
With the default values below, Septic Tank Calculator returns Tank Volume Required of 3,800. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of bedroomsaccepts 1–12
- 3
- Occupantsminimum 1
- 4
- Water use per personminimum 10 L
- 150 L
- Percolation rateminimum 0.01 min/mm
- 0.8 min/mm
- Tank retention timeminimum 0.5 days
- 2 days
Results
- Tank Volume Required
- 3,800 L
- Daily Flow
- 900 L
- Drain Field Area
- 161.00 m2
- Trench Length at 600 mm Wide
- 268.33 m
- Soil Rating
- Suitable for a conventional drain field
Step by step
- 1Values used
- 2Septic Tank
- 3Tank Volume Required= 3,800 L
- 4Daily Flow= 900 L
- 5Drain Field Area= 161.00 m2
- 6Trench Length at 600 mm Wide= 268.33 m
- 7Soil Rating= Suitable for a conventional drain field
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Septic Tank calculated?
Tank volume = daily flow × retention days + sludge allowance; field area = flow ÷ loading rate Tanks are sized on design occupancy rather than current occupancy, because bedrooms outlast households. Drain field area depends on how fast the soil accepts water, so a slow percolation rate can more than double the land needed.
Why does Septic Tank matter?
Percolation testing governs whether a conventional system is permitted at all, which makes it the first thing to establish on an unserviced plot.
What values do I need to enter?
This calculator takes 5 inputs: Number of bedrooms, Occupants, Water use per person, Percolation rate, Tank retention time. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.