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Septic Tank Sizing Calculator

Work out septic tank sizing instantly with clear inputs, formula shown and shareable results.

About Septic Tank Sizing Calculator

Septic tank capacity is the daily wastewater flow multiplied by the retention time needed for settlement, usually one to two days, plus an allowance for accumulated sludge between desludging visits. Dimensions follow from a liquid depth of about 1.5 metres and a length to width ratio of two to one, which promotes plug flow rather than short-circuiting.

Formulas

Tank volume

Litres = users x litres per person per day x retention days + sludge allowance

Dimensions

Plan area = volume / 1.5 m depth, with length = 2 x width

Worked example

With the default values below, Septic Tank Sizing Calculator returns Tank volume required of 2.420. Change any input above and every figure here is recalculated in your browser.

Inputs used

Number of usersminimum 1
8
Wastewater per person per dayminimum 1 L
120 L
Retention timeminimum 0.5 days
2 days
Sludge storage allowanceminimum 0 L
500 L

Results

Tank volume required
2.420 m³
Liquid retention volume
1,920 L
Suggested tank length
1.80 m
Suggested tank width
0.90 m

Step by step

  1. 1Values used
  2. 2Tank volume
  3. 3Dimensions
  4. 4Tank volume required= 2.420 m³
  5. 5Liquid retention volume= 1,920 L
  6. 6Suggested tank length= 1.80 m
  7. 7Suggested tank width= 0.90 m

Assumptions and limits

  • The result is derived only from the 4 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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. On-site wastewater systems must be designed to local health and environmental regulations and sized from percolation test results.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. On-site wastewater systems must be designed to local health and environmental regulations and sized from percolation test results.

Frequently Asked Questions

How is the Septic Tank Sizing figure worked out?

Litres = users x litres per person per day x retention days + sludge allowance. Septic tank capacity is the daily wastewater flow multiplied by the retention time needed for settlement, usually one to two days, plus an allowance for accumulated sludge between desludging visits.

Does the tank replace a soakaway?

No. The tank only settles solids and provides anaerobic treatment. The effluent still needs a soakaway or drainage field sized on soil percolation to complete disposal.

What values do I need to enter?

This calculator takes 4 inputs: Number of users, Wastewater per person per day, Retention time, Sludge storage allowance. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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