Septic Tank Size Calculator

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

About Septic Tank Size Calculator

Septic tank capacity is daily flow times retention time, plus sludge storage, plus a base allowance. The common British rule of 2,000 litres plus 150 litres per person gives a similar answer, and a 2 day retention is the minimum for adequate settlement.

Formula

Septic sizing

capacity = occupants x flow per person x retention days x (1 + sludge allowance) + base volume

Worked example

With the default values below, Septic Tank Size Calculator returns Tank capacity required of 4,100. Change any input above and every figure here is recalculated in your browser.

Inputs used

Occupants: 5 people
Flow per person: 150 L/day
Retention time: 2 days
Sludge storage allowance: 40 %

Results

Tank capacity required: 4,100 L
Tank capacity: 4.10 m³
Total daily flow: 750 L/day
Emptying interval: 4 months

Step by step

1Values used
2Septic sizing
3Tank capacity required= 4,100 L
4Tank capacity= 4.10 m³
5Total daily flow= 750 L/day
6Emptying interval= 4 months

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 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

Why does retention time matter?

Solids need time to settle and scum to rise. Too short a retention lets solids carry over into the drainage field and blind it permanently.

How often should a tank be emptied?

Annually is the usual recommendation, or when sludge reaches a third of the depth. Overfull tanks discharge solids that destroy the soakaway.