Septic Tank Size Calculator

People living there

Guests on average

Water per person, l a day

Retention, days

Reserve, %

Result

Tank volume, m³2.88
That in litres2,880
Effluent a day, l800
First chamber, m³1.44
A holding tank fills in, days3.6
What mattersFor a sealed holding tank read the same figure as a pump-out interval: it fills in as many days as the retention period.

Three days is not a safety margin, it is the process

Calculates the septic tank volume a household needs from the three-day settling rule, and how fast a sealed holding tank would fill.

The rule is a tank that holds three days of effluent. That is not caution: it is roughly how long solids take to separate out and break down before the water moves on to the next stage.

With the standard 200 litres a person a day, a four-person household needs about 2.4 cubic metres before any reserve — which is why the smallest sensible tank for a permanently occupied house is around three.

What this calculator shows

The tank volume in cubic metres and in litres
The daily effluent and the size of the first chamber
How many days a sealed holding tank of that size would take to fill

What to keep in mind

The 200 litres a person a day figure covers a house with a full set of plumbing. A cottage with a summer shower uses far less.
The first chamber is taken as half the volume, which suits a standard two- or three-chamber layout.

FAQs

Where does 200 litres a person come from?

It is the design figure for a home with running water, a shower, a washing machine and a WC. Metered use is often lower, but the tank is sized for the design load.

Why exactly three days?

Because that is how long the settling and initial breakdown take. A smaller tank pushes solids into the next stage and clogs the drainage field.

Do guests need to be counted?

If they come regularly, yes. A tank sized to the exact permanent household overflows every time the family visits.

How often does a septic tank need pumping?

A working septic tank with a drainage field needs the sludge removed once or twice a year. A sealed holding tank has to be emptied as fast as it fills, which is the figure shown here.

Worked example

A house for four

Input: 4 people, 200 litres each, 3 days of settling, a 20 percent reserve

Output: 2.88 m³

Note: That is 800 litres of effluent a day, so the same tank used as a sealed holding vessel would need emptying every 3.6 days — which is precisely why people build septic tanks instead.

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Septic Tank Size Calculator

Result

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