Septic Tank Size Calculator

Written byOleh Soloninupdated 25 September 2026

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 matters
For a sealed holding tank read this volume as a pump-out interval: it fills in 3.6 days — the retention period plus the reserve.

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.

How to use

Enter how many people live in the house all year and how many guests stay on an average day.
Keep 200 litres a person for a house with mains water, a shower and a washing machine, or lower it for a cabin without them.
For a septic tank leave three days of retention. For a sealed holding tank enter how many days you want between pump-outs.
The reserve defaults to 20 percent. The result gives the volume, the daily flow, the first chamber and how fast a tank of that size fills.

Tank size by household

The volume grows in step with the household: every resident adds 0.72 m³ — 200 litres over three days plus the 20 percent reserve. Two people manage with under one and a half cubic metres; six need more than four.

The three-day rule is Russia's SP 32.13330.2018, clause 9.2.13.3: a septic tank serving up to 25 people holds at least three days' inflow, and one serving more holds at least two and a half. Other codes size differently: the British BS 6297 uses 2,000 litres plus 180 a person, which gives 2,720 litres for four against the 2,880 here.

Every row comes from the calculator on this page at 200 litres a person, three days of retention and a 20 percent reserve.
Residents	Effluent a day, l	Tank volume, m³	First chamber, m³
1	200	0.72	0.36
2	400	1.44	0.72
3	600	2.16	1.08
4	800	2.88	1.44
5	1,000	3.6	1.8
6	1,200	4.32	2.16
8	1,600	5.76	2.88

Holding tanks: size against pump-out interval

A sealed tank lets nothing into the ground, so it is emptied in full. It is sized not by retention but by how often you are willing to call the tanker: put the interval in the retention field.

For four people with full plumbing, a weekly pump-out needs 6.72 m³ and a monthly one almost 29. A holding tank suits low flows: at 50 litres a person the same four manage with 1.68 m³ emptied once a week.

Every row comes from the calculator on this page for four people at 200 litres a day. The reserve stretches the interval by its own share: 20 percent turns a week into 8.4 days.
Pump out every, days	Volume with 20% reserve, m³	Without reserve, m³	Fills in, days
3	2.88	2.4	3.6
7	6.72	5.6	8.4
14	13.44	11.2	16.8
30	28.8	24	36

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 calculator takes 200 litres a person a day for a house with full plumbing; take the design flow your local standard requires. A cabin with a hand basin and a summer shower uses a fraction of it.
The first chamber is taken as half the volume, which is how a three-chamber tank is divided. In a two-chamber tank the first chamber takes three quarters.

FAQs

Where does 200 litres a person come from?

It is the calculator's design figure for a home with running water, a shower, a washing machine and a WC; your local standard may set another. Metered use can be lower, but the tank is sized for a design load so it copes with the busy days.

Why exactly three days?

That is how long settling and the first stage of breakdown take, and it is what the Russian code asks for below 5 m³ a day. A smaller tank pushes solids into the next stage and clogs the drainage field.

What size septic tank do two people need?

1.44 m³ at 200 litres a person, three days and a 20 percent reserve: 400 litres of effluent a day and a first chamber of 0.72 m³. If two more come every weekend, size it for four — 2.88 m³.

Do guests need to be counted?

If they come regularly, yes. Extra flow cuts the settling time and can overload treatment and drainage, so regular guests belong in the design load: four residents and two regular guests need 4.32 m³, not 2.88.

How often does a septic tank need pumping?

A septic tank with a drainage field is pumped for sludge, not for volume, so the interval depends on the design; the US EPA's guidance for a household tank is every three to five years. A sealed holding tank has to be emptied as fast as it fills, which is the figure shown here.

How big a holding tank for a weekly pump-out?

For four people at 200 litres each, 6.72 m³ with a 20 percent reserve or 5.6 without. At cabin use of about 50 litres a person, the same four need 1.68 m³.

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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