Septic Tank Size for UK Domestic Properties

Factors that determine the correct Septic Tank Size for a domestic property

The septic tank size must be adequate for the flow that you are producing.  The size, style and construction fo your septic tank or cesspit is crucial to its performance.

Correct size for a septic tank depends on the number of people using the system, which in turn is determined by the number of bedrooms in the house. 

Table of minimum number of persons allowed for the septic tank sizing based on the number of bedrooms in the house 

Number of Bedrooms Number of People
1 2
2 4
3 5
4 6
5 7


Table of required septic tank size based on the number of litres used per day

Litres/Day No. of Persons Minimum size required in litres
720 2 to 4 2720
900 5 2900
1080 6 3080
1440 7 3260

 As explained in Septic tank systems the main purposes of a septic system are to remove solids from the raw sewage and only allow liquid effluent to discharge into the soakaway system, which soaks the effluent into the ground. The natural soil bacteria then destroy the bacteria and viruses present in the effluent before they can contaminate groundwater, as this is abstracted for drinking water all over the UK.

The minimum septic tank size for a 4 person household is 2720 Litres, with 180 Litres added for every extra person.  If the septic tank size is too small then you are likely to reduce the life of your soakaway drainage field system, as the retention time in the tank will be too short and insufficient settlement will take place. The soakaway or drainfield will then become blocked with small suspended solids and you will have no alternative but to either replace it or add a septic tank conversion unit in order that the effluent can discharge to a ditch instead.

Table of size of septic tank required based on the number of bedrooms in a house

Bedrooms per House Minimum size in litres
2 2720
3 2900
4 3080
5 3260

Many older septic tanks are too small for modern lifestyles as we use far more water now than we did 20 years ago.  For example, Back in 1960, the average water use/person/day was only 90 litres.  Today it is around 150 to 180 litres, almost double the amount.  Back in 1960, the weekly laundry wash used a 'twin tub', using the same water for whites, then coloureds finishing off with the dirty working clothes.  Twin tubs used about 35 litres per washing day.  Now, using automatic washing machines, each load uses a fresh water supply of around 50 litres per washing cycle.

As a guide, a 2720L will serve a maximum of 4 persons, a 3600L serves 8 people and a 4500L is a 13 person tank.  It is unlikely that permission would be given for a larger septic system as the Authorities know that the soakaway would be too large and expensive.  It is very likely that they would insist on a sewage treatment plant system. Sewage treatment plants now cost less than a septic tank system.

If your population is borderline, or if you want to extend the de-sludging interval, order the bigger septic tank size.