septic tanks
What size do you need?
Depending on your habitation and how many people are living under your roof, defining your needs in terms of septic tank size could be quite challenging. Read the article below to choose what water treatment tank is more suitable for you.

septic tanks

Septic tank sizes

In the UK, a house not connected to the mains/sewers will require treatment using one of the following solutions:

In any such case, you, as the homeowner, are responsible for meeting the general binding rules.

To make sure your septic tank works as it is designed to, it needs to be capable of handling the flow of effluent that your premise is producing.

septic tank size

How to determine septic tank size?

Step 1: Determine “P” (Minimum Population) from the number of bedrooms you have.

The volume of effluent coming from your house is determined by the number of people using the system. This is calculated based on the number of bedrooms in your home.

*Reference: Flows & Loads from British Water

**Please note that the abbreviation for Minimum Population is “P”.


Small one and two-bedroom houses or flats

  • Flat with one bedroom = minimum 3P system
  • Flat with two bedrooms = minimum 4P system


Single house

Up to and including three bedrooms: P = 5, add 1 P for each bedroom thereafter.

  • House with three bedrooms = minimum 5P system
  • House with four bedrooms = minimum 6P system (5+1)
  • House with five bedrooms = minimum 7P system (5+2)
  • House with six bedrooms = minimum 8P system (5+3)

*Reference:  Flows & loads from British Water for larger domestic housing, commercial and industrial premises.

Step 2: Calculate the number of litres of wastewater produced per day

British Water estimates that one person creates 150L per day. To calculate the number of litres per day for your property, just multiply the number of people x 150L.

Example: three bedrooms house = 5P (see step 1)

5 P x 150L = 750L per day

Step 3: Add 2,000L to your litres produced per day

Carrying on with the example above: 750L (step 2) + 2,000L = 2,750L

Therefore, the minimum size septic tank required for this example is 2,750L. For more information on our septic tanks or on our

wastewater treatment solutions, do not hesitate to download our ebooks. 

Choosing the correct size septic tank

​Given the choice, it’s recommended to round up the tank size required. If a septic tank is too small, your soakaway system will suffer as a result. If insufficient settlement time occurs, small suspended solids might block your soakaway. In which case, you will have to replace it or convert to a new sewage treatment unit.


Septic tank size calculator

Number of bedroomsPMinimum size of a septic tankTricel recommendation
132450Tricel Vento 6
242600Tricel Vento 6
352750Tricel Vento 6
462900Tricel Vento 6
573050Tricel Vento 12
683200Tricel Vento 12
793350Tricel Vento 12
8103500Tricel Vento 12
9113650Tricel Vento 12
10123800Tricel Vento 12

For P greater than 12, please refer to the flows & loads from British Water.

For larger projects, Tricel Vento 20 is also available. Contact us to find out more.

​Regulation of septic tanks

​If you are buying a new septic tank, make sure it is certified to EU CE12566-1

Contact us to discuss your project or if you’re thinking of upgrading with a brand new septic tank or switch to a sewage treatment plant.

View Tricel Vento brochure
Get in touch if you have questions or need further information
Key Features


Strong & robust shallow dig tank. No electrical or moving parts

Fast delivery

UK manufactured ensuring short lead time


Tricel Vento is certified to EN12566-1

Get in touch

    Tricel Environment UK

    A trading brand of Dewey Waters Ltd.
    Heritage Works, Winterstoke Road, Weston-super-Mare, BS24 9AN, United Kingdom.
    Tel: +44 (0)1934 422 311