Septic Tank Size

What size tank 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. 

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 by making 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. It has to be sized correctly.

+44 (0)1934 422 311

If you have any questions, feel free to call us

Share on Facebook
Share on LinkedIn

How to calculate a 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
A house with four bedrooms = minimum 6P system (5+1)
House with five bedrooms = minimum 7P system (5+2)
A 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.

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 it to a new sewage treatment unit.

Use our Septic Tank calculator below to define the one you need:

Number of bedroomsPMinimum size of septic tankTricel septic tank recommendationTricel sewage treatment recommendation
132450Tricel Vento 6Tricel Novo UK6
242600Tricel Vento 6Tricel Novo UK6
352750Tricel Vento 6Tricel Novo UK6
462900Tricel Vento 6Tricel Novo UK6
573050Tricel Vento 12Tricel Novo UK8
683200Tricel Vento 12Tricel Novo UK8
793350Tricel Vento 12Tricel Novo UK10
8103500Tricel Vento 12Tricel Novo UK10
9113650Tricel Vento 12Tricel Novo UK12
10123800Tricel Vento 12Tricel Novo UK12

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. You can also download our brochure to see the full range. 

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.

Frequently Asked Questions

Yes, Tricel offers a range of different plants for different options. However, you will need to discuss this with your local council.

Please refer to our articles about the UK legislation & the 2020 regulationsRegulations in England & Wales and the homeowners responsibilities.  

You can also download our ebook.

Finally, our team is there to answer your questions, discuss your project, and do site visits. CONTACT US

  • Tricel Vento septic tanks have a 10 years warranty from date of purchase.
  • Tricel Novo sewage treatment plants have a 20 years SMC structure warranty and 3 years warranty on the mechanical parts. 

Get in touch