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Septic tank sizes

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

  • Septic tank
  • Sewage treatment plant
  • Cesspool
  • Non-standard system such as reed bed

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.


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 the 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 to a new sewage treatment unit.

Septic tank size calculator

Number of bedrooms P Minimum size of a septic tank Tricel recommendation
1 3 2450 Tricel Vento 6
2 4 2600
3 5 2750
4 6 2900
5 7 3050 Tricel Vento 12
6 8 3200
7 9 3350
8 10 3500
9 11 3650
10 12 3800

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.


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