How To Size A Septic Tank

How To Size A Septic Tank - A Guide To Septic Tank Sizing

How To Calculate UK Septic Tank Size by Owls Hall

If you want to know how to calculate septic tank size, look no further. It can be tough to work out exactly what size septic tank you need for either a commercial business property or your home. As such, we have put together this septic tank sizing guide which specifies which septic tank size a property would need, determined by the amount of people it needs to serve. With over 50 years’ experience in septic services, you can fully rely on the information provided by Owls Hall in this guide.

For context, a septic tank is generally used where no mains sewage connection is available, and the water can be discharged through the ground via a soakaway or drainage field.

Important septic tank information

Learning how to size a septic is essential for ensuring it conforms with building regulations and provides long term performance. The last thing you want is to realise down the line that you have chosen a septic tank too small for the amount of people in your property and have to install a bigger one.

Remember, septic tanks are not allowed to discharge to a ditch. Upgrading from a septic tank to a sewage treatment plant will allow discharging to a ditch and provide cleaner water, increasing the life of a soakaway should one be used.

4 steps to size a septic tank

The below is a guide to sizing a septic tank, with worked examples for single and multiple properties.

Step 1 – Find out how many people the property can accommodate.

British water flows and loads 4 document states that:

  • A single property of up to 3 bedrooms must be sized at 5 people.
  • A single property of 4 bedrooms must be sized at 6 people.
  • A single property of 5 bedrooms must be sized at 7 people
  • A single property of 6 bedrooms must be sized at 8 people
  • Multiple properties are sized individually and then added together.

Example A:

4 bedrooms = 6 people

A reduction factor is used where populations exceed 12 people This is to account for the fact that it is highly unlikely each property will be full. From 13-25 population, you can reduce the figure by multiplying the population by 0.9. From 26-50 population, you can reduce the figure by multiplying by 0.8.

Example B:

4 bedroom + 5 bedroom property = 6 people + 7 people = 13 people.
Then multiply by 0.9 and round up: 13 x 0.9 = 11.7 (12 rounded up)

Step 2: Calculate the number of litres produced from the property or properties

British water flows and loads 4 document states each person creates 150L a day. Therefore, take the number of people in step one and multiply by 150.

Example A:

4 bedrooms = 6 people. 6 x 150 = 900L per day

Example B:

4 bedroom + 5 Bedroom properties = 12 population: 12 x 150 = 1800L Per day

Step 3: Add 2000L to the answer in step 2. This will be the minimum size you must have to comply with building regulations and the environment agency general binding rules.

Example A:

4 bedrooms = 6 people. 6 x 150 = 900L + 2000L = 2900L

Example B:

4 Bedroom + 5 Bedroom = 12 people: 12 x 150 = 1800L = 2000L = 3800L

Step 4: Round the number up to the nearest available septic tank size. Under-sizing your septic tank will result in the water produced being high in suspended solids and risks blocking your soakaway.

For ease, we have created a simple table.

Septic tank size guide

Bedrooms Minimum size of septic tank Recommended product
1-3 bedrooms 2750L 3000L Septic Tank
4 bedrooms 2900L 3000L Septic Tank
5 bedrooms 3050L 4000L septic tank
2 x 1-3 bed properties 3500L 4000L septic tank
2 x 4 bedrooms 3800L 4000L septic tank
2 x 5 bedrooms 3950L 4000L septic tank
3 x 2 bedrooms 3800L 4000L septic tank
3 x 3 bedrooms 4250L 5000L septic tank

For sizing of a septic tank for a commercial building, more detailed calculations must be made to ensure the daily flow is accurate. This is done using the British water flows and loads 4 Document. For assistance with this, our technical team can help on 01844 202121.

Further Reading: