How to size a septic tank

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. Learning how to size a Septic Tank is important to ensuring it conforms with building regulations and provides long term performance. 

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.

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.

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.

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

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:

• How does a septic tank work
• What items should not go into a septic tank