Owls Hall, Farm Blackmore End, Braintree, Essex CM7 4DF
CONTACT OWLS HALL
01844 877 030 01844 877 030

How to size a septic tank

HOW TO SIZE A SEPTIC TANK

Owls Hall Environmental How To Guide

If you want to know how to calculate septic tank size, look no further. A septic tank is 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 tank is essential for ensuring it conforms with building regulations and provides long term performance. 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:

Contact us at Owls Hall: 01371 850 537