Septic Tank Size

Septic Tank Sizing: Your Essential UK Guide

How do I work out the right septic tank size for my home?

Calculating the correct size for your septic tank is crucial. Here’s the standard formula used in the UK, as outlined by BS6297 and British Water:

C = (150P + 2000)

  • C = Tank capacity (litres)
  • P = Potential occupants (not the current number of residents)

Understanding the formula:

  • Potential occupants: This means the maximum number of people your house could accommodate. Children count as adults for this calculation.
  • Desludging: Assume your tank will be emptied (desludged) annually.

Example: A 4-bedroom house (which could potentially house 6 people) would need a minimum septic tank size of 2900 litres (2000 + (150 * 6)).

Important Note: The minimum septic tank size allowed in the UK is 2700 litres.

What are the most common septic tank sizes in the UK?

Selecting the right septic tank size is important for the smooth operation of your off-mains drainage system. Here’s a look at what’s typically used across the UK:

Key Things to Remember:

  • Bedrooms Matter: The number of bedrooms in your house is the main way to estimate your needed septic tank size. UK regulations have standard occupancy figures per bedroom.
  • Real Residents: If more people live in your home than the bedroom count would suggest, you might need a larger septic tank.
  • Water Matters: Newer homes tend to use more water than older ones (think power showers, washing machines, etc.). A larger tank might be needed if your home has high water usage.

Popular Septic Tank Sizes in the UK:

  • Smaller Homes (1-3 bedrooms, up to 5 occupants): Tanks in the 2,700 – 3,750 litre range are frequently used.
  • Medium-Sized Homes (4-5 bedrooms, up to 7 occupants): 3,600 – 4,500 litre tanks are the norm.
  • Larger Homes (6+ bedrooms, or high-occupancy situations): You’ll likely need a 4,800+ litre tank. For particularly large properties, a sewage treatment plant might be required instead of a septic tank.

Get Expert Advice: Always speak to a qualified professional to get the perfect septic tank size for your home. They’ll take into account building regulations and local environmental factors specific to your area.

Septic Tank Sizing for the UK: A Guide

When sizing a septic tank for your UK property, the go-to resource is British Water’s Code of Practice “Flows and Loads – 4”. A copy can be found at:

Understanding British Water Guidelines

  • Bedrooms and Population: For homes with up to 3 bedrooms, the system should accommodate a minimum of 5 people. Add one extra person for each additional bedroom (e.g., a 4-bedroom home needs a 6-person system).
  • Occupancy Matters: Design the tank size around the maximum potential occupancy of the home, not just the current number of residents.
  • Water Use: British Water standards estimate 150 litres of wastewater per person, per day. Tank sizes may be reduced for older homes with low water usage, or increased if modern fixtures suggest greater water consumption.
  • Appliance Impact: If appliances like waste disposal units heavily impact wastewater volume or composition, factor this into the tank size for extra capacity.
  • Essential Maintenance: British Water’s capacity calculations assume the septic tank will be desludged (emptied) annually.

Key Takeaways

  • Bigger is Generally Better: An oversized septic system is preferable to one that’s too small, ensuring effective waste management.
  • Regulations Still Apply: Remember that British Water guidance complements, rather than replaces, Building Regulations and Environment Agency standards.
  • Getting Professional Help:
    • Qualified Installers: Seek advice from reputable septic tank installers or suppliers for site-specific guidance.
    • British Water Resources: Visit their website for more information and approved manufacturers.

Septic Tank Sizing: Understanding UK Building Regulations

If you’re planning a new off-mains drainage system, ensuring your septic tank is the correct size is crucial. Building Regulations set out the minimum standards to follow.

Key Requirements:

  • Minimum Capacity: For properties with up to 4 occupants, your septic tank needs a minimum capacity of 2,700 litres (2.7m³) below the inlet.
  • Scaling for More Occupants: Every additional resident means you’ll need an extra 180 litres of tank capacity.
  • Construction Standards: Your septic tank can be a prefabricated unit (typically glass-reinforced plastics, polyethylene, or steel) complying with BS EN 12566-1, or built onsite using engineering bricks and concrete.

Additional Considerations:

  • Leak Prevention: Septic tanks must be watertight to prevent both groundwater entering and sewage escaping.
  • Ventilation: Tanks must be vented well away from any buildings.
  • Maintenance Access: Adequate access points are necessary for inspection, cleaning, and emptying.

Important Note: Building Regulations are the bare minimum. Heavier water use or other factors may mean you require a larger system. Always get professional advice for precise sizing and to ensure full regulatory compliance.

Where to Find the Regulations:

Visit the UK government website for the official Building Regulations:

Sizing Examples (Domestic Dwellings):

  • 1-Bedroom House: 2450 litres is the smallest permitted, but many regulations assume a minimum occupancy of 3 people. Consult an expert.
  • 2-Bedroom House: 2600 litres minimum
  • 3-Bedroom House: 2750 litres minimum
  • 4-Bedroom House: 2900 litres minimum
  • 5-Bedroom House: 3050 litres minimum
  • 6-Bedroom House: 3200 litres minimum

Important Notes:

  • These are the minimum sizes recommended by UK Building Regulations. Your actual septic tank needs may be larger depending on your typical water usage.
  • Regulations are determined by the potential occupancy of the house (based on the number of bedrooms), rather than the number of people residing there at any particular time.
  • Always consult a qualified drainage professional for the most accurate assessment of your septic tank size requirements. They’ll factor in your specific circumstances beyond just bedroom count.

Septic Tank Sizing Guide for the UK

How big a septic tank do I need for 1 person?

Though less common, a 1-person dwelling still requires a minimum tank size of 2,700 litres. However, most professionals recommend sizing up slightly to accommodate occasional guests and fluctuations in water usage.

What size septic tank do I need for 2 people?

The minimum requirement remains 2,700 litres. Again, consider sizing up to provide flexibility for potential visitors.

What size septic tank do I need for 3 people?

The recommended minimum increases slightly to 2,880 litres (2.88 cubic metres).

Septic Tank Sizes for Larger Households

  • 4 people: 3,060 litres (3.06 cubic metres) minimum
  • 5 people: 3,240 litres (3.24 cubic metres) minimum
  • 6 people: 3,420 litres (3.42 cubic metres) minimum
  • 7 people: 3,600 litres (3.6 cubic metres) minimum
  • 8 people: 3,780 litres (3.78 cubic metres) minimum
  • 9 people: 3,960 litres (3.96 cubic metres) minimum
  • 10 people: 4,140 litres (4.14 cubic metres) minimum
  • 11 people: 4,320 litres (4.32 cubic metres) minimum
  • 12 people: 4,500 litres (4.5 cubic metres) minimum
  • 13 people: 4,680 litres (4.68 cubic metres) minimum

How are these sizes calculated?

The baseline is 2,700 litres for up to four occupants. Every additional resident increases the minimum requirement by 180 litres.

Additional Considerations:

  • Opt for a larger tank if you have high water usage habits or frequent visitors.
  • Properties with more than 13 residents might benefit from a sewage treatment plant instead of a traditional septic tank system.
  • Always seek the advice of a qualified drainage professional. They’ll provide the most accurate assessment and ensure your system best suits your household’s needs.